Common Tube Amp Malfunctions: My amp makes no sound.

If your amp makes no sound, it is first important to define what you mean by “no sound.” To that end, the first test you should perform is whether you can hear anything coming from the speakers. (This is, of course, after you have eliminated the possibility of trivial problems.)

Listening to what (if anything) comes out of the speakers can help you isolate the problem to specific parts of the circuit. If you hear no sound coming from the speakers — including hum, hiss, reverb crash, input cable pop, or any other incidental, non-musical sounds — the problem could be the speaker itself. But, if you hear non-audio sound coming from the speakers, the problem is likely, although not guaranteed, to be a fault in the preamp section of the circuit.

safety notes

Audio amplifiers, and tube amplifiers in particular, contain high voltages. Do not attempt to repair an amplifier without following the appropriate safety protocols. This includes (but is not limited to) discharging the electrolytic capacitors if you are troubleshooting the circuit within the amp chassis. Note that electrolytic capacitors can pose a shock hazard even in the amp is turned off and unplugged.

No sound comes from the speakers

If there is absolutely nothing coming from the speakers, you should distinguish between an amplifier that is not turning on and an amplifier that is not passing signal. If you see tube glow and the pilot light is on, that is an indicator that the early power supply is functioning correctly (i.e., your amp turns on). If you don’t see these things, you should read the guide My amp doesn’t turn on first.

Usually, in a no-sound scenario, you will hear some sound coming from the speakers, even if the signal path happens to be cut off. For instance, you may hear hum, hiss, reverb crash (if applicable), or even popping sounds when connecting or disconnecting inputs. This is because, in many cases, the signal is interrupted well before the speaker, leaving the speaker to transmit any incidental noises that enter the circuit outside of the fault area.

However, it is certainly possible for the speaker to be interrupted in a way that also silences the speaker. Possible causes include:

The speaker is completely blown
The output transformer is open or otherwise faulty
The speaker jack is not wired correctly
The speaker wiring has deteriorated, or the jacks have rusted to the extent that they can no longer make the proper connection

There might also be an interruption in the early power supply: for instance, a failing power transformer. These possibilities are discussed in the guide, My amp doesn’t turn on.

Notes

Listen to the speaker carefully, in a quiet room, to confirm that it is, indeed, not passing any signal whatsoever. Any audible signal, audio or otherwise, is a good clue. Give yourself every possible advantage when troubleshooting and always listen carefully before concluding that the speaker has no sound.

In some cases, the signal is extremely attenuated but very, very faintly audible. In this case, hum will also be attenuated, so you can’t necessarily make a determination of “no sound” by the absence of the usual noise floor. Turn the instrument up and make a thorough test before making your conclusions.

When testing, make sure that the speaker is pointed in your general direction. You may need to get physically closer to the speaker than you would otherwise position it during normal use. This isn’t to suggest that you should smash your face against the speaker cloth for a closer listening experience: if there is an unexpected loud noise, you may damage your hearing. Just be aware of the position of the speaker. If it is on the floor, elevate it. If it is facing away from you, turn it around. If you can tilt it upwards, do so.

Is the speaker blown? Many symptoms of a blown speaker imitate symptoms of a malfunctioning amplifier. For this reason, you should always be suspicious of your speaker until you can prove that it is functioning correctly. One way to do this is to test the speaker in a working amplifier. You can always do the reverse — test a known working speaker in the questionable amplifier — but there is a chance that the amplifier is malfunctioning in a way that damaged the speaker in the first place. Shorted output transistors or a faulty output transformer can destroy a speaker in an instant. So, do not use a valuable speaker for this test.

Some sound comes from the speakers

If you hear hum, hiss, or other incidental sounds coming from the speakers, but no signal, there is probably some fault in the amplifier that is interrupting the signal. In this case, you can assume that your speakers are likely working. If your speakers are reproducing these incidental sounds, it would probably reproduce signal as well, if signal were present. The problem is that signal is not present. Where did it go?

Step one: Isolate the problem.

The bad news: a “no signal” problem can dwell anywhere in the amp circuit. The good news is that most problems can be narrowed down to small portions of the amplifier. In most cases, you don’t even need test equipment to determine where the problem is hiding. You can use your knowledge of how an amplifier works, along with the evidence that your malfunctioning amp is giving you.

Listen to the noise floor. The first piece of evidence that you have is the sound that the amplifier actually is making: the noise floor. We’re giving this section probably more real estate than it deserves, because listening to the noise floor is a vague and inexact method of diagnosing an amplifier’s problems. However, the point is that everything that the amp is doing should be treated as evidence of its malfunction. The more you listen to the amp and think about its issues, the less you’ll rip out and replace perfectly good components and drive yourself to total frustration.

Think of it this way: if you are the detective, the noise floor is the witness that says, “A guy wearing a green hat robbed me.” This is a great piece of information as long as you don’t forget that 1) a lot of people own green hats; 2) people have the ability to take their hats off, and/or put new hats on; 3) a lot of people don’t know what the color green is (Is chartreuse green? Is teal green?); 4) if the witness was dishonest or forgetful, there might not even be a green hat in the first place. So, you can’t simply arrest the first person you see who is wearing a green hat. In order to make sense of the green hat as evidence, you ideally need other information from other sources as well.

Likewise, the noise floor is an unreliable witness. Although we can and should listen to it, we can’t let it dictate our method of troubleshooting. We can’t let it supersede our knowledge of the amplifier’s circuit or whatever other evidence the amp is offering. With that disclaimer out of the way, we’ll now discuss what to listen to when you listen to the noise floor.

When functioning correctly, every amplifier has a noise floor. So, if the noise floor coming from the speakers is identical or nearly identical to the amplifier’s usual noise floor, you can deduce that the amplifier is almost completely working correctly. The noise floor is present and it is thoroughly being amplified. Only the signal is absent. Therefore, you should start troubleshooting at the very beginning of the signal chain:

If the amplifier is a guitar amp, confirm that the instrument cable as well as your guitar’s electronics are working correctly.
If the amp is the internal Wurlitzer amp, confirm that the input cable is working, the reeds are not shorted to the pickup and the amplifier is sending sufficient power to the reed bar.
Confirm that the input jacks are in good shape.
Look for flaws or malfunctions around the input circuitry.

On the other hand, if the noise floor is lower than you would typically expect from this amplifier, it is likely that some of the amplifier’s hum is being interrupted, as well as the signal. This suggests a flaw or malfunction later in the preamp.

Sometimes, the noise floor coming from the speaker is louder or more aggressive than the amp’s working noise floor. This suggests that a component is malfunctioning in a way that is not only interrupting the signal, but introducing noise itself. This often happens when an active component (i.e., a tube or a transistor) fails, but it could also happen due to the failure of a crucial passive component as well. The takeaway here is that you are probably not looking at a simple mis-wire, or corroded or damaged jack. Instead, there is probably a component that needs to be replaced. In a tube amp, replacing the tubes is a good place to start.

However, there may be some problems with using the noise floor as a diagnostic tool:

If the amp has been broken the entire time that it is in your possession, you may not know what the working noise floor is supposed to sound like. You can’t make any accurate assumptions, because some vintage amplifiers have a louder noise floor than others.
Most working amps in good condition have a very unobtrusive noise floor. It is totally understandable if, back when the amp was actually working, you’ve never bothered to memorize exactly what the noise floor sounds like. So, you may not be able to identify whether the hum coming from your now-malfunctioning amplifier is the usual noise floor, or slightly lower.
The amplifier may have multiple problems that are affecting the noise floor in competing ways.

Bottom line: the noise floor is decent evidence, but it is important not to overstate the conclusions that you can draw from it. It’s not a form of code that, if cracked, will tell you exactly what your problem is. Rather, it will give you an idea of where to begin — which is certainly a valuable tool, if you do not have an oscilloscope and nothing in the amplifier is obviously malfunctioning. However, if this beginning doesn’t solve your problem, you have to let go of this initial guess, circle back around and investigate some of the possibilities that you may have set aside as unlikely.

Check all possible inputs and outputs. If the amplifier has two channels, try both. If neither channel gives you sound, the problem is likely somewhere in the circuit after the two channels merge. If one channel gives you sound, but not the other, the problem likely exists in the silent channel.

If the amplifier has an aux output, you should test the output. If it gives you sound, the interruption is probably in the circuit following the output. If you get no sound from the output, the interruption probably precedes the output circuit. There are various ways to design an aux output, so if you’re getting a different result from the aux than through the main speaker, you should pay careful attention to how and where the signal is tapped.

If the amplifier has a spare jack, but not aux output, you may want to wire a quick-and-dirty aux output in the preamp. True aux outputs should be buffered to minimize losses when you connect the aux to an external device, but for this test, you don’t need to worry about buffering. Just connect the tip of the jack to a safe point in the signal path (after a coupling cap connects one gain stage to the next, for instance), and the sleeve to ground. If you get sound from this aux, you know that the preamp circuitry before the aux is functioning properly.

Step Two: Investigate likely suspects.

If you are able to isolate the problem to a certain area of the circuit, you can think about how the components in that area work — and what might be causing them to malfunction. In general, common problems that interrupt the signal path include:

A failing tube, transistor, or passive component that is interrupting the signal
A tube or transistor is not receiving the adequate supply voltage, due to a failure in the power supply
The signal has become inadvertently grounded
A ground that is necessary to the circuit has become inadvertently lifted

A good order-of-operations in diagnosing the problem is to:

Replace the tubes with known good tubes.
Visually inspect the amplifier to ensure that all wires are in place and in good condition, and that there is no debris that might inappropriately bridge two leads.
Ensure that grounds are, in fact, connected to ground.
Ensure that the power supply is supplying tube plates and other critical areas with the appropriate voltage. If there is inadequate voltage, find where the voltage is being throttled.
Replace original problem components with new, working components. Problem components include any resistors that are under a lot of stress (including plate resistors and power supply resistors), electrolytic capacitors, and cathode resistors (which are essential to a tube circuit).

If you do this and the amplifier still does not pass signal, you should:

Check your previous work.
Try to isolate the problem with more specificity. For instance, try taking an output from an earlier point in the circuit. Or, if you are working in a Wurlitzer amplifier, try creating an input that you can test with guitar (i.e., one that is completely isolated from the 180v reed bar supply voltage). This will tell you if there is a fault in the Wurlitzer input or reed bar, which is extremely common, particularly if the Wurlitzer has recently been moved.
Reconsider the assumptions that you are making. For instance, if you are focusing on the later preamp circuit due to assumptions made about the noise floor, expand your search and look at the early preamp as well.