What has been done to this JCM 800?

[Ronniejackjones]

Hi Folks: Am looking for some assistance in identify changes made to a JCM 800 50W. To my ears, this amp sounds esepcially good, and I am hoping to have my tech copy it as closely as possible. Some things I and others have been able to make out: post phase inverter master volume has been added; small electrolytic bypass cap soldered across the 820R cathode resistor of v2a; a possible cascade of the two triodes of V1. Some things I still don't understand are the jumpering all over, and the caps on C2. Any help on this will be enormously appreciated. More, if anybody is up for drawing me up a schematic, I will gladly compensate via Paypal.  http://i66.tinypic.com/2q9zy81.png

[92Volts]

It would help to see more of the amp.

The area shown doesn't include a post-phase-inverter Master Volume. Those require a dual-gang pot to simultaneously adjust the volume of both phases. Based on what I believe the JCM800 panel layout is, I believe we see the original master volume in the picture you posted and it's unmodified... but it would help to clarify as much as you can about which pots in the picture serve which function.

[DummyLoad]

it appears to have been converted to a 2204 circuit preamp and V2b 820R was bypassed for more gain. C2 was lifted an and a conjunctive filter added in series to the signal path (probably 500pF/470k) and whoever did the work used the channel 1 input jacks as a tie points to do that: notice the absence of R6 & R7 which were the input grid stoppers for that channel. also note the 10K cathode resistor for R1. other things happened to R26,R27 along with the blue wire that ties to R27 and goes to parts unknown, do not appear to be stock.

respectfully, whoever did that, should've just bought a 2204 to butcher instead of butchering a superlead.




--pete