Tube Guitar Amp Troubleshooting
By Rob Robinette, edited 2/27/2017
This webpage can be downloaded as a .pdf file: Tube Guitar Troubleshooting.
Here's my technique for troubleshooting a tube guitar amplifier. Many of these techniques apply to solid state amps too.
If you are turning on a new build amp or heavily repaired/modified amp for the first time I recommend you use a light bulb current limiter and follow my Amp Startup Procedure. Following it will minimize damage due to a miswired amp.
The more you know about how tube amps work the easier they are to troubleshoot, so keep learning.
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. You must drain the filter caps before doing any work inside an amplifier chassis. See more tube amplifier safety info here.
Always suspect a bad tube as they are the most common failure point in a tube amplifier. Having a spare set of tubes to swap into the amp one at a time is a must for gigging tube amp users.
On the other hand the power and output transformers are the least likely cause of amp problems.
Always try another guitar and guitar cable.
Make sure a speaker or dummy load is connected to the amp every time it is powered up.
Click on your amp's symptom:
The amp is completely dead (no lights, no sound).
The Amp is Completely Dead
If the amp is completely silent (no speaker hum or hiss at all) then the problem can be just about anywhere in the amp but you should suspect a bad tube, blown fuse or the power supply in that order.
First try plugging in a guitar cable, turn up all the amp's volume, gain and master volume controls up a little and touch the tip of the cable's other end. You should hear loud noise. If you hear noise jump to The amp shows signs of life but does not put out any guitar audio.
If the pilot light does not light you can remove the bulb and test it for continuity across its terminals using a multimeter's continuity or "beep" function. A blown bulb will not show continuity.
If none of the tubes show any heater glow (dimming the lights can help you see the tube glow) the the problem is probably with a blown fuse or the power supply.
Most older amps have a mains fuse holder on the control panel or on the back of the amp chassis (usually with a push-and-turn cap). Most Fender guitar amp fuses are MDL type "slow blow" or "time delay" 1/4 inch (6mm) wide by 1 1/4 inch (30mm) long.
Many newer amps have the mains fuse built into the IEC power cord socket. You have to pry open a little cover to get to the fuse. Check the fuse for continuity with your multimeter. They can look good but still be blown.
IEC Power Cord Socket
Many amps have internal fuses mounted on the circuit board or in in-line fuse holders. Any one of these fuses can make the amp silent. Again, test all the fuses for continuity with your multimeter.
If after replacing the fuse the amp blows the new fuse see the If the Amp Blows Fuses section.
If you see heater glow in all the tubes then swap out all the tubes one by one.
If the amp is still dead move on to the Going Inside the Chassis section.
If the Amp Blows Fuses
If the amp is blowing fuses at power up replace the fuse and plug the amp into a light bulb current limiter and power it up. A light bulb limiter allows you to power up a new or problem amp and limit damage to the amp from miswiring or other defects. If an amp is blowing fuses the limiter will allow you to power up the amp, not blow a fuse and troubleshoot. The limiter keeps the current flowing through the amp low enough to not blow the fuse. The bulb should initially go bright as the power transformer and filter caps charge, then the bulb should dim noticeably if the amp is healthy. If the bulb burns at its full brightness then a problem is allowing too much current to flow through the amp.
If you don't use a light bulb limiter then substitute "blows a fuse" for "light bulb goes full bright" below.
Simple Light Bulb Current Limiter
Note: The amp will sound funky and voltages will be lower than normal when powered by the limiter so you must remove the limiter to check your amp for normal voltage. If you can't find a 100 watt bulb for your limiter a 150 is the next best thing. You can not use an LED or fluorescent bulb.
1. Plug the amp in to the light-bulb limiter and turn the amp on in 'standby' mode if available. If you don't have a standby switch jump to step 2.
If the limiter's bulb burns at full brightness in standby then a problem is in the power transformer, heater wiring or rectifier (the only things usually powered in standby). See power transformer.
If the bulb burns dim (normal) jump to step 2.
Unplug the amp and remove the rectifier tube. For amps with solid state rectifiers you must unplug the amp, drain the filter caps and unsolder and lift the connection just after the rectifier or diodes (disconnect the rectifier output). See Testing Diodes.
If the bulb goes dim then the rectifier is the problem. Check the rectifier tube wiring or replace the solid state rectifier (they are cheap).
If the bulb burns bright then the power transformer or its wiring is shorting voltage to ground. Measure the AC voltage of all the power transformer's secondaries--high voltage to the rectifier, 6.3v to the tube heaters, 5v to the rectifier tube heat, 50v to the fixed bias circuit. Also check the AC voltage from the power cord.
If you don't find a problem disconnect all the secondary wires and tape them off individually so they can't short to the chassis or one another. Power up the amp, if the light bulb burns bright then the transformer is shorting out internally and must be replaced. If the light bulb dims then there is a short in the wiring the transformer primary leads were connected to--high voltage to the rectifier, 6.3v heater wiring, 5v heater wiring or 50v bias circuit.
You can also test a power transformer by 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 center tap and HT wire 1 and 250 ohms from center tap to HT wire 2, 0.3 ohms between the 6.3v leads.
2. Power down the amp and remove the tubes one by one, starting with the power tube closest to the rectifier. After pulling each tube, power up the amp and check the light bulb.
If the bulb stays bright then power down, put the pulled tube back in its socket and pull the next tube. Repeat until the bulb goes dim.
If the bulb goes dim (normal) it means you have found the problem area.
Replace that tube with a known good tube. If the bulb stays dim with the new tube you should be good to go.
If the bulb goes bright with the known good tube then there is a short in the amp near that tube. Closely inspect the tube socket with a magnifying glass for anything that would short two pins or short a pin to ground. Use your meter to check for continuity from socket pin to ground. Closely inspect the components that are connected to that tube socket on the circuit board and check them for continuity to ground.
3. If you haven't found the problem yet, power down the amp and remove all the power tubes.
Power up the amp and if the bulb goes dim then the problem is probably with the fixed bias circuit. Check the voltage at the grid pins of all the power tube sockets.
If the bulb stays bright then switch the amp into standby mode (if available).
If the bulb goes dim then the problem is probably a shorted power supply filter cap or the associated wiring.
The goal of early testing is "divide and conquer." We'll try to find what general area of the amp is causing the problem to narrow our search.
With the amp powered up can you hear any hiss or hum coming from the speaker? If you can then you know the mains fuse, power transformer, power tubes and output transformer are powered and at least partially working. The problem is probably between the input jack and phase inverter (or driver for single ended amps).
Some amps have multiple volume, gain and master volumes so make sure they are all turned up for testing.
If the amp powers up but is quiet or sounds bad you should always suspect a bad tube. You should have a spare set of known good tubes on hand so swap out all the tubes one at a time and see if that fixes your problem.
Try another guitar and guitar cable.
Try all the input jacks.
For a new amp startup a very common "no guitar sound" problem is incorrect input jack wiring. To test your input jack wiring insert a guitar cable into the input jack, then measure the other end of the cable from tip to sleeve for resistance. You should measure 1M which is the standard value for the input resistor which is connected across the input jack's tip and ground terminals. If you measure near 0 ohms between the tip and sleeve the tip of the guitar cable is making contact with ground and sending all the guitar signal to ground. Inspect the input jacks carefully and make sure the shunt (switch) terminal is the middle terminal as shown on amp layouts. On some Switchcraft clone jacks the shunt terminal is not the middle terminal. Make sure the jacks' switch disconnects when a guitar cable plug is inserted.
If the amp has an effects loop try plugging the guitar into the FX loop Return jack. That will bypass the gain stages and circuitry upstream. If that works you know the problem is upstream of the Return jack.
Check the speaker and speaker cable. A speaker cable with a break in it can fry the output transformer. Make sure the speaker is plugged into the correct speaker jack. Many Fender amps ground out the signal if you plug only one speaker into the Aux speaker jack. Try all the speaker jacks.
Play another amp through the speaker to verify its tone and function. You can also connect another speaker to the problem amp, if the amp sounds good you know you have a bad speaker.
If the "no guitar audio" problem persists continue to: Going Inside the Chassis.
Weird noises, including squealing, swooshing, clicking, static, honking, motor boating, etc. can be caused by:
A bad tube so swap out the tubes one at a time for known, good tubes.
A microphonic tube. You can gently tap the tubes with a chopstick or wooden pencil with the amp on and listen for excessive noise.
Positive feedback from the negative feedback circuit. Swap the output transformer's secondary wires to make the feedback negative. When you start up a new build amp with NFB you have a 50% chance of getting a squeal from positive feedback.
Lead dress causing capacitive coupling. Chopstick the amp. You can use a non-conducting wooden chopstick to move wires around with the amp operating to find the source of noise. It can also be used to apply pressure to components and solder joints to identify weak components and joints.
Weak or failing filter caps can cause oscillation and squeal especially in high gain amps. You can alligator clip a new cap in parallel with an existing cap. If the oscillation goes away replace the old cap.
Intermittent noise or noise that begins or goes away when the amp warms up are sometimes caused by a weak component or bad solder joint that is affected by heat. You can sometimes identify the issue by waiting for the problem to occur then carefully 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 back you've located the problem.
For more info see Going Inside the Chassis
Nasty sounding guitar audio can be caused by many things including:
For a new or recently repaired amp using the incorrect value of a component can cause funky audio. Verify the value of all resistors and caps.
A bad tube so swap out the tubes one at a time for known, good tubes.
A bad solder joint. Applying pressure with a chopstick and hearing noise can identify a bad joint.
A disconnected component. Component leads can break and wires can pull loose. Chopsticking components and wires can help identify the loose connection.
A bad component:
Blown power tube screen resistor. This is a common cause of a weak sounding amp. When a power tube blows it can short the plate and screen and cause the screen resistor to burn and blow. Measure the resistance across the resistor.
Leaking coupling capacitor. A leaking cap can cause scratchy pots and affect preamp and power tube bias.
Shorted resistor. Measure the resistance across the resistor. A shorted resistor will show 0 ohms.
Open resistor. Applying light pressure to the resistor and hearing noise can sometimes identify a bad resistor. Measure the resistance across the resistor. An open resistor will show a very high resistance well beyond the resistor's rating.
If the output transformer shorts between windings it can lead to no output, weak output or funky sounding output. You can test the transformer by measuring the resistance between its leads. Drain the filter caps and remove the rectifier and power tubes before making the following resistance measurements:
For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.
A shorted primary winding will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.
The secondary windings will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.
If a transformer winding is open (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).
A short between the primary and secondary windings will typically show less than 10 ohms resistance between the primary and secondary leads.
Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a short to the transformer's iron core.
For comparison my healthy Hammond 125C push-pull output transformer measured: red enter tap to brown 150 ohms, red enter tap 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 53 ohms between the two primary wires. All of the secondaries measured .2 to .3 ohms. Both transformers measured an open circuit between their primary and secondary and to the transformer outer metal shell.
If any of the above faults are present your best bet is to just replace the output transformer.
For more info see Going Inside the Chassis.
Your first step in combating hum and buzz is to try another guitar and guitar cable. Also try the amp in another location because noise is often caused by dirty power (perhaps caused by a refrigerator's compressor motor) or radio frequency interference (RFI) caused by a noisy light dimmer or fluorescent lights. Before taking the cover off your amp try it in another location, preferably in another building.
The next step is to identify the type and frequency of the hum or buzz. Buzz has a sharp tone to it where hum has a smooth sound. The "buzz" is caused by noise that can be seen on an oscilloscope to have sharp spikes.
In the United States hum and buzz usually comes in two frequencies, 60 and 120Hz (Hz means cycles per second). If your power runs at 50Hz like in Europe you will have 50 and 100Hz hum. Determining the frequency of the noise will help you track down the source. See this Youtube video at 2:15 for samples of both types of hum.
"Chopsticking" the Amp
You can use a non-conducting wooden chopstick to move wires around with the amp on to find the source of noise and identify lead dress problems. On my first amp build I had the V1A plate and grid wires laying on top of one another which created a moderate hum. Simply moving those wires apart made the amp almost silent. A chopstick can also be used to apply pressure to components and solder joints to identify weak components and joints.
Sometimes you'll have to resort to using an oscilloscope and signal generator to track down the source of a difficult to identify hum or buzz. See the oscilloscope section for more info.
50 or 60Hz Buzz
50 or 60Hz buzz is usually caused by power line noise and can be addressed by applying a small, high voltage filter cap across the high voltage rectifier input wires. This .02uF 3KV (3000v) ceramic disc cap works well for this. I usually solder one across the rectifier tube socket where the two high voltage power transformer wires connect. For a solid state rectifier solder the cap across the rectifier inputs.
Buzz can also be caused by AC components. I had a nasty 60Hz buzz that was caused by the placement of a 120v pilot light too close to the amp's signal wires.
100 or 120Hz Buzz
100 or 120Hz buzz can be caused by a bad tube so swap in a new set of tubes. Buzz can also be caused by a noisy rectifier. This can usually be eliminated by placing .02uF 1KV caps in parallel with each rectifier diode. If the rectifier is inside a housing then running caps from the two rectifier input to the other two terminals usually works well.
50 or 60Hz Hum
50 or 60Hz hum usually comes from the power transformer or its wiring, the tube heater wires or from external RFI caused by fluorescent lights, dimmers and other sources.
Assuming you've tried the amp in a different location the hum must be generated by the power transformer or its wiring. Keep as much distance from the power transformer's wires and the amp's wires and circuitry as possible. Heater wires should be twisted and the untwisted wire that goes to each tube pin should be as short as possible with no big loops. Signal wires should cross the heater wires at 90 degrees to minimize coupling.
If your amp's power transformer has no 6.3v heater center tap then the amp needs an artificial center tap. Verify there is an artificial center tap installed and check the resistance of its two resistors. A missing 6.3v center tap or a burned out resistor will usually cause loud hum.
100 or 120Hz Hum
This is probably the most common hum in an amp. It can be caused by:
A bad tube so swap out the tubes for known, good tubes.
An input jack that doesn't ground out with nothing plugged in. If the jack's shunt switch does not make good contact with the jack's tip connector you will get loud hum when no guitar is plugged in but the amp will sound fine when you plug in a guitar.
Worn out filter capacitors. Electrolytic filter caps have a typical lifespan of 15 to 20 years. You can test for bad filter caps by clipping in a parallel cap. If the new cap reduces hum then replace the original cap. If your amp is fixed bias don't forget the fixed bias circuit filter cap(s).
Missing ground. Loud hum can be caused by components that should be grounded but aren't. Forgetting to solder a volume pot's ground wire or a cold solder joint on a cathode resistor's ground are two common causes of loud hum. I like to use an alligator clip wire and connect one end to the chassis and carefully probe all the amp's ground connections to see if I hear an improvement. Pressing solder joints with a chopstick can help find bad solder joints.
Loops in the ground circuit. If your input jacks are grounded to the chassis and you also run a ground wire between them then a ground loop is formed (chassis is one side, the wire is the other) which can act as an antenna and pick up RFI noise and hum. You will also form a ground loop if you use shielded cable and ground both ends. You should only ground one end of shielded cable.
Lead dress (wires too close to one another causing capacitive coupling) Chopstick the amp's wires around to see if you can decrease the hum. Pay close attention to the tube grid and plate wires--keep them as far apart from one another as possible.
Chasing Down a Nasty Buzz
After I finished testing and tweaking the Deluxe Micro prototype in bare circuit boards I mounted them in a Hammond blank chassis. When I fired up the newly mounted amp I had a nasty buzz at max volume that wasn't there before I put it in the chassis. It didn't sound like smooth 60Hz heater wire hum or 120Hz power supply ripple hum so the investigation began.
The amp with no guitar cable plugged in was absolutely silent at max volume. The buzz could be controlled by both the volume and master volume controls. The amp's tone control could almost entirely eliminate it when turned full down. The guitar's tone control could completely eliminate the noise when turned full down. Hmmm. The buzz didn't change with guitar movement or when touching the guitar's strings or grounded metal bridge.
Since the volume control affected the noise I knew it was entering the amp before the control--somewhere between the guitar and the volume pot.
My first instinct was I had a bad cable or a guitar issue. I tried another cable--no change. I tried my Strat--no change. I tried a cable with no guitar connected and got the expected crescendo of noise.
My next suspect was the fluorescent lights that light the basement workshop. I turned them off and the amp looked cool in the dark as it spewed forth The Buzz.
Maybe it was power line noise. I diligently moved the amp to another part of the house and tried--no change.
Time to get into the chassis. My first thought was a bad ground connection so I alligator clipped a wire to the chassis and carefully probed all the ground connections in the amp to see if I heard an improvement but found no success.
My next step was lead dress. I used a wooden chopstick to push around wires while listening for an improvement in noise. I did find that the input jack wires that ran by the power switch and across both circuit boards were picking up some 60Hz hum from the power switch. I replaced the input jack and headphone jack wires with RG174 coax which eliminated the hum but had no effect on The Buzz.
Time for the oscilloscope. I started with the input jack, zoomed in on the signal and played with the guitar tone control to change the volume of The Buzz but couldn't see any signal artifact change with the guitar tone change. I jumped to the speaker output jack and tried again with no success. Next I tapped into V1A's output just after the coupling cap (to avoid high voltage DC on the probe) and repeated the tone change and again I couldn't see any change on 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 control's movement. I zoomed in the scope's display and confirmed it was THE BUZZ.
I used the scope's horizontal scan knob to move the waveform left and right so I could measure the time between spikes--17.4 milliseconds. Off to the internet to convert the time interval to frequency and the answer was--60Hz, it was wall power related after all, or so I thought. I pulled out my trusty H&K Tubemiester 5 and plugged it into the same wall power socket and fired it up--no buzz. I figured the H&K might have some power line filtering built in so that test proved nothing.
I have seen some amps with a .01uF 3KV (3000V) disk capacitor across the rectifier tube socket's high voltage input pins to pre-filter high freq noise so I gave that a try. It did help a little but THE BUZZ still lived.
I sat there staring into the amp's chassis wondering what it could be. I noticed the 120V pilot light (not a standard 6.3V light) sitting next to the input jack. This was my first build using a 120V light so I had no experience with them. I hauled the amp back to the solder station and de-soldered the light's neutral wire and fired up the amp--success! THE BUZZ was no more. It was that damn noisy bitch of a pilot light.
The fact that the amp was silent when no guitar cord was plugged in was a big clue here that I missed. With no cord plugged in the circuit from the input jack to V1A's grid is grounded so THE BUZZ was being shunted to ground. I should have focused the investigation on the input circuit from the start. 20/20 hindsight and all that. . .
When I stuffed the prototype B9A boards into the chassis I put them to the far left to make room to mount the power transformer on the far right. After getting the boards mounted I realized I had no front panel real estate on the left side of the chassis away from the inputs and preamp stages. I ended up having to mount the power switch, pilot light, guitar input and headphone jack right next to each other--which is not good--it's a cardinal sin of amp building actually and I had to pay a penance to atone for my sin.
Notice the power switch, input jack, 120v pilot light and headphone jack. Not a good idea. If possible keep the power switch and light on the power transformer end and put the input jack on the preamp end of the chassis.
The little prototype is now nice and quiet and just sounds killer.
Going Inside the Chassis
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here.
Look for internal fuses. Many amps have several fuses inside the chassis, both in-line and circuit board mounted to protect the power transformer and other circuitry.
If an amp is completely dead take a look at the tubes to see if they have any heater glow. Dimming the room lights can help see the glow. Most amp pilot lights are powered by the heater circuit, but not all so don't assume a lit pilot light means the tubes have heat. If a tube isn't lit measure the AC heater voltage from heater-wire-to-heater-wire, it should be around 5.7v to 6.9v which is 6.3v +/- 10%.
Dirty or loose tube sockets can cause everything from no signal at all to intermittent static, pops and hum. If wiggling the tubes causes noise then put some contact cleaner on the tube's pins and insert them into the socket two or three times. You can also re-tension loose sockets for solid pin contact.
My next step is to take DC voltage readings starting at the filter caps (B+1, B+2, B+3). It's good to have a baseline voltage chart for your amps so you know what voltage to expect. A higher than normal B+ reading can be caused by a non-functional tube that's not drawing current from the power supply. A lower than normal reading can be caused by a tube that is pulling too much current which can be caused by a short, bias problem or in new build amps an incorrect component value.
If voltage is completely missing there is a break in the power supply. Back up toward the power supply and find where there is voltage and where there is no voltage and look for the cause. A blown power resistor between the filter caps is a common cause of a break in the flow of voltage.
Next measure the voltage at the power tube socket closest to the power 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 can 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. Arrow points to the insertion index notch. Pin numbers start after the index notch and increase clockwise around the socket.
The power tube pins 3 (plate) and 4 (screen) should have high voltage DC and pin 5 (grid) should pop when probed with the meter. For fixed bias amps the grid should show a negative voltage but cathode biased amps will indicate near 0 volts on the grid. Pin 8 (cathode) will show 0 volts in fixed bias amps or show voltage between 10 to 25 volts DC in cathode biased amps. Nine pin power tubes like the EL84 have different pin functions so Google their data sheet to see their pin functions. You should always hear a pop when probing the grid of any tube in the signal chain. Tubes that run the tremolo may not pop when probed. If you get no grid pop then you have a problem somewhere between that power tube and speaker. The first thing to try is to replace the tube. Next measure the voltage at every pin of the troubled tube's socket for clues to the problem.
Higher than normal voltage on the plates can be caused by no electron flow through the tube. This can be caused by no heater voltage or a disconnect between the tube cathode, the cathode resistor (if used) and ground. High voltage on the plates can also be caused by a higher value cathode resistor like using a 15k resistor instead of the specified 1.5k.
Lower than normal voltage on the plates is caused by too much current flow through the tube. This can be caused by an incorrect bias voltage on the grid. A leaking coupling cap can let DC voltage through to the grid and heat up the bias current. A lower value cathode resistor can also allow too much current to flow through a tube.
Cathode biased power and preamp tubes should show the bias voltage on the tube's cathode pin. If there is voltage on the plate but the cathode is zero this can be caused by a bad cathode resistor connection or no heater voltage. With the amp off measure the resistance from the cathode pin to ground. It should equal the cathode resistor value.
Fixed bias power tubes should show the cathode as connected to ground and show a negative voltage on the grid, usually between -30 to -50 volts DC. An incorrect grid bias voltage can make an amp sound bad.
Blown power tube screen resistors are a common cause of weak or nasty sounding output. When a power tube blows it often shorts the tube's plate to the screen which results in a blown screen resistor. A blown screen resistor will normally fail open with infinite resistance. The amp can operate with one power tube but it will sound weak and funky. When you replace the blown tube it will still function poorly because the blown screen resistor will not allow any voltage to the screens so the amp will sound different than with the blown tube but still not sound good due the huge output imbalance between the good tube and the tube with no screen voltage. Verify the resistance across the screen resistor.
If the first power tube checks out then move to the next tube up the circuit which will be another power tube for push-pull circuits or the driver tube for single ended amps. Make sure all the volumes, gains and master volumes are turned up a little when you check the preamp tubes so you can listen for a pop when probing the preamp tubes' grid.
Again listen for a pop when probing the grid and plate pins and look for voltage anomalies. The pop should get louder as you test each tube up the amplification chain toward the input. Continue moving toward the first preamp tube. If you don't get a pop, or a weaker pop than expected when probing a tube's grid then you have found a problem area. Carefully check the voltages of that tube to find clues to why it's not working.
For 9 pin preamp tubes verify 5.7 to 6.9v AC between the heater pins 4 and 9, and 5 and 9 and verify you have DC voltage on pins 1 and 6 (plates), 3 and 8 (cathodes). You should hear a pop when probing the grids, pins 2 and 7. Nine pin tubes should have 0 volts on their grids unless the triode is being used as a phase inverter.
If you have unexpected voltage on a tube's grid you may have a leaky coupling capacitor upstream allowing DC voltage through to the grid.
Troubleshooting Spring Reverb
The reverb driver tube is pushed very hard with high voltage in many amps so the tube tends to burn out sooner than all the other small tubes. A bad tube can also cause excessive hum in the reverb. Swap out both reverb tubes with known good tubes and test the amp.
The reverb tank cable connectors (all 4 of them) can corrode and cause hum, weak or no reverb. Clean the connectors (male and female) with contact cleaner. Spray the male connectors and insert them into the female connectors several times.
The fine wires in the reverb tank are a common failure point and they are difficult to repair. Replace the tank if any of the wires are broken.
If you bang on the amp with the power on and reverb turned up and hear the reverb spring crash you know the reverb tank's springs, output transducer and reverb recovery amplifier are functioning.
You can switch the tank's input and output cables and bang on the amp again. If you hear the spring crash (it will be quieter this time) you know the input transducer is working. After this test you will know the problem is upstream of the tank, which is the reverb driver circuit including the reverb transformer.
Loose transducers can cause funky problems. If they are loose put some glue on them to hold them solid.
Isolating the reverb pan with a pan bag, rubber grommets and foam padding beneath the cab and pan can help cure reverb problems, especially those that occur at high volume and high reverb settings.
"Chopsticking" the Amp
You can use a non-conducting wooden chopstick to move wires around with the amp operating to identify lead dress problems. On my first amp build I had the V1A plate and grid wires laying on top of one another which created a moderate hum. Simply moving those wires apart made the amp almost silent. A chopstick can also be used to apply pressure to components and solder joints to identify weak components and joints.
Freeze Spray for Intermittent Problems
Intermittent problems are sometimes caused by a weak component or bad solder joint that is affected by heat as the amplifier warms up. You can sometimes identify the issue by waiting for the problem to occur then carefully 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 back you've located the problem.
Break Out the Oscilloscope
For really tough troubleshooting tasks I break out the tone generator and inject a 500Hz 100 milliamp (.1 amp) signal at the input jack and trace it through the amp with an oscilloscope by probing the tubes' grids. Start at the input jack and work towards the speaker and watch for the wave shape to disappear or change shape. There are tone generator apps for your phone you can use but you will need an adapter with a 1/4" mono TS plug to plug into the amp's input jack.
I also use a dummy load when doing this test so I don't have to listen to the damn tone coming through the amp's speaker. Be careful with your oscilloscope probes because the very high voltage on the tube plates can damage it unless you use a high voltage rated probe. The grids normally have zero or low DC voltage so I take my signal sample from the grids.
To test a diode make sure the circuit is powered down (drain the caps) and measure the diode's resistance both ways across it (measure, then reverse the meter probes, measure again).
A good diode will show resistance of 7 to 15 ohms in one direction (forward resistance) and almost infinite resistance in the other direction (reverse resistance).
Reading an open circuit (infinite resistance) both ways means the diode is open.
A zero resistance reading in either direction means the diode is shorted.
A diode with a resistance reading below 7 ohms should be replaced.
Measuring a series of diodes in a row should show about 12 ohms of resistance per diode in one direction and infinite resistance in the other.
The striped end of a diode is the cathode end, the other end is the anode.
This webpage can be downloaded as a .pdf file: Tube Guitar Troubleshooting.
By Rob Robinette