Jcm 800 2203 undoing previous mods.

[BmfToolco]

I am curious if you can tell something about the mods on my jcm 800. It’s a 1986 2203. It has an extra preamp tube and it seems to be missing a couple of the stock parts on the board. I will attach some photos. Is it more work than it’s worth to put this amp back to stock. I will be in to it to change the caps so I’m curious what else could be involved. Thanks in advance.

 

[Chris-in-LA]

Definitely missing caps in the preamp but the missing F1 and F2 are not missing. Here’s a pic of my mostly stock 1986 2203. There is an extra 10k dropping resistor on mine that was added after I started burning through 12ax7’s.

 

[BmfToolco]

Do you happen to know what the specs are of those two caps? And possibly what they do?
I didn’t mind the way the amp sounds. I just went in to replace caps and saw some stuff on the board that had been messed with and started thinking about putting it back to stock.

 

[Chris-in-LA]

Schematics show:
C4 - 470p
C5 - 1nf
C8 - 470p

C4 is tied to R5 and is part of the cascaded circuit
C5 is the bright cap
C8 is usually tied to the 2 mixing resistors.

It is possible that the small cap (C4) is the 1nf and the larger cap next to it (C5) is actually the 470p. I'm just going by size as C8 is 470p.

 

[BmfToolco]

That actually helps so much. I’m just starting to see how the extra gain tube is working. I’ve been running it on different tubes and bias settings. I’m not sure I want to change it at this point. She is a bad ass. Haha.

Thanks so much for showing me the board. It helped me make some sense of the schematics.