Oscillation or not enough filtering

[islandamp]

My hi-gain modded Plexi build has an overpowering bass. Clean and powerfull, but I have to keep it on 1. The problem is, the distortion, especially on the bass, sounds like one of those 1980's harmonizers is running at all times. Thats the best description I can come up with. Sounds like the fake 5th or 7th is riding with the signal.

[Jana]

can you hook it up to a signal generator, scope and dummy load and see what is going on?

can you post a schematic so we can see what you are trying to do?

[Luthierwnc]

You might need to de-fart the couplers or bypass caps. Likely test spots are to halve the first coupler, old school 2k7/0.68 first cathode or drop the treble cap.

Like the other poster, I'd be interested in knowing your slope resistor value, coupling values and what combination of feed resistor and bypass values on the first and second stages. Also, what treble cap (value and composition) you are using?

I've built this amp (I can't get the entire URL to turn blue. You'll need to type it in):

http://www.lynx.bc.ca/~jc/JCM2chanPream ... 300dpi.gif

It is so hot it is unstable when dimed. Note the second stage doesn't use a bypass cap. It is just too woofy. FWIW,

Skip

[islandamp]

Its basically a Koch Powertone with a plexi power section. Has seperate gain controls for both p.a. tubes. So close but still not right. It still has the same hum that the amp had when it was a straight Plexi. This harmonizer sound is all thats keeping it from being a great sounding amp, if that makes sense. I have a dpdt switch thats both standby and on so I'm thinking theres possibly some 60 hertz bleed through, or maybe the p.i to p.a wires are too long even though they are shielded cable. If I can find this problem which is probably causing the hum also, I'll have a great amp, however, I'm losing patience with it and may end up gutting it and starting another amp with the chassis and transformers.