A note from 2026: I originally published this article in 2020. I’ve worked on many more amplifiers and electronic instruments in the years since, but the basic troubleshooting philosophy described here is still fundamental to the way that I work. I’ve left the article largely as written.
Troubleshooting a vintage amplifier involves a skillset that is distinct from designing new circuits. The prime advantage of troubleshooting — the fact that the circuit has already been created for you, in its entirety — is also its biggest challenge.
When you are designing a new amplifier, you have complete control over the materials used. You choose the transformers. You choose the supply voltage. You choose the tube complement. You probably have a toolset that you come back to again and again: favorite design strategies, go-to circuits, preferred tonal character. You can trust your components. They’re brand-new and you just installed them.
When you are troubleshooting an existing circuit, all of the choices have been made for you. Some of these choices, like the tube complement, were made by the circuit designer. Other choices, like the physical condition of the amp, or the presence or absence of mods, were made by its previous owners. Some choices — like the extent of aging in the passive components — aren’t choices at all, but simply a consequence of the passage of time.
All of these choices were made years and years ago — possibly before you were born. You would not agree with many of these choices, because they were made in a different time, with access to different technology, by people with different goals than you do. Your goal, probably, just just to make music, preferably with the broken amp in question. But the manufacturers’ goals were, probably 1) to make money, and 2) easily churn out amps by the hundreds or thousands. Maybe they had other goals as well: for instance, to create amps with a particular popular sound. That sound may now be timeless, or it may be irrelevant. Times change.
It is likely that the people who made the instrument never thought that anyone would use it fifty or sixty years later. Nor would the manufacturers have dreamed that you would have access to such sophisticated recording gear in your home—even in your pocket. These were commercial products, built for the inevitability that something new and “better” would eventually come along. If you can get the amp working again—let alone if you can make music with it and integrate it into your current studio rig—you are giving the amplifier a second life that its designers likely never fathomed.
Troubleshooting an amp is cleaning up a mess that you had no hand in making. This is why many people hate it. But it’s wrong to think of the amp as a broken piece of lo-fi garbage. Its natural lifespan ended sometime long ago. It did what it was meant to do — or maybe it didn’t, but, after all this time, the outcome of both is basically the same. The amp has been discontinued. The company likely doesn’t exist anymore.
So, the amp is broken, but in a sense you are the one behaving abnormally. You are expecting it to work in a setting that is totally different from the one it was built in. You are giving it a second life, but you shouldn’t necessarily expect the rebirth process to be easy.
The good news is that, in troubleshooting, you actually do have a lot of control. You decide whether to respect the choices of the original designer, or whether to discard all the old ideas. You decide what components to replace, and what you will be replacing them with. You decide whether to simply fix the problem and keep the rest of the circuit original, or whether to rebuild the entire thing. You decide where repairs begin and where they end.
This amplifier was definitely not made for you to record with. Just look at that aux output jack.
Where do you draw the line between troubleshooting an amp and modifying it?
The longer you look at an amplifier circuit, the more ideas you may have for improving it. Some of these ideas may be completely necessary to the amp’s functionality. Others might be straight-up mods. Most fall into a gray area: fixes that would make the amp sound better, but aren’t strictly necessary to make the amp work properly.
So, when does something cease to be a fix and start to become a mod?
There are two reasons why you might be concerned about the answer. First of all, the amplifier is very cool. If it wasn’t, why are you trying to fix it? It makes sense to preserve some of that vintage magic.
Second, if you don’t define the scope of your work, you’ll be tinkering with this amplifier forever. This point goes both ways. On one hand, you don’t need to apply endless mods to make the amplifier do everything an amplifier has ever been capable of doing since the beginning of recorded music. It’s okay for an amplifier to be great at one thing and somewhat flawed in another respect. (We are obviously talking about tone and not safety. If the amp literally produces the voice of angels but shocks you every time you touch the chassis, keep troubleshooting.)
On the other hand, if you feel the need to preserve everything in amber except for parts that are unambiguously broken, you have also signed up for a never-ending task. The fact is that “broken” is often subjective. An open filter capacitor might make a lot of hum, but so does an AC wire routed underneath an input jack. Once upon a time, somebody approved the location of that wire and decided that the noise floor was acceptable. But it’s not acceptable to you when you close mic the speaker and send the signal to your DAW, and it’s not even acceptable to you when you’re listening in the room because all you can think about is how bad it would sound if you were to close mic the speaker and send it to your DAW. But the engineer didn’t have a DAW. A lot of people will stand in the room and listen to that speaker and think it sounds amazing, because they are listening to the natural tone of the amplifier and not its noise floor. So—is the wire layout “broken” or not?
Here are some “mods” that we often add while troubleshooting an amp:
Add cathode bypass capacitors where needed
Add grid stopper resistors, if the opportunity arises
Reorganize the grounding scheme
Add power supply filtering if the original was extremely weak
Add buffered outputs, if space permits
Replace ceramic capacitors with mica or polypropylene
Replace carbon composition resistors with metal film in critical areas
All of these things will change the character of the amp. However, we don’t do all of these things to every amp that we fix. We appreciate the variety of vintage circuits, and we don’t believe that every amp needs to have the exact same sound. By the same token, we want the amps that we restore to be as functional as possible. Why spend hours bringing an amp back to life when its original tone leaves a lot to be desired? So, if we see something that we consider to be a design flaw, we will address it. You may have a different idea of what a design flaw is, and you may want to address different aspects of the amplifier.
There will always be purists who disapprove of your mods. But, at the end of the day, purists aren’t going to be playing your amp: you will. If the mod benefits your music, go for it. Our golden rule is that the mod should be something that makes the instrument more desirable to at least some players, keeping it in circulation and out of the trash.
General principles of troubleshooting
Start with the easiest fixes
There are a lot of silly things that can go wrong with an amplifier. But even though the source of the problem might be trivial, the result can mimic a very serious problem. For instance, a broken cable can cause the amp to stop passing signal. If this happens, the correct solution is obviously to replace the cable. But if you overlook the cable, you may end up making pointless and potentially destructive changes to the amplifier. Our guide on trivial problems has more info here.
Don’t get sentimental about any particular component
Vintage components look cool. (We would probably tie vintage capacitors together and string them around the studio like garlands if this wasn’t a horrible, hazardous idea.) But if any component is broken, leaky, or drifted in spec, it has to go. Cut it out, throw it away, and put a new, working component in its place.
Take pictures of the circuit before you begin
Taking pictures is a very important step. There are many instances where it is useful to see how the circuit used to be hooked up. For instance, vintage transformer wires can have confusing color codes. If you clip those wires, you should be sure to have clear photos of where they were originally terminated. This will help you be sure that you are putting the wires back where they belong. In general, photos of the original circuit will help you put everything back in the right place.
Taking pictures also helps with the previous bullet point. It is easier for us to clip out cool-looking components when we know that we already have documentation of the amp’s original appearance. One day, all of the mid-century electronics will be restored, modded, or thrown away: it might be useful to have some photos somewhere of what their circuits used to look like.
Think before you act
It is important to think twice before making any changes to the amplifier circuit. Troubleshooting an amplifier can be frustrating, and becoming too frustrated can cloud your judgement. Clipping out a component takes a fraction of a second; if you change your mind, putting it back in is a lot more time-consuming. And if you clip things out in frustration, you will probably have to end up putting a lot of them back.
Similarly, if an idea doesn't work, stop and evaluate before moving on to the next idea. Why didn’t the idea work? Did you execute it properly? What have you learned from it? What is the next move? Don’t make any circuit changes unless you can articulate to yourself what you are about to do and why.
If you feel that you need to clear your head, there is nothing wrong with taking a coffee break or even moving on to a different project for a while. Keep what you are doing in perspective. This is a vintage amplifier and it has probably been broken for decades before you ever laid eyes on it. Putting it aside for an extra couple of days isn’t the end of the world, and can give you the breathing room to ensure that your final repair is the best that it can be.
Document everything that you attempt
There are a few reasons why taking notes is important. First of all, notes keep you organized. If you have a lot of ideas, your notes will help you keep track of what you’ve already done and what you haven’t tried yet. This is particularly helpful if you are interrupted in the middle of troubleshooting and can’t pick back up for a few days or weeks. Also, if you ever encounter the same problem again, maybe even months or years later, you will be very glad that you took notes the first time.
Check your work as often as you need to
For many reasons, it is better to test one thing at a time. If you try four things at once, and the amp started working once you turned it back on, how do you know which of your four ideas was the correct one? If you don’t know how you fixed this amp, you are at a disadvantage next time you encounter the problem.
It is also possible that you attempt four things at once, and then you turn the amp back on and discover that it now has a new problem. You probably made a wiring error, and now you need to backtrack four things to find it.
You should also check your work as you go by visually tracing your new circuit connections. It is easy to make minor wiring mistakes, particularly in a point-to-point circuit. Checking your work periodically will help you catch them before you power the amp on.
Things that you must know before troubleshooting an amplifier
How to stay safe
Tube amplifiers (and many solid state amplifiers) contain high voltage circuits. Even if you are not literally working on the amp circuit, it is essential that you know how to keep yourself safe. For instance, in a Wurlitzer 200 or 200a, the switch terminals are exposed when the lid is off. If you touch them while the amp is plugged in, you will shock yourself with line voltage straight from the wall. For this reason, you absolutely must unplug the keyboard anytime you are making any adjustment under the lid, whether you are working in the amplifier or in the mechanical action.
If you are performing some test that requires the amplifier to be powered on, there is a whole other set of safety protocols that you need to follow. The specifics of electronics safety is outside the scope of this article. The point is that you must do your research, and always ask yourself: What are the hazards of the test that I am about to perform? How do I keep myself safe?
Electricity is not the only hazard in a vintage amplifier. There are a lot of vintage devices that people had full confidence using back in the day, but are banned or restricted today because of safety concerns. Lead paint, asbestos, various toxic chemicals: all of these things were extremely common and could even be found in your amplifier. For instance, asbestos is sometimes found in the insulation around vintage power cords. Also, certain vintage motor assemblies, for instance in some Leslie-style cabinets, contain carcinogenic chemicals. If you don’t know what something is, or what it is made from, you should handle it as if it were hazardous.
How an amplifier works
The more you know about how an amplifier works, the easier it is to repair. When an amp malfunctions, understanding the circuit will help you zero in on the likeliest suspects faster, with less trial-and-error. It is also easier to trace point-to-point circuits when you know the major features of their topography.
Tube amplifiers look much more complicated than they actually are. Solid state amplifiers are found in more variations, because advances in solid state components have branched out in different ways over the years. However, it is still very possible to understand solid state amps with the right background.
If you’re not clear on how an amplifier works, you can start with our article on how vacuum tubes work. More articles on how certain components or circuits work can be found in our Guides section.
What parts are easy to replace? What parts are difficult to replace? Some components, such as resistors and capacitors, are extremely easy to replace. Even if you can’t find an exact value, equivalents are plentiful and easy to calculate. Other components, such as transformers, are more difficult to source and trickier to replace. Although you can usually spec an equivalent, you will probably need to create new mounting holes. Finally, there are obsolete components, such as germanium transistors and selenium rectifiers. If you can’t find the same part, you will likely need to modify the circuit in order to accommodate a different solution.
It is important to acknowledge which components are difficult to replace, because you need to treat them with more care than ordinary components. If you clip out the wrong resistor, no big deal; if you clip out the wrong transistor, you may be in trouble if you didn’t leave the leads long enough to easily replace it. You should also avoid performing tests that put undue stress on these components, unless they are absolutely necessary.
Further Reading
Browse all of our articles on restoring vintage gear. Or, click on an image below.
