Filter Caps: What you need to know
If you own tube amps, you’ve probably heard the term “filter caps,” at some point, and it was probably followed by dire warnings about lethal voltage and catastrophic amp failure.
So, let’s talk about what filter caps— or electrolytic capacitors— are and what you need to know about them. First, the purpose of filter caps in your amp. When pseudo-DC power flows into the amp from the rectifier, it needs to be “smoothed out” or “filtered” before it hits the tubes, or what you hear will mostly be noise. That’s what your filter caps do. They add capacitance to the pseudo-DC in order to make it more orderly when it gets to the tubes. They also act as a battery, which is why they are typically indicated on schematics as B+1, B+2, etc.
When amps are in use, the amount of power they consume varies depending on how loud and hard it’s being played. As a result, the amp can require more power than the power supply can offer at a given moment. The filter caps store power for these occasions when the amp needs a reserve of power to maintain performance.
This is also why filter caps can be dangerous. Since they store voltage, they are charged with electricity even after the amp is turned off. They can store power for weeks or months depending on the amp. Typically, in a properly wired circuit, the filter caps should mostly discharge themselves when the amp is shut down. If you’ve ever turned off your amp’s power and kept playing, that fizzle sound is the caps discharging their remaining power.
When it comes to amp repair, most people are warned to stay away because of the potentially lethal voltage these caps may contain. And because you can’t know if the caps discharged correctly when the amp was last shut down, there is no way of knowing how much juice is still loose in there.
This is why it is strenuously recommended that you never open up your amp and poke around. Again, we are talking about lethal levels of DC voltage. In some cases, more than 700 volts (it will kill you).
Amp techs know what to look our for when we open an amp, and we always have a discharge tool ready to go to drain the caps before we touch anything.
If you own a tube amp long enough, you will need to change the filter caps, or more likely have someone do it (safely) for you. The best analogy I’ve heard for filter caps is the tires on your car. They last a long time, but we all know they won’t last forever. And if you ignore this fact, something bad will happen. Same idea with filter caps. Even high-quality filter caps are bound by the laws of science, and thus they all have a reliable lifespan of 15-20 years. If you are buying a new amp, this isn’t something you need to think about right now, but if you have an amp that’s a little older, it’s important to verify how long those caps have been in there.
The first sign of trouble with caps is noise. Hum, buzz, things like this. If left untreated, however, the caps will degrade to the point where the can blow leaving other parts of the amp vulnerable to permanent failure. That would be a cool band name.
Replacing filter caps is just part of owning an amp, like replacing tires is part of owning a car. Any qualified amp tech should be able to do this, and for most amps, it shouldn’t take more than an hour of their time.
So, now you know the deal with filter caps. As always, if you have any questions, feel free to send us an email.