How to Troubleshoot Any Amplifier: Start with trivial problems.

How-ToGuitar Amps

Apr 19

The most efficient way to troubleshoot an amplifier is to investigate common trivial problems first. Complicated problems can have complicated solutions, but trivial problems can often be solved in a matter of minutes. By starting with potential trivial problems, you can cover a lot of ground quickly, without unsoldering a single connection. This reduces wear-and-tear on your amplifier, saves time, and prevents you from replacing components that still have plenty of life left in them.

What is a trivial problem? Trivial problems include anything that can be addressed quickly, without making invasive modifications to the device. Trivial problems tend to be easily-overlooked and often masquerade as catastrophic problems. They have a way of making you feel very stupid, because they were under your nose the whole time. We know what it feels like; we’ve all been there. When this article is over, we won’t mention it or even think about it ever again.

Trivial problems include:

The amp didn’t turn on, because it was unplugged.
The amp didn’t turn on, because the outlet you chose was on a switch that was flipped into the off position.
The amp didn’t make sound because the 1/4” cable was broken. (A corollary: the amp didn’t make sound because the second and third 1/4” cables you tried were also broken. Throw your broken 1/4” cables away!!)
The amp didn’t make sound because a tube needed to be replaced.
The amp didn’t make sound because the speaker was disconnected.
The amp is distorting due to an incomplete mechanical connection; for instance, a bent switching jack.
The amp was noisy because it was plugged into a noisy outlet.

These are all real problems, because they prevent the amplifier from working as intended. They are trivial because they can usually be addressed in minutes, and because there is no glory in diagnosing them. Often, there is only pain.

Seriously though: trivial problems are a really underrated source of amp malfunction. Everyone loves pointing to the transformers and making other dire snap judgments when something goes wrong, but we have definitely experienced more unplugged speaker jacks and switched-off outlets than broken transformers. This is particularly true when you are working on a Wurlitzer. Tinkering with the mechanical action adds hours to the restoration: you get tired, you’re staring at the same keyboard for what seems like forever, your attention is split and all it takes is one unplugged cable to start an hours-long wild goose chase looking for an amplifier bogeyman that doesn’t actually exist. The bottom line is always to take a moment to think and observe the instrument before taking any destructive actions, such as unsoldering or clipping out components.

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Observations on Trivial Problems

Trivial problems can mimic serious issues. Imagine that you have plugged a broken 1/4” input cable into your amplifier. The broken cable interrupts the signal, so that the amplifier passes no audio. To fix the problem, all you have to do is replace the cable with a working one. Unfortunately, cables are so trivial that it is easy to overlook them as a possible failure point. Plus, it is logical to assume that an old amplifier may have problems; on the other hand, your cable is more likely to evade suspicion, since it was probably manufactured much more recently. However, if you begin troubleshooting by investigating potential trivial problems, you will likely realize that the cable is broken right away. And, if the problem isn’t trivial at all, you only wasted seconds by trying a new cable.

Misdiagnosing a trivial problem can lead to hours of labor. Lack of audio is an extremely vague problem that can be caused by many factors. But, if the source of the problem is a broken input cable, nothing that you do inside of an amplifier will fix the problem. If you aren’t careful, you can actually make the problem worse.

Every time you make an adjustment to an amplifier circuit, you risk damaging components. Being skilled with a soldering iron certainly minimizes the risk, but we’re all human and there is always room for error. Plus, components don’t like excessive heat, and soldering irons…are hot. You can easily melt wire insulation, burn a component, install something backwards, connect things to the wrong point — and the more frustrated you are, the more likely these mistakes become.

The best way to avoid unnecessary work is to investigate trivial problems first.

Investigating trivial problems is not wishful thinking. Some people avoid addressing trivial possibilities first because it seems too easy: almost like wishful thinking. Certainly it is more likely and a better use of time to consider serious problems, right? Wrong. It would be “nice” if the problem was something simple, like a broken input cable, right? Wrong again! What would actually be nice is if time worked like a vending machine: you put x number of hours in, and you receive y improved amplifier quality in return. The fact that an amp could malfunction indefinitely because of something so stupid as a bad input cable is a vicious truth from the ninth circle of hell.

Nor is troubleshooting trivial problems easy. Pessimism is easy. “That amp is a piece of garbage,” you might say, and you’d be right, regardless of whether the amp was plugged into a switched-off outlet or had a broken input jack or was literally on fire. Anything can be garbage; if you seem to have higher standards than other people, who can argue with that? Plus, you can’t go wrong by declaring an amp to be garbage. If the amp isn’t broken today, it may become broken in the future, in which case your “garbage” designation was in fact prophetic. Best case scenario: you’re correct. Worst case scenario: you’re technically correct.

On the other hand, starting from the assumption that the amp is fine doesn’t quite work the same. It’s risky. It feels like foolish optimism. There is the possibility of being “wrong,” because the amplifier will never deteriorate into a better state and retroactively prove you right. And yet, optimism is the most efficient mindset for troubleshooting.

In the beginning, we were pessimists, and because of our suspicious mindset every troubleshooting effort was drawn-out and unnecessarily frustrating. Of course, we weren't suspicious of everything. We believed in the cables. We believed in the outlets. We believed that we’d never forget to plug a speaker or a power plug back in.

It was only when we let ourselves believe in trivial problems and easy solutions that we started fixing things faster. It became possible to work more methodically. By testing the simple things first, we remembered more accurately what we already did and what we had yet to try. There was less need to backtrack. Our repairs looked cleaner and overall became more complete.

It only takes a trivial amount of time to eliminate trivial problems. At this point, testing for simple fixes only takes us a few minutes: much, much less time than the length of this article would suggest. Swapping out a 1/4” cable takes less time than describing how to do it. This article is long, not because the topic is complicated, but because we have attempted to describe all of the simple problems that you may encounter.

Types of Trivial Problems

Connections to and from the amplifier: power cords, input cables, etc.

The most basic thing that you can do is examine the amplifier and ensure that everything is plugged in. There are many connections that, if disconnected, may cut the signal or otherwise prevent the amp from working correctly. They include:

General amplifier connections:

The power cord, which may be hardwired or connected via an IEC receptacle
The speaker cable, which may be hardwired or connected with an RCA or 1/4” plug
Connections to and from the reverb tank (usually RCA)
Tubes, which should be present, firmly seated in their sockets, and free of cracks or other signs of damage

Connections specific to Wurlitzer keyboards:

The RCA input cable, which originates at the reed bar and plugs into the amplifier
The wiring harness. In early keyboards, this is connected to the amp with a large round socket. Later keyboards use rectangular plastic Molex connectors. The 140b uses two: a small one specifically for the switch, and a larger one that contains the wiring for the potentiometers and headphone jack. The 200 and 200a both have one molex connector that contains power plug connections and the 200a has an additional molex plug connecting the speakers.

Sometimes, connectors may be firmly plugged in, but too deteriorated to make an adequate connection. This is common for RCA connectors in particular. You can confirm whether plugs are making adequate connections by performing a continuity test with a multimeter.

Outlets

Outlets can be noisy, either due to dirty power or because they are on a circuit with other noisy devices (such as a dimmer switch or fluorescent lights). Or, they can be straight-up broken.

Problems with outlets include:

Power is inadequately filtered at the source, introducing noise
The outlet shares a circuit with a noisy device
The outlet is on a switch, and the switch is turned off
The outlet is not working correctly

If you are using the amplifier in an old or unfamiliar building, it is worthwhile to try a different outlet if your amplifier isn’t turning on or is otherwise not performing to your standards. Even if you are in a familiar place, some vintage electronics are more sensitive than others. If you have a noise problem in particular, it is worthwhile to test a few different circuits in your building to try to determine which one is the quietest.

You should avoid plugging vintage gear straight into a wall outlet. It is better to use a power strip on a switch, preferably one with some level of filtering and surge protection. Our favorite power strip, the Furman SS-6B, does not provide surge protection, but it is a great budget offering.

Inputs and outputs

Although inputs and outputs are inside of the amplifier, and certainly a pain to replace, they can be considered sources of trivial problems. Although they are possibly under more mechanical stress than any other part of the amp (thanks to how often people plug in and out of them), they are an underrated source of malfunction. Second, because most amplifiers have multiple inputs and outputs, they are usually easy to test.

Visually inspect all inputs and outputs. Input and output jacks depend on a firm mechanical connection between the jack connectors and the plug. If the metal connectors are bent or broken, it is impossible to make a mechanical connection. This will either interrupt or distort the signal.

Pay close attention to the switching jack. This type has an extra length of metal that is touching the tip connector when nothing is plugged into the jack. When a cable is plugged into the jack, these two metal pieces separate. If these two metal connectors are always separated, or always connected, the jack is not working as designed. If the circuit depends on the jack’s switching function, it will not work correctly.

Test all inputs and outputs. Here, you are testing whether the problem occurs no matter which of the amplifier’s inputs or outputs you are using. This is an important test for two reasons.

First of all, deteriorated jacks are a common source of problems. If the amplifier has multiple inputs and outputs, testing all of them can rule out problems with a specific jack. If the issue happens regardless of what input or output you are plugged into, the fault is probably somewhere in the circuit. However, if the issue only happens when connected to certain jacks, the jack itself or its corresponding circuit might be malfunctioning.

Furthermore, wiring a jack can be tricky, particularly switching jacks and vintage jacks. If you have recently modified the amp’s jacks, or installed new ones, you should double-check your work before exploring other possible sources of your problem.

(If your amp only has one input, or one output, feel free to file it as a “complicated problem” that can be explored later.)

Issues a broken jack might cause:

No sound, possibly because the signal path is interrupted or inappropriately grounded
Distortion, if conductors are making an incomplete mechanical connection
Noise, particularly if the jack is loose

Common problems with jacks:

Rust and corrosion is preventing conductors from making a firm mechanical connection
Switching jacks might be bent apart, essentially taking the switch portion of the jack out of circuit
Cold or broken solder joints, or deteriorated wiring leading to or from the jack
If the amplifier was recently serviced or modified, the jack may have been wired incorrectly

The second reason you should test all inputs and outputs is so that you can narrow down the source of the problem to a specific part of the circuit. For instance, in an amplifier with two channels (as distinguished from an amplifier with two inputs), each channel typically has some wiring that is isolated from the other channel. If the problem is present in one channel and not the other, you now know to focus on the circuit in the problem channel, and not necessarily the shared circuit later in the signal path.

Input cables and speaker cables

Input and speaker cables may be disconnected, missing, or faulty. If possible, try using a different input or speaker cable. Many speaker cables (and even some input cables: in Wurlitzer keyboards, for instance) are hard-wired, so this isn’t possible in every case. But if it is, you should try a different cable in order to eliminate the possibility that it is the cable, and not the amplifier, that is malfunctioning.

It is best to use a cable that you absolutely know is good: for instance, because you have recently used it in another, working amplifier. There is a small but irritating possibility that both the first and second cables that you try are broken. (Maybe you are in someone else’s studio and did not realize that all the cables you are borrowing are from their broken cable stash…true story unfortunately………………)

Broken cables can…

Interrupt the signal, so that no sound comes out of the amplifier
Cause intermittent operation
Introduce noise or distortion

Tubes

You should always try replacing the tubes before performing any invasive tests. Failing tubes can cause problems ranging from subtle distortions to complete loss of signal.

Note that a tube that is cracked cannot work, because the vacuum has been compromised. If a tube has lost its vacuum, the getter (the silver coating inside of the tube) will have turned white. Note that it may take several hours or days for the getter to turn completely (or even noticeably) white.

The downside to replacing tubes is that tubes are expensive. If you don’t have a replacement set on hand, you may be hesitant to invest in one if you aren’t sure whether new tubes will solve your problem. However, it is a good practice to have a spare set of tubes for every amplifier that you own.

Note on tube testers. When you are buying vintage tubes, more reputable sellers will first test them for you using a tube tester. Tube testers will inform you whether the tube works to the machine’s standards. Whether the tube works to your standards is another question. Tubes that passed a tube test can still have subtle problems — a constant crackling sound, for instance — even if they are otherwise functional.

If you want to try vintage tubes in your amp — with the least chance of surprises — you should have first establish an accurate baseline of the amp’s sound quality by using new-production tubes. This is not to imply that all new-production tubes are perfect. However, they are a lot more predictable than vintage tubes, which are usually a black hole of information. How was the vintage tube stored for the past 50 years? Was it ever used? Was it previously tested in an amp and then discarded because the user found a better-sounding tube?

Just because it arrived in its “original box” doesn’t mean its completely new-old-stock and unused. We put used tubes back in their original box (or at least, an original box) all the time, as long as they have a little life left in them. Obviously, we wouldn’t sell them as new, unused tubes - but if someone else acquired our collection 20 or 30 years from now, are they going to know the specifics of our tube-storage system? Probably not. More likely, they will sell them as “new-old-stock vintage tubes ca. 2020” and call it a day.

Disconnected ground wires or loose screws

This section is for Wurlitzer users, since, in most amplifiers, all ground wires are completely enclosed within the chassis. (Checking inside the amp chassis is outside the scope of this article, but, if you are able to do so safely, it is a good idea to make sure there are no disconnected wires there either.) Wurlitzer keyboards are different, because they have several ground wires that are connected to various parts of the amp rail or reed bar. If any of these wires are loose or disconnected, the amp may become noisy.

In a Wurlitzer, ground wires are typically green and end in a connector that is fastened to the amp rail with a screw. If your Wurlitzer has a noise problem, it is worthwhile to tighten all ground screws that you can find. (The number varies depending on the model.) Note that you must do this with the keyboard turned off and unplugged, since in most models the wiring leading to the switch is exposed when the lid is off. Otherwise, if you accidentally touch the switch, you will get shocked.