Common Tube Amp Malfunctions: How to accurately diagnose what is wrong with your vintage amplifier.

If your amplifier isn’t working, the best thing you can do is describe the problem as accurately as possible. If you’re looking for DIY solutions, using accurate terms will help you google your problem and give you the best search results. If you’re asking a tech or a friend for help, describing your problem with specificity helps them offer more relevant suggestions. If you’re getting in touch with us through our Wurlitzer Hotline, an accurate description of the problem will help us give you the best suggestions possible.

Describing the problem is a crucial first step to diagnosing it, and diagnosing the problem is almost as useful as solving it. A correct diagnosis often leads to the part of the circuit that is malfunctioning, so you can focus your attention there instead of wasting time examining components that aren’t at issue. It’s the difference between looking for a needle in a haystack, and looking for a needle in a box of needles.

This guide describes common tube amp malfunctions and their subtleties, including various types of hum and noise. It is intended to help narrow down the source of seemingly vague and general problems, so that you can determine a solution faster.

Repairing Your Amp, Step Zero: Describe its malfunction accurately

Describing the problem itself is rarely enough information to fix it. It’s often just as important to describe the context of the problem: that is, where and when the problem occurs. Even an amplifier in perfect working condition can be made to malfunction if it is placed in the wrong environment. For instance, if plugged into a poorly filtered outlet, or placed next to a source of interference, even a working amp will produce noise. Also, problems that are intermittent often have different causes than continuous problems, even if the symptoms are the same.

Describing the last time the amplifier worked can also provide important clues. Wiring errors, for instance, are common in amps that have never once worked, but are unlikely in newly-malfunctioning amplifiers that have previously worked without issue for years. Amps that are heavily used also have different issues than amps that just came out of storage.

A complete description of an amplifier malfunction answers these three questions:

What is the problem?
When does the problem occur?
When was the amplifier last working correctly, and what happened after that?

Each question will be discussed in the following sections of this guide.

Note that there is no limit to how much detail you can provide when answering these three questions. If you are asking someone else for help, they will appreciate more (rather than less) information. If you are troubleshooting your own amplifier, it is important to think carefully about the problem before you dive into soldering.

Questions to help diagnose your broken amp

Because all amplifiers are unique, it is impossible to come up with a one-size-fits all approach to repairing them. We have, however, made a list of questions that should put you on the right track. These are questions we always ask ourselves before we even open up any broken amplifier’s chassis.

QUESTION ONE: What, EXACTLY, is wrong with your amplifier?

Most amplifier problems fall into one of the following categories:

The amp doesn’t turn on
The amp makes no sound
The amp passes signal but makes strange, non-musical sounds
The amp passes signal that sounds distorted
The amp is extremely noisy
The amp has low volume
The amp smells like it’s burning
The amp’s effects don’t work

We are gradually writing guides that go into more detail about each of these problems. We will link to the articles here as we complete them.

QUESTION TWO: When does the problem occur?

Taking notice of when the problem occurs can help you narrow down its cause. Although many problems appear at start-up, others are intermittent or only happen after a certain amount of usage.

Does the problem happen as soon as you turn it on? if not, how long does it generally take for the problem to arise?
Does the problem happen every time you turn it on? Or only sometimes?
If the problem only happens sometimes, does it happen with increasing frequency?
Have you ever tried using the amp in a different location? Does the problem arise no matter where you use the amp?

Notes

If the problem only happens after the amplifier has been on for a certain amount of time, it may be triggered by heat. Many components get increasingly hot the longer an amplifier is on. Under normal circumstances, these components are designed to tolerate or dissipate that heat, but if the amp is malfunctioning their ability to do so may become diminished.

Note that there are a few stages of warm-up for amplifiers. First, the heaters must warm up. Preamp and power amp tubes cannot amplify unless their filaments are hot. If there is a tube rectifier, the power supply does not operate until that filament is hot as well. So, if the problem doesn’t start for 10-30 seconds or so after startup, it clearly cannot manifest unless the tubes’ cathodes are hot and capable of conducting. However, if the problem appears long after the heaters warm up — that is, after several minutes or more of operation — it is probably heat-induced. Power tubes and other components become very hot over time. In a functioning amp, this is not a problem, but heat can exacerbate problems in an aging, flawed amplifier. Is a component getting unexpectedly hot? Has a component deteriorated to the point where it can no longer tolerate the heat that it is rated for?

If the problem is intermittent or happens only when the amp is in a certain location, it may be environmental. That is, the amplifier may be interacting with another piece of electronics that is plugged into the same circuit or a conductive surface that is in the room. (For instance, we get a lot of weird noise when we place an amp directly on our concrete floors.) Outlets can also provide dirty power that might make an amp noisy.

QUESTION THREE: When was the amp last working correctly, and what happened after that?

It’s often hard to remember exactly what happened before an amp starts malfunctioning, but, if you can, this information can provide important troubleshooting clues.

This question doesn’t imply that it was something you did that broke the amp. Instead, it asks whether you witnessed the approximate moment that the amp broke. But if it was something you did that broke the amp, it’s okay: it happens to the best of us. On the upside, your breaking the amp is better for troubleshooting, because now you have a lot of first-hand info about the source of the problem.

Did the amp start malfunctioning as soon as you turned it on? Or did it malfunction after being on for a certain amount of time?
If it started malfunctioning as soon as you turned it on: did anything happen to the amp between that moment, and the last time you turned it on? Was the amp moved? Was it sitting unplayed for a long period of time?
If the amp was already on when it started malfunctioning, what was going on when it malfunctioned? Were you playing the amp? Was it idling? Were you adjusting any controls? Were you plugging anything into the inputs or outputs? Was it connected to any other devices?
If the amp is vintage and you recently purchased it, was the problem present from the moment you bought it? Or, was it working correctly at first, and only started malfunctioning later?

Notes

If the amplifier started malfunctioning as soon as you turned it on, and it was working fine the previous time, considering what happened between then and now is a vital source of clues. For instance, if you moved the amp, check for signs of physical damage. If the amplifier was stored for a long period of time, check for moisture damage or other evidence of improper storage.

If the amplifier was on, and broke while you were adjusting a control or plugging something into it, think about what happens in the circuit when you take that action. What does the control do? Does the jack you are plugging into take components in or out of circuit? Side note: in 200a model Wurlitzers, we have found that plugging devices into multiple outputs can cause the amp to blow an onboard fuse.

If the amplifier is vintage and has never worked correctly in your possession, you obviously don’t know the history of its malfunctioning. Additionally, you should note whether the amplifier has been modified or not. The presence of mods is a red flag in a broken amplifier, because the mods might have been installed in an attempt to fix the problem. Since they clearly did not, you may need to reverse the mods in order to accurately diagnose the circuit. It is also possible that the mod itself is causing or exacerbating the problem.

Question Four: Is the problem trivial?

Once you have defined the problem, but before you do any invasive troubleshooting, you should ensure that the problem isn’t what we call a trivial problem. Trivial problems are those that seem like serious problems, but in fact can be addressed quickly without making major modifications to the amplifier. They are usually environmental (i.e., broken cables; noisy outlets) or caused by deteriorated hardware (i.e., broken jacks). Read more about trivial problems here.