AF’s Weblog

July 25, 2012

Diagnosing and Fixing a Tube Amp

Filed under: Amps — Tags: , , , , , — audiofanzine @ 10:23 am

To read the full detailed article see:  Diagnosing and Fixing a Tube Amp

Guitar amps are, no matter how you cut it, black boxes. You may think you have control over them, but when it gets down to it, you can’t really see inside the black box, and even if you could, there aren’t any moving parts. Electricity is largely non-mechanical. Or in other words, magic. And when the magic stops, most people think all they can do is resort to prayer. Or an amp technician.

Fig. 1. My trusty Fender Vibro-King was on the fritz, and the suspects were the output tubes and the speakers.

When your amp isn’t performing up to snuff, there’s still a lot you can do without having an EE degree, or even knowing how to operate test equipment. Here are some holistic approaches—and solutions—you can try yourself, as I did when my tube amp went on the fritz. Warning: Some of the following procedures involve messing around with the components of the amp, so be careful. Electricity can kill you. Proceed at your own risk.

Creeping Death

Failure in a tube amp is often creeping and insidious, more like the wearing of the tread in your tires than a light bulb blowing. Because the changes generally occur over time, you can become inured to little degradations in performance. Then one day, maybe after you’ve been away and come back to it, you realize something’s not right. Such was the case with me and my favorite tube amp, my Fender Vibro-King (see Fig. 1). The following procedures, though, will work on many tube amps. Just swap out the specifics or make the necessary adjustment for your model accordingly.

Testing….is this thing killing?

The first test I made was just to see how loud my amp got, and if it was ear-splitting—as nature intended—at its highest volume. The Vibro-King has no master volume, so this is fairly easy to determine: wind the lone volume knob up to 10, stand the hell back, and play your guitar with the controls full out. Disappointingly, the amp, in its maxed-out state, did not rattle the windows, disturb the neighbors, and risk injury to my eardrums. I knew the amp wasn’t firing on all cylinders even without the benefit of test equipment or knowing which link in the chain was falling down on the job (if you’ll forgive the mixed metaphor). Next step: Just how is it not delivering full power?

Let’s take a closer look…

Conclusion

It’s a fine line to determine what you should monkey with and what you should leave alone when something goes wrong in a tube amp. Amps are much more difficult than guitars for two reasons: 1) They’re all electrical and not mechanical and electrical; and 2) you’re dealing with high voltage, which is dangerous to you and your amp. But that doesn’t mean that you should be afraid of your amp, just that you need to take the necessary precautions. If your amp doesn’t work and you find out it’s because of a user-replaceable blown fuse, you’re going to feel pretty foolish if you have to learn this from the tech at the shop. That’s perhaps the simplest example, but even being able to swap out tubes and diagnose problems can help you understand better your amp and even lead you to solutions—even if they’re incomplete and temporary. But that’s sometimes all you need to get you through that last gig before you can pay a visit to the shop.

To read the full detailed article see:  Diagnosing and Fixing a Tube Amp

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