This photo shows the bridged guts of a Dual Reverb JCM900 amplifier that's been modified with my 2-channel circuit (see Marshall amp page) ... In point-to-point fashion I've bridged sections of the hybrid amp in order to introduce the all-tube Mod circuits shown on this Marshall amp page ... to elliminate noise and instability problems the same "system" power and ground system is retained to avoid introducing ground loops and since only one triode tube section needs rewiring in "either" version of the JCM900 circuits all main rewiring of this circuit is done on only one of tube sockets ... the one version is the non-reverb 2500 series and the other being the reverb 4100 series ... For the 2550 model the second 12ax7 is rewired on its socket and the Effect Loop is disabled - For the 4100 the first 12ax7 is rewired on its socket and ALL the sillicon is easily bypassed on the fly (notice how the second and third gain stage just happens to be lying nearly stock amidst all them op-amps, and the tone stack needs no modding in either amp !!

*** 2500 Series JCM900 Schematic

*** 4100C Series Dual-Channel Reverb JCM900 Schematic

2-Channel Relay Switching Circuit

the switching circuit that makes this all work is based on a road tested supply circuit that is robust, quiet, minimal part and therefore maximized in reliability (I once made the mistake of trying to feed these coils off the zenerized 15+ volt source in a JCM900 and found that it didn't hold) ... I use a 15volt regulator with 12v relay coils ... this leaves room for an LED and some resistance to trim current levels so they match nominal closed position coil current levels, or yu could vary resistance until voltage across coil was exactly 12v in closed position including LED ...

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