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By Rob Robinette

Here's my methodology for troubleshooting a tube guitar amplifier. Many of these tactics follow to cast state amps too.

If you are turning on a brand new build amp or heavily repaired/modified amp for the primary time I like to recommend you employ a light bulb current limiter and apply my Amp Startup Procedure. Following it will decrease damage because of a miswired amp.

The extra about how tube amps paintings the better they're to troubleshoot, so continue to learn.

WARNING: A tube amplifier chassis accommodates deadly top voltage even if unplugged--sometimes over seven hundred volts AC and 500 volts DC. If you haven't been educated to work with prime voltage then have an amp technician carrier your amp. Never touch the amplifier chassis with one hand while probing with the opposite hand as a result of a lethal surprise can run between your fingers through your middle. Use only one hand when running on a powered amp. You must drain the filter caps earlier than doing any paintings inside an amplifier chassis. See extra tube amplifier protection data right here.

Preliminary Troubleshooting

Divide and conquer. Does a quantity, tone or master quantity management impact the noise? When a control affects the noise it generally signifies the noise is coming into the amp earlier than that control however I have seen a quantity management that caused noise in a bad power tube build up so this is not a hard-and-fast rule.

Always suspect a foul tube as they are the most common failure point in a tube amplifier. Having a spare set of tubes to change into the amp one after the other is a should for gigging tube amp customers. A nasty tube may cause a variety of different symptoms together with the whole lot from complete signal loss, hum, hiss, static to one thing that feels like whale sounds. You identify it and a nasty tube may cause it. Power tubes generally wear out quicker than preamp tubes however as they say, "tubular morghulis," "all tubes must die." Don't forget to check out a brand new rectifier tube--they could cause a wide variety of problems including weird noises. Reverb motive force tubes frequently have over Four hundred volts on their plates and die sooner than preamp tubes.

Loose or grimy tube socket pins can cause a wide variety of challenging to diagnose intermittent problems including entire loss of sign, purple plating due to lack of bias voltage, weird noises, static, crackle & pop, weakening or thinning of sign output, lack of tube heat, higher than typical plate voltage, zero voltage on the cathode of a cathode biased amp--almost anything. Sometimes you'll gently wiggle a tube and concentrate for noise. You can clean the socket by means of spraying touch cleaner on a tube's pins and putting it into the socket a few instances. See this for how to re-tension the tube pin holders.

In many Fender 1960 and more recent two channel amplifiers the first tube on the right, V1, is for the Normal channel handiest and may also be pulled out and used to switch the entire different small tubes in the amp to find a dangerous tube. Play through the Vibrato channel and test the amp after each and every tube switch.

Contrary to well-liked belief, the facility and output transformers are the least most probably reason for amp issues.

Always check out every other guitar, guitar cable, speaker and speaker cable.

Make positive a speaker or dummy load is attached to the amp each time it's powered up.

Click in your amp's symptom:

The amp is totally useless. No lights, no sound, no speaker hiss.

The amp is blowing fuses.

The amp shows indicators of lifestyles but does not put out any guitar audio. Lights are on or you'll be able to hear speaker hiss.

The amp powers up but makes weird noises likes squeals, oscillation, static or motor boating.

The amp powers up but the guitar audio sounds weak, dangerous or drops out totally.

The amp has over the top hum or buzz.

Power tube purple plating.

Troubleshooting spring reverb.

Tremolo Ticking

Testing Diodes.

If the amp is completely silent (no speaker hum or hiss in any respect) then the issue will also be with reference to any place in the amp however you should suspect a nasty tube, blown fuse or the power supply in that order.

Try a brand new rectifier tube.

Try plugging in a guitar cable, flip up all the amp's volume, gain and grasp quantity controls up a little and contact the tip of the guitar cable's other finish. You will have to listen loud noise. If you hear noise jump to The amp shows signs of life but does no longer put out any guitar audio.

If the pilot light does not mild you'll be able to remove the bulb and test it for continuity around the bulb terminals the usage of a multimeter's continuity or "beep" function. A blown bulb won't display continuity.

If none of the tubes show any heater glow (dimming the room lights assist you to see the tube glow) the problem is almost certainly with a blown fuse or the ability provide.

Most older amps have a mains fuse holder on the management panel or on the back of the amp chassis (in most cases with a push-and-turn cap). Most Fender guitar amp fuses are MDL "slow blow" or "time delay" 1/Four inch (6mm) wide via 1 1/4 inch (30mm) lengthy.

Replace the fuse with one with the precise amp rating listed at the back of the amp, usually 1 or 2 amps. The voltage ranking of the fuse will also be anything at or above your wall voltage (125v USA or 240v Europe). A 250V AC rated fuse will paintings high-quality worldwide.

Many newer amps have the mains fuse constructed into the IEC power cord socket. You have to pry open somewhat cover to get to the fuse. Check the fuse for continuity together with your multimeter. They can glance good however nonetheless be blown.

IEC Power Cord Socket

Many amps have interior fuses fixed on the circuit board or on wires in in-line fuse holders. Any one of these fuses can make the amp silent. Again, test all of the fuses for continuity with your multimeter. You will have to take away a fuse to test it as a result of if left in-circuit you can get a false "good" reading.

If after changing the fuse the amp blows the brand new fuse see the If the Amp Blows Fuses segment. 

If the amp is blowing fuses at energy up replace the fuse and plug the amp right into a mild bulb present limiter and tool it up. A mild bulb limiter permits you to energy up a new or problem amp and restrict damage to the amp from miswiring or different defects. If an amp is blowing fuses the limiter will will let you energy up the amp, not blow a fuse and troubleshoot. The limiter helps to keep the present flowing during the amp low enough to not blow the fuse. The bulb must to begin with go brilliant as the facility transformer and filter caps price, then the bulb will have to dim noticeably if the amp is healthy. If the bulb burns at its complete brightness then an issue is allowing too much present to flow throughout the amp.

If you don't use a gentle bulb limiter then substitute "blows a fuse" for "light bulb goes full bright" underneath.

Simple Light bulb Current Limiter

Note: The amp will sound funky and voltages will likely be less than typical when powered by way of the limiter. If you can't discover a 100 watt bulb in your limiter a A hundred and fifty is the following perfect thing. You cannot use an LED or fluorescent bulb.

1. Plug the amp in to the mild bulb limiter and turn the amp on in 'standby' mode if available. If you should not have a standby switch jump to Power Transformer.

If the bulb glows dim, which is traditional, jump to Step 2.

If the limiter's bulb burns at full brightness in standby then an issue is within the energy cord, power transformer, heater wiring, fastened bias circuit (mounted bias amps simplest) or rectifier. These are the only things most often powered in standby.

If your amp has a rectifier tube then remove it and power up the amp.

If the bulb is going dim then the rectifier tube or socket is the problem. Try any other rectifier tube and check up on the rectifier tube socket.

If the bulb glows vibrant take a look at the rectifier tube socket and its wiring. If you do not find an issue move to the Power Transformer.

Power Transformer

Remove all the tubes and power up the amp with the facility and standby switches on (absolutely powered). If the bulb glows dim (which is standard) then soar to Step 2.

Warning: With no tubes installed many amps will go away their filter capacitors fully charged. You should measure for DC voltage on the clear out caps and drain the caps if voltage is over 30v ahead of doing any work throughout the chassis.

If the bulb glows brilliant there is a brief so hint the voltage from the facility transformer and search for a short--and do not put out of your mind the bias circuit.

If you don't to find the problem, drain the caps and disconnect all the power transformer secondary wires and tape them off individually so they can't brief to the chassis or make touch with one another.

Power up the amp, if the sunshine bulb doesn't glow at all or glows dim then bounce to the following paragraph. If the bulb glows vivid then the ability cord or transformer winding is shorting out internally. Drain the caps and disconnect all three power cord wires, wrap every one in electrical tape and plug within the amp. If the bulb doesn't light then the transformer is shorted internally and will have to get replaced. If the bulb glows vivid the ability cord is shorting so try a brand new power cord.

If the sunshine bulb does not glow with the entire secondary wires disconnected then there is a brief in the wiring after the transformer secondary leads downstream in the rectifier, 6.3v heater wiring, 5v heater wiring or bias circuit.

Power down the amp and drain the caps. One at a time attach the power transformer secondary cord pairs. Start with the top voltage wires to the rectifier. Power up, if the sunshine bulb doesn't glow then its downstream wiring is OK. If the bulb goes vivid you realize there's a downside downstream of the connection in either the rectifier tube socket wiring or in amps with a forged state rectifier the issue may also be in the rectifier, filter out caps, output transformer or tube socket connections. If your bias circuit is powered via the high voltage faucets then this is a suspect too. Unsolder the connection from the last clear out cap that supplies the preamp. If the bulb does not mild the preamp wiring is OK, disconnect the facility tube display screen connection and check. If it's good enough then if in case you have a shorting capacitor. Unsolder them and test.

If the top voltage connection examined nice power down, drain the caps and attach the 6.3v heater wires. Power up and if the bulb doesn't gentle the heater wiring is OK. If the bulb burns vibrant then there is a wiring problem in the heater wires, heater artificial center faucet or tube sockets. Using a magnifying glass moderately check out every heater twine connection. If you don't find a problem and your amp has an artificial heart faucet then exchange both resistors.

If the 6.3v circuit tested nice power down, drain the caps and fasten the 5v heater wires that power the tube rectifier. If your amp has a cast state rectifier skip this step and transfer to the fixed bias circuit. Power up, if the bulb doesn't gentle then move directly to the bias circuit. If the bulb is going bright in moderation investigate cross-check the rectifier tube wiring--there's a quick someplace.

If the 5v circuit exams nice and your amp has an influence transformer fixed bias faucet, power down, drain the caps and fix the fastened bias. If the bulb doesn't light the circuit is OK. If it is going brilliant there's a drawback within the bias circuit. Carefully check out the circuit looking for shorts to floor. If your fixed bias circuit runs wires underneath the circuit board then that cord may be shorting to the chassis. Test the bias circuit components.

You can additionally test a power transformer through measuring the resistance between leads. A healthy Hammond 270AX 240-0-240v power transformer measured: 14 ohms between primary leads, 223 ohms between the secondary heart tap and HT cord 1 and 250 ohms from heart tap to HT twine 2, 0.3 ohms between the 6.3v leads (that is low sufficient resistance to beep all through a continuity check). Your power transformer must measure someplace near those values and now not show an open (mega ohms) or shorted (Zero ohms) connection.

2. Power down the amp and insert the tubes one at a time, starting with the rectifier, then preamp tubes, then energy tubes. After inserting every tube, energy up the amp and check the sunshine bulb.

If the bulb is going dim (traditional) pull that tube and insert the next tube (do not remove the rectifier tube, it's needed to energy the opposite tubes). Repeat till the bulb goes vibrant.

If the bulb goes brilliant then power down, it manner you will have discovered the problem area.

Replace that tube with a recognized nice tube. If the bulb stays dim with the new tube you had a bad tube and will have to be good to move.

If the bulb goes vivid with the identified nice tube then there's a quick within the circuit near that tube. Closely investigate cross-check the tube socket with a magnifying glass for the rest that would brief two pins or quick a pin to ground. Use your meter to take a look at for continuity from socket pin to floor. Read the ohm score color stripes of all of the resistors in the circuit to ensure their value. Closely check out the components which might be connected to that tube socket at the circuit board and check them for continuity to floor.

The function of early testing is "divide and conquer." We'll attempt to to find what common area of the amp is causing the problem to slim our seek.

With the amp's power and standby transfer on (totally powered) and all volumes grew to become full up are you able to hear any hiss or hum coming from the speaker? If you can you then know the mains fuse, energy transformer, power tubes and output transformer are powered and a minimum of partly running. The downside is most definitely between the enter jack and section inverter (or driving force for single ended amps).

Some amps have multiple quantity, achieve and grasp volumes so ensure that they are all turned up for testing.

If the amp powers up however is quiet or sounds unhealthy you must always suspect a foul tube. You will have to have a spare set of identified nice tubes readily available so switch out all of the tubes one at a time and spot if that fixes your drawback.

Try any other guitar and guitar cable.

Try the entire enter jacks. If one channel works and the other doesn't you already know the issue is within the dangerous channel between the input jack and the place the two channels sign up for.

Dirty or bent FX loop jacks. Insert a jumper or guitar cable from FX Out to FX In to peer if that remedies the issue. If the jumper cable does repair your downside then take a look at cleansing all the jack contacts, particularly the transfer contacts. If that does not fix it exchange the dangerous jack.

For a new amp startup a very common "no guitar sound" problem is improper input jack wiring. To check your enter jack wiring, with the amp off, insert a guitar cable into the enter jack, then measure the opposite finish of the cable from tip to sleeve for resistance. The Hi jack will have to measure about 1 megaohm which is the standard worth for the input resistor which is hooked up across the enter jack's tip and ground terminals. The Low jack will have to measure around 136k.

Fender Style Hi-Low Input Jack Tests

If you measure near Zero ohms between the tip and sleeve the tip of the guitar cable is making contact with floor and sending the entire guitar sign to ground. Inspect the input jacks carefully and make sure the switch (shunt) terminal is truly the center terminal as shown on amp layouts. On some Switchcraft clone jacks the switch terminal is not the center terminal. Make positive the jacks' transfer disconnects when a guitar cable plug is inserted.

A unethical input jack switch (shunt) terminal can short audio to floor. Visually ascertain that the end and switch terminals separate when a plug is inserted. You can generally bend the terminals so that they close with out a plug and open with a plug inserted.

If the amp has an effects loop check out plugging the guitar into the FX loop Return jack. That will bypass the achieve stages and circuitry upstream. If that works you understand the problem is upstream of the Return jack.

Check the speaker and speaker cable. A speaker cable with a destroy in it might fry the output transformer. Make positive the speaker is plugged into the correct speaker jack. Many Fender amps ground out the signal should you plug just one speaker into the Aux speaker jack. Try all the speaker jacks.

Play every other amp in the course of the speaker to verify its tone and function. You too can connect another speaker to the issue amp, if the amp sounds nice you recognize you have got a nasty speaker.

Inspect the speaker connections at both ends of the speaker cable. Slide-on spade connectors could make intermittent connection and cause buzz, static and audio drop out. I don't like spade connectors--solder that speaker cable to the speaker terminals as a result of you can blow the output transformer and/or energy tubes if that connection breaks.

If the "no guitar audio" downside persists proceed to: Going Inside the Chassis.


Finding the Noise

Start by means of pulling the primary preamp tube, powering up and seeing if the noise is still provide. If it is then you recognize the noise is being generated downstream of the tube you pulled. Power down, pull the next tube and continue until the noise goes away, then you recognize the noise is coming from between the final tube you pulled and the former tube.

You can sometimes identify the problem by ready for it to happen then in moderation spray freeze spray to cool amp parts and solder joints. If spraying a part or solder joint reasons the problem to depart or come back you might have located the problem.

Weird noises, including squealing, wooshing, clicking, static, honking, motor boating, etc. can also be brought about by way of:

A bad tube so change out the tubes one after the other for known, good tubes.

A microphonic tube or other part could cause rattles, buzzes, rings and other odd noises in combo cab amps. If you connect your amp to an extension cab and disconnect the inner speaker and the noise is going away it's most probably brought about by way of a microphonic component or perhaps a cab rattle. You can gently tap the tubes with a chopstick or wooden pencil with the amp on and concentrate for excessive noise. You too can put mild drive on each and every tube the usage of a chopstick or gloved hand after which play the offending word to check out to spot the noisy tube. High temperature o-rings put on preamp tubes can assist in combo cabs however metal tube shrouds won't fit over them.

Dirty or free tube sockets could cause the whole lot from no signal at all to intermittent static, pops, whooshing and hum. If wiggling the tubes causes noise then put some touch cleaner at the tube's pins and insert them into the socket two or 3 times. You too can re-tension free sockets for solid pin touch.

Inspect the speaker connections--both ends of the speaker cable. Slide-on spade connectors could make intermittent connection and reason buzz, static and audio drop out. I do not like spade connectors--solder that speaker cable to the speaker terminals as a result of you can blow the output transformer and/or energy tubes if that connection breaks.

In combo amps the cab, speaker or speaker baffle can vibrate, rattle and buzz. Try sitting on the cab while taking part in or have anyone put force on the baffle board to identify the problem space. Try a separate extension cab to ensure it is a cab issue. You can glue and screw small braces to tighten up unfastened joints. To assist troubleshoot you can use a sign generator (or phone ap) hooked up to the amp and play with the signal frequency to seek out the resonant tone that causes the rattle or buzz. Once you get a continuous buzz it is going to be simple to find the rattle the usage of your palms to hose down other portions of the cab. You too can use a chopstick to position drive on amp components to search for a microphonic element like a tube or capacitor.

Electric sounding arcing or zap noises can be caused via tube arcing or free grounds. You can infrequently locate an actual arc by means of opening the chassis and enjoying the amp at nighttime. Look for little flashes of sunshine when the "zap" sound occurs. Look for free, damaged or unsoldered joints, especially on the floor bus and the place the bus ties to the chassis.

Dirty FX loop jacks. Insert a jumper or guitar cable from FX Out to FX In to look if that treatments the drawback. If it does repair your problem try cleaning all of the jack contacts, particularly the switch contacts. If that does not fix it change the jacks.

Dirty or worn pots. Dirty pots can cause intermittent contact, signal dropout and static. Most pots have a hollow in their shells so you'll be able to squirt touch cleaner into them. Run the back and forth through their complete shuttle in no time several occasions right after you squirt them.

Positive Feedback from the unfavourable comments circuit. When you get started up a brand new build amp with an NFB loop or replace an output transformer you could have a 50% chance of getting a noisy squeal from sure comments (most often no longer affected by the volume control). Swap the output transformer's number one wires to make the comments damaging. The number one wires run from the output transformer to the ability tubes. You can most often change the secondary wires (speaker jack wires) as an alternative however you'll be able to now not change them if you have got a multi-tap secondary (4-8-Sixteen ohms for instance). Positive comments doesn't always cause a loud squeal. In my 5F6A Bassman it simply caused extraordinary noise layered on most sensible of all of the notes.

Lead dress causing capacitive coupling which is able to cause hum, noise, squeals and oscillation. Chopstick the amp. You can use a non-conducting wood chopstick to transport wires around with the amp operating and volume full as much as find the supply of noise. It may also be used to use power to elements and solder joints to spot susceptible or broken components and joints.

Silent oscillation (above human listening to) could cause less than typical amp voltages, not up to typical (or no) output volume and brief sign dropout. Use chopsticking to seek out lead get dressed problems that may purpose oscillation. Snubber caps can be utilized to clear out above human hearing frequencies. Temporarily alligator clipping a 1nF (1000pF) or 500pF snubber cap around the preamp tube plate load resistors, separately, can lend a hand find the issue.

Weak or failing filter caps may cause hum, oscillation, squeal and motorboating, particularly in prime achieve amps. You can temporarily alligator clip a brand new cap in parallel with an existing cap. If the hum or oscillation is going away then replace the outdated cap.

Defective plate or cathode resistors can cause constant static, crackle and dad noise. Sometimes placing some drive at the resistor with a chopstick or spraying it with freeze spray will exchange the noise and help you determine the bad resistor. I love to use 2 watt resistors for all preamp plate resistors to lengthen their lifespan and scale back amp hiss.

Popping or squealing when a guitar cable is plugged in. A foul 1M input resistor (in most cases mounted on an input jack) will allow the first gain degree grid to go with the flow in the couple of minutes between the jack shorting switch opening and the cable plug making full touch. You can test this by means of opening the enter jack switch, if it squeals the input resistor is dangerous. Measure the resistance across the resistor and if it measures nice re-flow its solder joints and check the enter jack ground.

Intermittent noise, crackling static or noise that begins or goes away when the amp warms up is sometimes brought about by way of a vulnerable part or bad solder joint that is affected by heat. You can occasionally identify the problem via ready for the issue to happen then in moderation spray freeze spray to chill amp elements and solder joints. If spraying a part or solder joint reasons the problem to depart or come back you could have positioned the problem.

Motorboating (amp sounds like a boat engine) is normally led to by a vulnerable or damaged power provide filter out cap. You can briefly alligator-clip an additional filter out cap in parallel with the caps, starting with the primary filter out cap. If the temporary cap fixes the issue then exchange the original cap. Also check the facility supply voltage losing resistor values.

Ghost Notes are brought about by a noise blending with a note to create a harmonic be aware that shouldn't be there. Hum, oscillation and different noise can cause them.

Conductive circuit boards in vintage amps may cause a mess of issues. Many classic Fender amps from the '50s to the '70s have been manufactured from a subject matter that may dangle moisture and behavior electrical energy from one eyelet to any other. This can cause all varieties of problems and noise. Some previous circuit boards were covered with wax to fight moisture but after 60 years the wax is not holding up. You can sometimes measure voltage from a high voltage eyelet to the circuit board immediately across the eyelet to test for conduction. You can use a hair dryer to dry the circuit board for a short lived repair and troubleshooting. Replacing the circuit board is the most productive long run fix.

For extra data see Going Inside the Chassis

Nasty sounding guitar audio may also be brought about by way of many stuff including:

Strange noises and entire guitar audio dropout will also be caused through oscillation. It may also be led to by a dying tube but the conventional perpetrator is an interaction between two wires. Ultrasonic oscillation (above human hearing) can block out guitar audio and silence the amp. Chopstick the amp wires, especially the tube socket wires and separate the grid and plate wires. Do this whilst the amp is in oscillation to see if you'll get it to stop--if it is ultrasonic oscillation then feed a sign into the amp so you'll be able to know when the oscillation stops. Use a "filter out cap on a stick" (500pF cap with a floor clip on one finish) to clear out all of the tube grids to see if you'll be able to kill the oscillation. When you to find the in charge tube you can upload a filter out to the grid or bypass its plate load resistor with a cap. Use the smallest cap that solves the issue for the least aspect effect.

A bad tube so change out the tubes one at a time for known, good tubes.

Dirty FX loop jacks. Insert a jumper or guitar cable from FX out to FX in to look if that treatments the problem. If it does repair your downside take a look at cleansing all the jack contacts, particularly the switch contacts. If that does not fix it change the jacks.

Dirty or worn pots. Dirty pots can cause intermittent contact, signal dropout and static. Most pots have a hole of their shell so you'll be able to squirt touch cleaner into them. Run the pot back and forth via its complete shuttle very quickly a number of instances right after you squirt them.

A bad solder joint. Applying pressure with a chopstick and hearing noise can identify a bad joint.  Freeze spray can also lend a hand establish unhealthy solder joints by way of cooling them and inflicting a change in noise.

Ghost Notes are caused through a noise blending with a note to create a harmonic be aware that should not be there. Hum, oscillation and other noise can cause them.

Cone Rub. The speaker voice coil rubs its encompass. Typically happens right through loud, low notes. Try every other speaker to spot a nasty speaker. Sometimes mounting the speaker upside down will stop the cone from rubbing. You could have to have the speaker re-coned to get rid of cone rub.

Inspect the speaker connections--both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and reason buzz, static and audio drop out. I don't like spade connectors--solder that speaker cable to the speaker terminals because you'll blow the output transformer and/or energy tubes if that connection breaks.

For a brand new or lately repaired amp using the wrong worth of a component can cause low output or funky audio. Verify the value of all resistors and caps.

A disconnected component. Component leads can damage and wires can pull loose. Resistors can crack yet glance completely common. Chopsticking components and wires can lend a hand determine the loose connection or unhealthy part.

Conductive circuit boards in vintage amps could cause a multitude of issues. Many classic Fender amps from the '50s to the '70s were manufactured from a subject matter that can dangle moisture and habits electricity from one eyelet to another. This could cause all sorts of problems and noise. Some outdated circuit boards had been lined with wax to combat moisture but after 60 years the wax isn't keeping up. You can sometimes measure voltage from a prime voltage eyelet to the circuit board instantly around the eyelet to check for conduction. You can use a hair dryer to dry the circuit board for a temporary repair and troubleshooting. Replacing the circuit board is the most efficient long term fix.

A bad element:

Blown energy tube display screen resistor. This is a not unusual reason behind a vulnerable sounding amp. When a power tube blows it will probably brief the plate and screen and purpose the display resistor to burn and blow. A blown or cracked resistor can glance normal--gentle force from a chopstick can help discover a dangerous resistor. Measure the resistance around the resistor.

Leaking coupling capacitor. A leaking cap can cause scratchy pots and impact preamp and power tube bias. Measure the DC voltage on tube grids and look for anomalies.

Shorted resistor. Measure the resistance around the resistor. A shorted resistor will show very low and even Zero ohms.

Open resistor. Applying mild drive to the resistor and hearing noise can occasionally determine a nasty resistor. Measure the resistance across the resistor. An open resistor will show a very prime resistance way past the resistor's ranking.

Output Transformer

If the output transformer shorts between windings it might result in no output, vulnerable output or funky sounding output. You can test the transformer through measuring the resistance between its leads. Drain the filter out caps and remove the rectifier and power tubes sooner than making the next resistance measurements:

For push-pull transformers you should see roughly the similar resistance between each secondary cord at every power tube and the middle tap--typically somewhere between 10 to two hundred ohms.

A shorted number one winding will have much lower resistance compared to the other winding and customarily measure at less than 10 ohms.

The secondary windings will ceaselessly measure not up to 1 ohm between all the secondary leads so it is tricky to detect a shorted secondary.

If a transformer winding is open (destroy) it's going to display a very prime resistance between the primary leads or between the secondary (speaker) leads (usually 500k or upper).

A brief between the principle and secondary windings will usually show less than 10 ohms resistance between the principle and secondary leads.

Measure the resistance between the entire leads and chassis ground--low resistance of less than 10 ohms indicates a brief to the transformer's iron core.

For comparison my wholesome Hammond 125C push-pull output transformer measured: crimson center faucet to brown 150 ohms, crimson middle faucet to blue 116 ohms, brown to blue 268 ohms. All secondaries measured .2 to .6 ohms between them. A Hammond 125GSE single-ended output transformer measured Fifty three ohms between the two number one wires. All of the secondaries measured .2 to .3 ohms. Both transformers measured an open circuit (no connection) between their primary and secondary wires and between all wires to the transformer outer steel shell.

If any of the above faults are present your very best wager is to just change the output transformer. For treasured vintage transformers it is possible to have them rewound.

 For extra information see Going Inside the Chassis.

"Teach it the words and it will stop humming."

Start with the basics. Does any volume or master management affect the hum or buzz? If yes then the noise is coming into the circuit sooner than the control.

Is the noise in either channels? If sure then the noise is entering the circuit after the channels merge. If no, the noise is getting into upstream of the channel merge.

If the amp has an FX go back jack plug your guitar in there and see if the noise remains to be present (turn the guitar volume management down ahead of plugging in then flip it up for the take a look at). If no noise the noise is coming from upstream of the FX go back jack.

Loud hum from a newly built amp is almost all the time caused by way of a bad or missing ground. Check for continuity between everything that are supposed to be grounded and the chassis. A joint and not using a solder could cause intermittent loud hum. Inspect each solder joint with a magnifying glass. It's quite common to go away a joint or two without solder.

Hum and noise can also be led to via the chassis being out of the chassis and open to environmental electromagnetic noise.

Be positive and take a look at any other guitar and guitar cable. Also check out the amp in every other location because noise is regularly brought about by way of grimy power (in all probability caused by a fridge's compressor motor) or radio frequency interference (RFI) caused via a mobile phone, noisy mild dimmer, fluorescent lights or different electrical apparatus. Before taking the duvet off your amp check out it in every other location, ideally in another building.

An input jack whose transfer tab is not making contact when no guitar is plugged in may cause loud hum or buzz (see jack picture above). Try pushing the transfer tab closed and see if it kills the noise. This is particularly true if plugging in a guitar decreases amp noise.

In combo amps the cab, speaker or speaker baffle can vibrate, rattle and buzz. Try sitting on the cab or hanging drive on different parts of the cab, speaker baffle and speaker frame (it is helping to have a friend play whilst you prod).

A loss of life tube may cause 50 or 60Hz hum from heater-cathode leakage but can additionally motive A hundred or 120Hz hum so swapping out every tube should be performed ahead of going to any extent further. I have had emblem new preamp tubes hum loudly so try swapping them all.

After trying different tubes the next move is to spot the kind and frequency of the hum or buzz. Buzz has a pointy tone to it the place hum has a easy sound. A "buzz" is led to through noise that may be seen on an oscilloscope as a waveform with sharp spikes.

In the United States hum and buzz generally is available in two frequencies, 60 and 120Hz (Hz method cycles in keeping with second). If your power runs at 50Hz like in Europe you will have 50 and 100Hz hum and buzz. Determining the frequency of the noise will let you observe down the source. See this Youtube video at 2:15 for samples of both sorts of hum. 50 and 60Hz hum generally comes from the power transformer circuit, mounted bias circuit or heater wires. 100 and 120Hz hum comes from the power provide after the rectifier.

Dirty, corroded or loose tube socket pins may cause hum and buzz. You can clean them by means of spraying contact cleaner on a tube's pins and insert it into the socket a couple of instances.

Microphonic tubes can rattle, buzz and ring. Touching the tubes gently with your chopstick or a gloved hand while taking part in something that makes it buzz can assist you to establish the dangerous tube.

A hot energy tube bias may cause loud hum.

A big bias imbalance between power tubes could cause 60Hz heater hum.

Inspect the speaker connections--both ends of the speaker cable. Slide-on spade connectors can make intermittent connection and motive buzz, static and audio drop out. I don't like spade connectors--solder that speaker cable to the speaker terminals as a result of you can blow the output transformer and/or energy tubes if that connection breaks.

Adding aluminum or copper foil tape to an amp cab to hide the chassis opening can lend a hand cut back RFI. The tape will need to make good contact with the chassis for it to behave as an RFI shield. For the 5E3 Deluxe and different tweed amps the tape can be carried out to the interior of the wooden back panel to hide the chassis opening. For more trendy Fender amps you possibly can apply the tape to the underside of the top of the cab.

Connecting the ability transformer prime voltage middle faucet without delay to the first filter cap damaging terminal can scale back hum and buzz by means of minimizing pulsing 120Hz floor return current.

Connecting the (purple/yellow) high voltage center tap on the first filter cap unfavourable terminal can scale back 120Hz hum and buzz.

If your amp has an unused triode you will have to attach the unused plate, cathode and grid to ground.

"Chopsticking" the Amp

You can use a non-conducting picket chopstick to move wires round with the amp on and the volume as much as in finding the supply of noise and determine lead get dressed issues. On my first amp build I had the V1A plate and grid wires mendacity on most sensible of each other which created a reasonable hum. Simply transferring the ones wires aside made the amp practically silent. A chopstick can also be used to apply comfortable force to parts and solder joints to spot weak or broken elements and joints.

Sometimes you'll have to lodge to the usage of an oscilloscope and signal generator to trace down the source of a hard to identify hum or buzz. See the oscilloscope phase for more information.

50 or 60Hz Buzz

50 or 60Hz buzz is infrequently brought about by means of power line noise and may also be addressed by making use of a small, prime voltage filter out cap around the high voltage rectifier input wires. This .02uF 3KV (3000v) ceramic disc cap works well for this. I in most cases solder one across the rectifier tube socket the place the 2 top voltage power transformer wires connect. For a forged state rectifier solder the cap around the high rectifier inputs.

Buzz will also be caused by means of AC components. I had a nasty 60Hz buzz that was led to via the placement of a 120v pilot mild too with reference to the amp's signal wires.

Radio Frequency Interference or RFI can occasionally be heard as a 50 or 60Hz buzz. Adding copper or aluminum foil tape to the again of a chassis duvet can help reduce RFI. Moving assets of RFI like mobile phones, mild dimmers and fluorescent lighting fixtures clear of the amp can also lend a hand.

One hundred or 120Hz Buzz

A hundred or 120Hz buzz may also be caused by way of a bad tube so change in a new set of tubes. Buzz can even be led to by way of a loud rectifier. This can most often be eradicated by means of placing .02uF 1KV caps in parallel with every rectifier diode. If the rectifier is within a housing then running caps from the 2 rectifier inputs to the + and - terminals typically works neatly.

50 or 60Hz Hum

50 or 60Hz hum in most cases comes from the ability transformer or its wiring, the fixed bias circuit, the tube heater wires or from exterior RFI led to by way of fluorescent lighting fixtures, dimmers and other assets.

Assuming you have got attempted the amp in a special location, 50 or 60Hz hum will have to be generated by way of the ability transformer circuit or the mounted bias circuit (if your amp has one). Keep as a lot distance from the power transformer's wires and the amp's wires and circuitry as conceivable. Heater wires should be twisted and the untwisted twine that goes to each and every tube pin must be as quick as imaginable with out a giant loops. Signal wires must cross the heater wires at 90 levels to minimize coupling.

You can temporarily power your 6.3v heaters with a 6 volt lantern battery (to be had at Walmart). If the objectionable hum is going away below battery power you know your hum is coming from the 6.3v heater circuit.

Higher than standard heater voltage can increase hum. If your heater voltage is higher than approximately 6.6v it would be a good suggestion to reduce the heater voltage to 6.1 to 6.Five volts. See this link for info on the best way to scale back heater voltage.

If your amp's energy transformer has no 6.3v heater middle tap then the amp needs a man-made heart tap. Verify there's an artificial center faucet put in and check the resistance of its two resistors. A lacking 6.3v center tap or a burned out resistor will in most cases reason loud hum.

To troubleshoot the fastened bias circuit examine its floor connections are good and you'll be able to take a look at the caps by means of quickly alligator clipping any other cap in parallel with the circuit's caps. If adding the cap reduces hum then substitute the cap.

You can most often get the lowest amount of 60Hz heater hum via replacing an actual 6.3v heart tap or artificial heart faucet with a Humdinger pot. A Humdinger pot permits you to modify the resistance between the 2 6.3v heater lines to ground to achieve minimal hum. You modify it via ear with the amp became up to max to listen to the hum easiest. You too can connect a Humdinger's ground connection (wiper) to a cathode biased amp's cathode resistor to lift the heater floor reference for much more hum removal. See this link for additional information on putting in a Humdinger pot.

Push-pull amps must have their energy tubes stressed out in segment so the tubes can use commonplace mode noise rejection. For in segment wiring every heater wire must connect to the similar heater pin at the two energy tubes. This is why it is a good suggestion to use two colours of heater wiring so you'll hold the segment right kind.

Parallel energy tubes must be stressed out of section to help cancel hum between the energy tubes.

100 or 120Hz Hum

This is probably the most commonplace hum in an amp. It can be led to via:

A bad tube so change out the tubes for recognized, nice tubes.

An enter jack that doesn't ground out when not anything is plugged in. If the jack's shunt transfer does now not make nice contact with the jack's tip connector you will get loud hum when no guitar is plugged in but the amp will sound nice whilst you plug in a guitar.

Worn out filter out capacitors. Electrolytic clear out caps have a normal lifespan of 15 to twenty years. You can check for bad clear out caps through alligator clipping in a parallel cap. If the new cap reduces hum then substitute the original cap. If your amp is mounted bias don't overlook the fixed bias circuit filter out cap(s).

Missing floor. Loud hum can also be caused via elements that are supposed to be grounded however don't seem to be. Forgetting to solder a quantity pot's ground twine or a chilly solder joint on a cathode resistor's ground are two not unusual reasons of loud hum. I like to use an alligator clip cord and fasten one finish to the chassis and moderately probe all of the amp's ground connections to peer if I hear an growth. Touch the ground probe to all volume pots' ground terminal, the bottom side of preamp cathode resistors, etc. Pressing solder joints with a chopstick can also assist in finding unhealthy ground solder joints.

Loops in the floor circuit. If your input jacks are grounded to the chassis and also you also run a floor wire between them then a floor loop is formed (chassis is one side, the twine is the opposite) which can act as an antenna and pick up RFI noise and hum. You will also form a floor loop when you use shielded cable inside the amp and floor both ends. You must simplest ground one end of shielded cable.

Lead dress (wires too shut to one another inflicting capacitive coupling) Chopstick the amp's wires around with the amp on and the volume as much as see if you'll be able to lower the hum. Pay shut attention to the tube grid and plate wires--keep them as a ways excluding one every other as imaginable. My first amp build had a noisy hum since the first preamp level's grid and plate wires have been sitting on most sensible of each other. Separating the wires silenced the amp.

Chasing Down a Nasty Buzz

After I stopped checking out and tweaking the Deluxe Micro prototype in naked circuit boards I fastened them in a Hammond blank chassis. When I fired up the newly fastened amp I had a nasty buzz at max quantity that wasn't there ahead of I put it within the chassis. It didn't sound like easy 60Hz heater cord hum or 120Hz power provide ripple hum so the investigation started.

The amp with out a guitar cable plugged in turned into completely silent at max quantity. The buzz may well be managed via either the quantity and master volume controls. The amp's tone control may just almost fully eliminate it when became complete down. The guitar's tone management could totally eliminate the noise when grew to become full down. Hmmm. The buzz didn't change with guitar motion or when touching the guitar's strings or grounded steel bridge.

Since the quantity and tone controls affected the noise I knew it was coming into the amp ahead of the control--somewhere between the guitar and the quantity pot.

My first intuition become I had a foul cable or a guitar issue. I attempted any other cable--no exchange. I attempted my Strat--no alternate. I attempted a cable without a guitar connected and got the anticipated crescendo of noise.

My subsequent suspect changed into the fluorescent lighting that mild the basement workshop. I grew to become them off and the amp appeared cool at midnight because it spewed forth The Buzz.

Maybe it turned into power line noise. I diligently moved the amp to every other part of the area and tried again--no alternate.

Time to get into the chassis. My first idea turned into a foul floor connection so I alligator clipped a wire to the chassis and moderately probed all the ground connections within the amp to see if I heard an development but discovered no achievement.

My next step changed into lead dress. I used a wooden chopstick to push round wires while listening for an improvement in noise. I did in finding that the enter jack wires that ran by means of the ability switch and throughout both circuit forums were picking up some 60Hz hum from the facility switch. I changed the enter jack and headphone jack wires with RG174 coax which eradicated the hum but had no effect on The Buzz.

Time for the oscilloscope. I injected a 500Hz .1v signal into the amp's input jack. I hooked up the oscilloscope's ground probe to the preamp ground and attached the probe to the enter jack tip. I zoomed in at the 500Hz signal and performed with the tone management to change the volume of The Buzz however couldn't see any sign artifact change with the guitar tone alternate. I jumped to the speaker output jack and attempted back without a success. Next I tapped into V1A's output just after the coupling cap (to avoid high voltage DC at the probe) and repeated the tone trade and back I couldn't see any change at the oscilloscope. I moved the scope probe to V1B's output and repeated the guitar tone knob dance--and there it was. A vertical spike that grew and shrank with the tone management's motion. I zoomed in the scope's display and confirmed it was THE BUZZ.

I used the scope's horizontal scan knob to transport the waveform left and right so I may measure the time between spikes--16.Five milliseconds. I took the reciprocal of 16.5ms (1/.0000165) to get the frequency and the answer was--60Hz, it turned into wall power comparable after all, or so I believed. I pulled out my trusty H&Okay Tubemiester Five and plugged it into the same wall power socket and fired it up--no buzz. I figured the H&Okay may have some power line filtering built in so that test proved nothing.

I have noticed some amps with a .01uF 3KV (3000V) disk capacitor across the rectifier tube socket's prime voltage enter pins to pre-filter prime freq noise so I gave that a take a look at. It did assist just a little however THE BUZZ still lived.

I sat there staring into the amp's chassis questioning what it may well be. I realized the 120V pilot mild (now not a normal 6.3V mild) sitting next to the input jack. This became my first build the usage of a 120V light so I had no revel in with them. I hauled the amp again to the solder station and de-soldered the light's impartial cord and fired up the amp--success! THE BUZZ turned into no more. It was that damn noisy bitch of a pilot gentle.

The indisputable fact that the amp was silent when no guitar cord turned into plugged in was a large clue right here that I neglected. With no cord plugged in the circuit from the input jack to V1A's grid is grounded so THE BUZZ changed into being shunted to floor. I should have centered the investigation on the input circuit from the beginning. 20/20 hindsight and all that. . .

When I filled the prototype B9A boards into the chassis I put them to the a long way left to make room to mount the ability transformer on the some distance right. After getting the forums fixed I spotted I had no entrance panel actual estate on the left side of the chassis clear of the inputs and preamp phases. I finished up having to mount the facility transfer, pilot light, guitar enter and headphone jack proper subsequent to every other--which is not good--it's a cardinal sin of amp building in reality and I had to pay a penance to atone for my sin.

Notice the ability transfer, enter jack, 120v pilot gentle and headphone jack. Not a good idea. If conceivable maintain the ability transfer and light-weight at the energy transformer end and put the enter jack at the preamp finish of the chassis.

The little prototype is now nice and quiet and just sounds killer.


WARNING: A tube amplifier chassis contains deadly high voltage even when unplugged--sometimes over seven hundred volts AC and 500 volts DC. If you haven't been skilled to paintings with high voltage then have an amp technician provider your amp. Never contact the amplifier chassis with one hand whilst probing with the other hand because a lethal surprise can run between your fingers through your center. Use only one hand when running on a powered amp. See extra tube amplifier protection data right here.

Look for internal fuses. Many amps have several fuses within the chassis, either in-line and circuit board fastened to protect the facility transformer and other circuitry.

Inspect the circuit board, sockets, pots and controls using magnifying glasses and flashlight to help spot dangerous or damaged solder joints.

If an amp is completely useless check out the tubes to see if they have got any heater glow. Dimming the room lighting fixtures can help see the glow. Most amp pilot lighting are powered by means of the heater circuit, however now not all so don't assume a lit pilot mild approach the tubes have heat. If a tube isn't lit measure the AC heater voltage from heater-wire-to-heater-wire, it will have to be around 5.7v to 6.9v which is 6.3v +/- 10%.

My subsequent step is to take DC voltage readings starting on the rectifier output and filter out caps (B+1, B+2, B+3). It's good to have a baseline voltage chart on your amps so you realize what voltage to be expecting. The next than regular B+ studying will also be led to by means of a non-functional tube that is not drawing current from the facility supply. A not up to common reading may also be caused through a tube this is pulling an excessive amount of present which may also be brought about via a quick, bias problem or in new construct amps an fallacious component price.

If voltage is completely missing there's a wreck within the power supply. Back up towards the facility provide and in finding the place there is voltage and the place there's no voltage and glance for the purpose. A blown power resistor between the clear out caps is a commonplace motive of a ruin in the circulate of voltage.

Next measure the voltage at the power tube socket closest to the facility transformer. For octal power tubes (8 pins) I look for 5.7 to 6.9v AC heater voltage between socket pins 2 and 7.

Warning: The power tube plates could have over 500 volts DC on them.

Octal Tube Socket Numbers

Plate is pin 3, Screen pin 4, Grid pin 5 and Cathode is pin 8. Heaters are on pins 2 and 7. Arrow issues to the insertion index notch. Pin numbers get started after the index notch and building up clockwise around the socket.

The power tube pins 3 (plate) and four (display screen) must have prime voltage DC and pin 5 (grid) must pop when probed with the meter. For fixed bias amps the grid should display a detrimental voltage but cathode biased amps will indicate close to 0 volts at the grid. Pin 8 (cathode) will display 0 volts in fastened bias amps or display voltage between 10 to twenty-five volts DC in cathode biased amps. Nine pin power tubes just like the EL84 have different pin purposes so Google their information sheet to look their pin purposes. With all the volumes became up you will have to at all times pay attention a pop when probing the grid of any tube in the sign chain. Tubes that run the reverb or tremolo would possibly not pop when probed.

If you get no grid pop then you have a problem somewhere between that tube and the last tube that popped. The very first thing to try is to exchange the tube. Next measure the voltage at every pin of the bothered tube's socket for clues to the problem.

Higher than ordinary voltage at the plates may also be led to via no electron flow throughout the tube. This may also be caused by no heater voltage or a disconnect between the tube cathode, the cathode resistor (if used) and floor. High voltage at the plates will also be brought about by a higher value cathode resistor like the usage of a 15k resistor as a substitute of the required 1.5k.

Lower than usual voltage at the plates is led to via too much current flow in the course of the tube. This can be led to through an wrong bias voltage on the grid. A leaking coupling cap can let DC voltage thru to the grid and warmth up the unfairness current. A decrease value cathode resistor can also allow too much present to move via a tube.

Cathode biased energy and preamp tubes will have to show the unfairness voltage at the tube's cathode pin. If there may be voltage at the plate however the cathode is zero this will also be led to by way of a bad cathode resistor connection or no heater voltage. With the amp off measure the resistance from the cathode pin to ground. It must equivalent the cathode resistor price.

Fixed bias power tubes will have to display the cathode as hooked up to ground and display a detrimental voltage on the grid, typically between -30 to -50 volts DC. An mistaken grid bias voltage can make an amp sound dangerous.

Unwanted Grid Voltage Most preamp tubes must show no voltage on their grids (12A*7 tubes' pins 2 & 7) except tubes that serve as as phase inverters or cathode followers where voltage at the grid is standard. If the preamp tube's grid leak resistor is tied immediately to floor it should not have any voltage on it. If there may be voltage on the grid it is generally led to by a leaky upstream coupling cap (that must get replaced) or grid present. To examine the source of grid voltage you can pull the tube and see if the voltage on the tube socket grid pin is going away. If the grid voltage is still provide then it is coming from a leaking capacitor or a wiring mistake. No voltage on the grid pin way the unwanted grid voltage become being generated by grid current. Try some other tube and look for incorrect element values or wiring errors related to that tube circuit.

Blown power tube display resistors are a not unusual reason behind vulnerable or nasty sounding output. When an influence tube blows it often shorts the tube's plate to the display screen which leads to a blown screen resistor. A blown screen resistor will generally fail open with countless resistance. The amp can operate with one power tube however it is going to sound susceptible and funky. When you substitute the blown tube it is going to nonetheless serve as poorly since the blown display resistor won't permit any voltage to the screens so the amp will sound different than with a blown tube but nonetheless not sound good due the huge output imbalance between the good tube and the tube with no display screen voltage. Verify the display screen voltage and the resistance around the display screen resistor.

If the first power tube tests out then move to the next tube up the circuit which will be any other power tube for push-pull circuits or the driving force tube for single ended amps. Make sure all the volumes, beneficial properties and grasp volumes are became up a little bit whilst you test the preamp tubes so you'll be able to concentrate for a pop when probing the preamp tubes' grid.

Again concentrate for a pop when probing the grid and plate pins and search for voltage anomalies. The pop should get louder as you take a look at each and every tube up the amplification chain toward the enter. Continue moving toward the first preamp tube. If you do not get a pop, or a weaker pop than expected when probing a tube's grid then you have found an issue house. Carefully check the voltages of that tube to search out clues to why it's not operating.

For Nine pin preamp tubes verify 5.7 to 6.9v AC between the heater pins Four and 9, and Five and Nine and test you will have DC voltage on pins 1 and 6 (plates), Three and eight (cathodes). You will have to listen a pop when probing the grids, pins 2 and seven. Nine pin tubes should have Zero volts on their grids except the triode is being used as a phase inverter, cathode follower or uses grid bias.

If you may have unexpected voltage on a tube's grid you'll have a leaky coupling capacitor upstream allowing DC voltage through to the grid or a nasty tube producing grid present.

"Red plating" occurs when an excessive amount of present flows via a tube. Too many electrons pound on the steel plates, warmth them and make them glow crimson.

A nasty tube can red plate so take a look at any other tube.

If your amp has two or extra energy tubes and just one tube red plates then move it to some other energy tube socket. If the same tube pink plates in the new socket then the tube is bad. If the similar socket pink plates then we suspect the upstream coupling cap and a unfastened socket grid pin because we all know the other socket has nice bias voltage so the bias circuit is purposeful. Re-tension the grid pin of the pink plating socket and check the upstream coupling cap.

If the entire energy tubes purple plate then the problem is with the mounted bias circuit.

The tube's bias voltage, which is the voltage difference between the cathode and grid, controls the current circulation in the course of the tube.

If your amp has adjustable bias then measure the voltage on an influence tube grid (typically pin Five for 8-pin tubes) and modify the unfairness pot for the most important imaginable unfavorable voltage (usually around -60v DC) for the good bias. If this solves the pink plating downside then alter the bias and spot if the problem remains solved.

If the power tube cathode has a connection downside the amp will quit passing current so the cathode generally doesn't cause bias and pink plating problems so the grid is the place we will have to focus our consideration.

The fixed bias circuit is lovely easy so we wish to intently check out each and every solder joint and component with a magnifying glass.

Fixed bias amps need between -30 to -50 volts DC at the grid pins.

With the power tubes removed, energy up the amp and measure the ability tube grid voltages on all of the tube sockets. This is pin Five on typical 8-pin tubes.

If your amp has adjustable bias then set the best possible negative voltage conceivable, most often around -60 volts. This will give us the best bias possible.

If you'll be able to't set a bias voltage between -Forty to -60v DC then you may have a leaking coupling cap just upstream of the energy tube grid. Un-solder one end of the coupling cap and notice if you'll be able to set a bias voltage of -40 to -60v. This is especially true should you get different bias voltages on the energy tube sockets. If nothing changes after lifting the coupling cap then the cap isn't the problem.

In the prejudice circuit we will have to have AC voltage where the ability transformer connects to the circuit. We will have to have detrimental DC voltage after the unfairness diode. Move downstream and follow the detrimental DC voltage the entire option to the tube socket. If you discover a damage in the voltage then search for the motive.

A nasty solder joint, cracked resistor, dead, disconnected or reversed filter cap and dangerous bias pots may cause bias issues. A short to chassis will even kill the unfairness voltage. Gently push on bias parts with a chopstick and search for motion because of loose solder joints or component cracks.

A loose socket grid pin can prevent bias voltage from getting to the tube which can cause instant crimson plating. Re-tension the tube socket pins.

The reverb motive force tube is driven very demanding with prime voltage in many amps so the tube has a tendency to burn out faster than all the different small tubes. A foul tube can also purpose excessive hum within the reverb. Swap out either reverb tubes with recognized good tubes and test the amp.

The reverb tank cable connectors (all Four of them) can corrode and motive hum, pops, crackling, vulnerable or no reverb. Clean the connectors (male and female) with contact cleaner. Spray the male connectors and insert them into the feminine connectors several occasions. If the connectors are heavily corroded a toothbrush, small brass brush or steel wool may be needed to get them clean.

Try some other set of tank cables. They may cause intermittent sign drop out.

Mixing up the reverb input and output cables will lead to very susceptible reverb. Swap the cables on the tank and take a look at back. If it does not help or lowers reverb volume then change the cables back.

The effective wires within the reverb tank are a common failure level and they're tough to fix. Replace the tank if any of the wires are broken. I'm keen on the MOD tanks for round .

If you bang at the side of the cab with the facility on and reverb turned up and hear the reverb spring crash you already know the reverb tank's springs, output transducer and reverb restoration amplifier are functioning (the second part of the reverb circuit).

You can switch the tank's input and output cables and bang at the amp again. If you pay attention the spring crash (it will be quieter this time) you realize the input transducer is working. After this test you'll know the problem is upstream of the tank, which is the reverb motive force circuit including the reverb transformer. Swap the tank cables back to normal.

Loose transducers could cause funky problems. If they're unfastened put some glue on them to hold them solid or just exchange the tank. MOD replacement tanks are my favorites.

Isolating the reverb pan with a pan bag, rubber grommets and foam padding between the cab and pan can lend a hand treatment reverb problems, especially those that occur at top quantity and top reverb settings.

Adding a noise cap to the reverb circuit can occasionally quiet a noisy reverb. See this for more information.

A weak or bad V4A (Reverb Recovery) cathode bypass cap may cause a reverb oscillation (squeal) due to sure comments between V4A and V4B (Vibrato channel third gain degree).

A foul pan ground could cause the reverb depth knob to add quantity and acquire (upload dirt). Check cables and cable plugs and sockets.

Reverb Hum

Try turning the reverb pan 180 levels to scale back interaction between the ability transformer and the pan's output transducer.

There are many reverb pan grounding schemes and an wrong set of cables can lift the pan's ground or purpose a ground loop. Look at your amp's schematic to verify what part of the tank and cables should be grounded and what should not.

Temporarily clip a floor connection to the tank and see if that is helping with the hum.

See How Spring Reverb Works for more information.

You can use a non-conducting wood chopstick to move wires round with the amp on and all the volumes up to determine lead dress problems which will purpose hum and oscillation. On my first amp build I had the V1A plate and grid wires lying on best of one another which created a reasonable hum. Simply shifting those wires aside made the amp nearly silent. A chopstick can be used to use pressure to elements and solder joints to spot weak components and joints. You too can lightly tap on tubes to identify microphonic tubes. Don't overlook to chopstick the speaker cable connections.

Freeze Spray for Intermittent Problems

Intermittent issues are occasionally brought about by means of a weak part or unhealthy solder joint this is suffering from heat because the amplifier warms up. You can sometimes establish the problem by ready for the issue to happen then sparsely spray freeze spray to cool amp components and solder joints. If spraying a part or solder joint causes the issue to go away or come again you could have located the problem.

For in point of fact tricky troubleshooting tasks I break out the tone generator and inject a 500Hz One hundred milliamp (.1 amp) sign at an amp's input jack and trace it through the amp with an oscilloscope by way of probing the tube grids. Start on the input jack and paintings against the speaker and wait for the wave form to disappear or trade form. There are tone generator apps on your telephone however you'll need an adapter with a 1/4" mono TS plug to plug into the amp's input jack. You will chance your 0 dollar phone each time you plug in although. You can play a single string on a guitar to generate a waveform however it's roughly a ache in the butt and a guitar's harmonics and timbre will make it more difficult to spot anomalies at the scope.

I additionally use a dummy load when doing this take a look at so I do not need to pay attention to the damn tone coming in the course of the amp's speaker. Be cautious with your oscilloscope probe because the very top voltage at the tube plates can damage it except you utilize a high voltage rated probe. The grids most often have 0 or low DC voltage so I take my sign pattern from the grids unless I in point of fact want to examine the plate sign.

When in search of low-level noise flip down the tone generator signal to around 5 millivolts (.005 volt) to make seeing the noise more straightforward in the early preamp phases. The tone generator sign will likely be a easy sine wave in your scope. The noise can be just a little "hair" or small spikes on the tone sign wave.

60 Hz sign with noise spikes.

Oscope Basics

Set the oscilloscope to AC coupling. It's most secure to attach the scope probes to the amp chassis with the amp's energy off.

Examine the amp from amp enter to output and search for the tone generator or guitar sign to modify form (distort) or disappear (lack of all sound).

To read about the guitar or tone generator sign earlier than amplification clip the scope probe onto the input jack heart conductor and the probe ground clip to the preamp floor bus. This would be the smallest, lowest voltage sign you are going to encounter. Amplification throughout the amp will make the sign higher (upper voltage).

Adjust the scope time and voltage atmosphere knobs until you'll be able to see at least one full wave at the screen. If you place the scope's cause line to simply above zero volts the signal wave should stabilize on the screen (the cause line needs to the touch the on display screen sign to stabilize the sign for viewing).

To see the sign on the first gain level grid move the probe to the tube socket grid pin. Tube grids are normally at 0 volts DC (segment inverters and cathode followers will have as much as 80v DC) so grids are more secure to probe than plates which will have over 500v DC on them.

Move the scope probe to the 2d gain level grid and increase the scope's voltage setting knob to shrink the signal to fit on the scope screen.

When probing the power tubes move the scope probe floor clip to the power amp ground.

Oscope Tips

Pay consideration to the max AC & DC voltage scores of your scope and probe. This is especially vital when probing tube plates with top voltage DC on them.

Set the scope to AC coupling. This inserts a cap between the probe and scope circuit to keep prime voltage DC out of the scope.

Set the X10 transfer on the probe if needed for upper voltage signals. With the X10 switch on a 100v sign will display as 10 volts on the display.

Clip the bottom clip to an appropriate ground level with the amp turned off. For instance use the preamp floor bus or last clear out cap floor when probing the preamp and use the facility amp floor or first filter cap floor when probing the power amp.

When chasing an oscillation or different sign artifact it may be helpful to disconnect the worldwide destructive feedback as a result of NFB will attempt to right kind and reduce the oscillation or artifact.


The very robust tremolo oscillator signal can induce ticking into the amp's signal movement. First attempt to separate the tremolo motive force plate (V5 pin 6) twine from any grid wires to stop the ticking. Pay shut attention to the nearby V6A segment inverter grid (pin 2) cord.

Another choice is to make use of a shielded cable like RG174 for the tremolo driving force plate wire (V5 pin) to stop the transmission of the tremolo signal. This is almost a 100% repair for tremolo ticking. Ground the cable defend at just one finish (either end) to stop a ground loop.

If the above do not paintings then adding a .02uF cap to the tremolo roach should give up it.

Add a .02uF 400v (or higher voltage) cap across the left facet of the tremolo roach to cure tremolo tick.

See this to gradual the tremolo.

To take a look at a diode be certain that the circuit is powered down (drain the caps) and measure the diode's resistance both ways across it (measure, then opposite the meter probes, measure again).

A good diode will display resistance of seven to fifteen ohms in one direction (forward resistance) and practically countless resistance in the other direction (reverse resistance).

Reading an open circuit (countless resistance) both ways manner the diode is open.

A 0 resistance reading in both course approach the diode is shorted.

A diode with a resistance studying under 7 ohms will have to be replaced.

Measuring a series of diodes in a row should show about 12 ohms of resistance in line with diode in a single path and infinite resistance within the different.

The striped finish of a diode is the cathode finish, the opposite end is the anode.

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By Rob Robinette


RCA Corporation, RCA Receiving Tube Manual, RC30.  

Merlin Blencowe, Designing Tube Preamps for Guitar and Bass, second Edition.


Merlin Blencowe, Designing High-Fidelity Tube Preamps


Morgan Jones, Valve Amplifiers, 4th Edition.


Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, third Edition.


Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Preamps, second Edition.


Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Power Amps


Robert C. Megantz, Design and Construction of Tube Guitar Amplifiers


Neumann & Irving, Guitar Amplifier Overdrive, A Visual Tour It's fairly technical but it's the only guide written specifically about guitar amplifier overdrive. It comprises many graphs to assist in making the material more straightforward to understand.


T.E. Rutt, Vacuum Tube Triode Nonlinearity as Part of The Electric Guitar Sound

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