Converting a Marshall to earlier specs.


I've got a JCM800 1987, from 1987.

The power filtering is significantly different on this amp I am told post-1986. Since this amp actually didn't loose any filtercaps like the 2203 and 1959, I was wondering if it was possible to convert this amp back to earlier specs?
If so how would it be done?

Can anyone write me a step by step or point me in the right direction?


Silver Supporting Member
I hate to answer questions with questions but which features do you want revert to? IIRC, those Marshalls use double 50 caps at the nodes. That is a lot of filtration. If you were going back to a Plexi type circuit, you should have plenty of caps.

The specs only get you half-way there. For a vintage sound you probably need to use some old school components too. You can get mustard type caps (or Dijons or Mallory 150's) and carbon resistors lots of places. These are especially important on the supply resistors and coupling caps.

One last thing, I don't recall there being bleeder resistors on the power rail. Be sure to drain the charge out of those caps without using body parts. sh


Silver Supporting Member
AdmiralB, I think the bolded part is what he's already did the italicised part.;)

Can anyone write me a step by step or point me in the right direction?

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