If you’ve owned a tube amp for pretty much any amount of time, you’ve been there: Your amp starts making noise—probably at the worst possible moment, whether onstage, in the studio, at rehearsal, or when you just really want to play—and you don’t know what to do. Maybe you keep it cool, but maybe you panic: Is my amp broken for good? Is this going to cost me hundreds of dollars? Fortunately, there are a few basic things you can check before you call your amp tech in a frenzy.
Ultimately, an all-tube guitar amp is actually a very durable device. But it’s important to remember that these amps need maintenance and have parts that are going to need to be replaced over the course of their lives—and tubes are the most likely place to check when you’re having a problem.
Many vacuum tubes are “military grade,” which means they can withstand shock, vibration, humidity, temperature, and many other harsh conditions. That doesn’t mean they are fail-proof, though. So, when your amp is making noise, here are some ways to check out your tubes.
Look
Do a visual inspection. Are your amp’s tubes bright and shiny, or do they look weathered and dirty? If they look old, they may be old. This doesn’t have to be a sign of something wrong, but tubes wear out like lightbulbs—they work until they don’t. I usually say power tubes have a two- to five-year lifespan (for professionals, it’s more like six months to two years). Preamp tubes have more like a five- to 20-year life. They can function longer but start to get problematic before they just die.
Look at what’s going on inside. Is the clear part of the glass still clear? If there is a smoky look on the inside of the glass, that is a sign of heavy wear. Look at the shiny mirror-looking part; that is the getter flashing. It should look bright, though sometimes there is a little fuzziness along the edges. If it is getting smoky, then the tube is tired. And if it has gone completely white, the tube is toast.
The last visual is best done in darkness. Look for the heater element inside. It is a wire in the very center of the tube structure that glows red when operating. Don’t worry about what shape it is, how much you can see it, or if it looks exactly like the other tubes. If it is glowing, it’s good; if it is dark in there, it’s toast.
In his final PG column, the amp maker to the stars teaches you how to troubleshoot your guitar’s loudhailer if it’s raising a sonic ruckus.
Listen
Let’s move on to audible diagnostics. The amp works but is making extraneous noises. The first thing to do is unplug the guitar cable from the input of the amp, so nothing is plugged in, the amp is fully on, and the tone controls, volume, and master (if applicable) are full on. What noises do you hear? If it is silent with a tiny little bit of background noise, that is as good as a guitar amp gets.
Are you hearing an electronic, almost morse-code sound? A common modern problem is RF (radio frequency) noise from routers, cell phones, and transmission towers. It is not the amp and there is nothing to fix, but if you try moving the amp around the room or building, you may find a quieter place.
Is the amp making a constant “shhhhh” sound? Does it get louder if you turn the volume knob up? That is thermionic emission, the noise of electrons flowing. It is a byproduct of gain. Low-gain amps have less, and high-gain amps suffer greatly. Some tubes can add to this, so if you replace the first couple tubes in the signal path and it helps drop the noise floor, then keep the noisy ones for less critical positions.
Is the amp sitting there doing the snap-crackle-and-pop thing? This can be a tube issue, but most of the time it is a component issue inside. If you have a known good tube, you can replace the tubes one at a time, seeing if the noise changes. If you find that it was a tube, that tube is toast, so don’t use it as a spare.
Now, let’s talk about microphonics. Because of the amount of gain we have in guitar amps, microphonic tubes can be problematic. Start turning the amp volume/gain all the way up. If you get a howl, like with an open microphone, you have a microphonic tube. If it is still behaving but you can tap on the amp and get a sound like tapping on a microphone, that can be acceptable.
Finding the Cause
There are multiple tubes in an amp, so let’s find the root of the noise. The first thing is tap testing. The best tool is a pencil with an eraser. One by one, tap on the tubes with the amp on and turned up. If you get a significant microphonic sound from one, it should be swapped out with a quieter one. Now, this is the thing: A little bit of microphonics is normal, so it will take a little getting to know your amp to learn what is acceptable or not. Certain brands/models of tubes are better than others, so just because it’s new doesn’t mean it’s better or even good. As you go down the line of the signal chain, the gain is less, so you are looking for slight noises. Once you get to the power tubes, they can be microphonic, but they don’t howl like a preamp tube. They make a thud sound when tapped.
The other thing we’re looking for with this tap testing is rattling. Again, there is a certain level of acceptable rattle. But if you can hear it through the speaker, that’s unacceptable. If you have power tubes that rattle, this sometimes does not come through the speaker, but you can hear it when playing the amp at lower levels. If that bothers you, then they need to be replaced even though they’re functioning properly.
The last diagnostic is pulling the preamp tubes (not the power or rectifier tubes). Starting with the first tubes in the signal chain, pull them out one at a time and listen to see if the noise goes away. If you pull a tube and the noise disappears, then either that tube is the culprit, or your problem lies in that part of the circuit.
If you’re looking for a quick fix to get you through your gig or session and you narrow your problem down to a tube, go ahead and swap it out. If your amp is fixed bias, you may still need to head to your tech tomorrow and have it biased, but you’ll get your work done today. And when in doubt, replace your old tubes. If it turns out that wasn't the issue, you now have good spares for when a tube does eventually give you problems.
