Wednesday, May 12, 2010

Black Hole of Tone


Is your amp noisy? Does it cut out? Does it make an odd sound when pushed, or does it just run hot and no one can figure out why? If so, you may have a "Tone Black-Hole".

I first encountered this problem years ago, while working on a Fender Vibroverb. The amp sounded awful unless it was out of the speaker cabinet with the reverb unplugged, which would be a completely impractical way - at best - to play your amp. The problem that I found was in the reverb circuit, and the only way to see it was with my oscilloscope. There are no better ears than test equipment. If it looks like garbage on the oscilloscope, then it sounds like garbage, as well.

I've been seeing a good number of brand new mas-production amps with high frequency instability or - what we call - the Tone Black-Hole. The above picture is the output from just such an amp: A brand new Fender Blues Jr.

The worst offender I've seen so far has been the Peavey Valve King. Hartley, I'm calling you out! Your engineers have designed the only amplifier that I have ever seen that is more stable with the Negative Feedback disconnected. Part of the power amp circuit meant to improve its operation is actually harming its ability to produce a linear signal.

Luckily this is a simple problem to repair. A few small parts or a change in wire lead dress and it's gone. In most cases, it's very simple to test for; I may not even have to disassemble the amp.