First Amp Mod - JCM800

ivan H said:
I think I would start by maybe doing as Sysco suggested in documenting the changes that have been done to it, but then I'd probably put it back to a stock 800. It appears to have the original transformers, which is a good thing. I'd best not comment on added tubes positioning. Correct Piher type resistors are easily obtained & not too expensive. Year of manufacture would dictate what type caps "should" be in there, Philips KT-347 series (radial leaded) mustards, Chicklets etc etc, if you wanted to go that route (I'm not a fan of orange drops in Marshalls, even the 715P series that they appear to be). If you are going to add the (hot mod) Lynch Mod, putting the circuit back to stock would be advisable, at least to start with.
A reasonable amount of extra gain can be had from the 2203/2204 type pre-amp circuit without resorting to adding tubes or bypassing (or lifting the ground on) the tone stack. The trick to doing this well is to go for small gains spread through the various stages of the pre-amp circuitry, rather than going for a large gain in one area.
It'll be interesting to see the outcome of your endeavour. Cheers
Click to expand...
I think we would find that the circuit board underneath is pretty torn up...
It would take a lot of effort to undo that mess.

But I also think that the stock amp would sound / work better.

Bypassing the tone stack is like letting the genie out of the bottle.
I come to the conclusion that the tone controls are what wrecks the potential of many amps...

Besides the obvious:
this style of Marshall / Fender tone stack does very little and has a very small adjustment range. Not very useful even when it's working correctly.

I think the next amp I build might have one knob.
Master volume / loudness...
and I'll just leave out all those silly useless controls.

I see the need for expanding the usefulness of the effects controls.
 
ivan H said:
I think I would start by maybe doing as Sysco suggested in documenting the changes that have been done to it, but then I'd probably put it back to a stock 800. It appears to have the original transformers, which is a good thing. I'd best not comment on added tubes positioning. Correct Piher type resistors are easily obtained & not too expensive. Year of manufacture would dictate what type caps "should" be in there, Philips KT-347 series (radial leaded) mustards, Chicklets etc etc, if you wanted to go that route (I'm not a fan of orange drops in Marshalls, even the 715P series that they appear to be). If you are going to add the (hot mod) Lynch Mod, putting the circuit back to stock would be advisable, at least to start with.
A reasonable amount of extra gain can be had from the 2203/2204 type pre-amp circuit without resorting to adding tubes or bypassing (or lifting the ground on) the tone stack. The trick to doing this well is to go for small gains spread through the various stages of the pre-amp circuitry, rather than going for a large gain in one area.
It'll be interesting to see the outcome of your endeavour. Cheers
Click to expand...
Oh yeah
always document any changes.
 
Found this interesting image and decided to post it...

FB_IMG_1638978936884.jpg

Here's the text of the author's post:

This is the layout of the multiple push-pull pots in my “sleeper” Plexi/2204/hot rodded JCM800 head! I hope it might be useful for someone who doesn’t want to drill holes in their Plexi, like me...

PS: when all pots are pushed (Plexi operation), there is no cathode cap at V2A, which I prefer for clean “Bassman” tones.

PPS: combining the 2200p first gain stage peaker and the .68u V2A cathode cap on the same switch is useful because in Plexi operation (bright channel inputs) it turns into a simple cathode cap switch, as that peaker cap is out of the circuit in Plexi mode.
By the way, I highly recommend giving a try to the 2200p peaker in JCM800/hot rodded JCM800 circuits, it adds very musical mids.
 
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