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Re: JCM 800 4210 Boost channel
I would check the cathode resistors. They often get way out of spec after heating and cooling so many times. There's a 2.7k and a 1.5k. Also investigate any other resistors mounted close to the two tubes in the boost circuit.
As for the resistors, I have been experimenting a lot with different resistors, and adding capacitors in parallel with the diodes to round out the harshness the diodes give you. IMHO the diode clipping circuit is a bastardization of a great amp circuit. A small cap in parallel with those diodes would probably help a lot. You could even add a pot in series with it to control how much it rounds out the sound. I also like other diodes, especially glass diodes and germanium diodes.
If you need a schematic I have one.
Tim
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