On Troubleshooting
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.
This amplifier was definitely not made for you to record with. Just look at that aux output jack.
It is probably likely that the people who made the instrument never thought that anyone would use it fifty or sixty years later. Nor could the manufacturers have dreamed of the sophisticated and highly accessible recording gear that you probably want the amp to interface with. They made the amps, they made the money: case closed, objective achieved. 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 time. You decide where repairs begin and where they end.
Another upside: troubleshooting can actually be very simple. It doesn’t require math. It often doesn’t even require soldering. In many cases, you can successfully troubleshoot with the common-sense skills that you already have. Listen to the amplifier; think about the environment that you are in. Use your five senses before you ever even look inside the chassis.
If you do not have a lot of electronics experience, you may come to a point where you meet the limit of your abilities. That’s fine: there are people who can help. (In fact, we have options for you.) Or, you may be very unlucky, and the amplifier truly does need high-difficulty work before it can be used again. However, you should know that there are many simple problems that have simple solutions. The trick is to know your abilities, learn some amplifier basics, and familiarize yourself with simple fixes. This will save you time and money.
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.
Here’s an eternal question: When does something cease to be a fix, and starts to become a mod? There are two concerns that you must juggle here. First, the amplifier is very cool. (If it wasn’t, why are you trying to fix it?) For that reason, it is important to preserve some of that vintage magic. However, you definitely want the amplifier to sound good and play well when you are finished working on it.
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 to eliminate ground loops
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. On 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.
We also try to distinguish between things that provide vintage character, and things that are consequences of cheap design. For instance, the presence of cathode bypass capacitors will make the signal louder and a little more intense. However, clean, mellow, medium-gain tone is very characteristic of a vintage amp, so omitting those capacitors is very authentic. On the other hand, absurdly low-value filter capacitors were used mostly because bigger capacitors were expensive and so large that they may not have physically fit in the original chassis. Today, bigger capacitors are much smaller and more affordable than they used to be, and there aren’t too many tonal advantages to sticking with the original values. So, we’ll usually inch those values up (as long as the rectifier tube can handle it).
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, you should 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. More on our good mods guidelines here.
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. These are extremely rare and have no straightforward equivalent. 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.
