Unwanted Distortion

[RightLurker]

I'm a newbie. I have serviced three Fenders - a '62 Bassman, a '67 Princeton Reverb and a '67 Super Reverb. By serviced I mean cleaning jacks and pots, changing all the electrolytic caps, changing resistors where necessary, replacing the tubes, setting the bias, etc. I've noticed that in all three amps, and particularly the PR and SR, that they sound very good until you start to push them - turning the volume up a little past 4 on the SR, for example. At that point I start to hear a fizzy or rattling type distortion, particularly on lower notes. It almost sounds like faint grille cloth rattle, but it isn't. I'm going to make sure all the components in the PI circuit in the SR are as close to spec as possible (waiting for a matched and balanced 12AT7 to arrive) to see if that helps. Anyone familiar with the harsh/fizzing/rattling distortion I'm trying to describe, and how to correct it? Thanks in advance.

[xtian]

It might be best to rule out mechanical noise first. Plug the amp into an external speaker cab and see if you can still hear the fizzy buzzing.

[Cliff Schecht]

I think what you are suffering from is a side-effect from the fact that these amps weren't designed to be pushed into distortion. What happens is as the signal swings too large, the grids actually take some of that excess signal swing and "average" it into a DC voltage that shifts the bias of the tubes way cold momentarily. This causes the push-pull pair to have a nasty crossover distortion that is exacerbated by the speaker/negative feedback impedance not being constant with frequency. So as those low notes die off, the signal pushing the power stage clips it less and less and so the bias again is shifting. That's why the buzzing/fizzing you are hearing sometimes sounds like it's "moving" with your signal, because it is!

The fix is about $0.10. This PDF does a decent job explaining the circuit, but the essential thing is to have Zener's with a Zener voltage that is higher than your bias by a few volts. I had this same problem with a 5C3 build and this little trick cleaned it right up.

[RightLurker]

Thanks for the tips. I tried different speaker cabinets, and the distortion is present regardless of which cabinet is used. Cliff - I didn't see a pdf attached to your reply, but thanks.

[Cliff Schecht]

http://www.18watt.com/storage/18-watter ... fo_311.pdf

[RightLurker]

Thanks, Cliff. Interesting reading, and it sounds like it could be the source of the problem and the solution. Thanks again.