Hammerhead

Hi all, I'm new here to the forum and it looks like it’s the place to be if you have a Marshall question.

I have a 1983 JCM 800 4210 combo with a boost channel problem. The problem? The sound sucks! The clean channel sounds excellent; I keep the channel volume on 10, crank the master between 7-9, put an OD pedal in front, and let her rip. Fantastic sound. However, when I switch to the boost channel the sound just sucks, too thin, fuzzy, not enough bass, you name it. I have head about the problems with the boost channel in this model 800, clipping diodes and all. But I guess my question is, does anyone know of a good Mod for the boost channel?

Any info anyone could give me would be great.

timstoel

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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Alexmc1988

Hey I have a 1982 JCM 800 4210 and I have the same problem, the boost channel sucks! I have been looking at some mods and at the schematics but the schema's dont seem to line up with my amp at all! Do you think you could help me possibly?