The photo above shows the bridged guts of a Dual Reverb JCM900 amplifier that's been modified for pure all-tube distortion and classic stock Marshall tone- all sillicon circuits bypassed ... this circuit rocks in the 70's sense, nice lively compression and overdrive characteristics- tunable from light to extremely and heavily saturated distortion ... aside from noise-free stage-grade relay switching this JCM800-derivative is set for maximizing the in-amp hardware already present- that is, this circuit takes full advantage of the fine power supply and output stage structure in these amps (many of the ideas here are influenced by Rick Onslow's work in Montreal) ... the schematic shown below is for a single footswitch two-master version while the photo above shows an alternative version in which the "B" channel can have two selectable MV settings (Masters B2, B1, and A which accompany VolB and VolA) ... the circuit makes the use of Ken Fischer Master-Volume circuits in the output stage schematic and micro-relays or a central multi-relay depending on the amp model and room in the chassis ... though more easily installed in a JCM800 or Bashed Metalface Marshall amp this circuit sounds especially gnarly in a JCM900 for some kind reason ... :)~

In the JCM900 case I prefer to disable the Effects Loop and the Reverb circuit if present- all op-amps and diode circuitry have been bypassed and isolated from the signal path- the original Pentode wired Output Stage is unaltered aside from the KenFischer MV network which leaves the amp stock when dialed to 10 ... the same 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 circuit all rewiring of this circuit is done on the tube socket itself ... the goal is to provide the amp with two basic signal path variations: one which is a single-channel version of the close-to-classic/stock Marshall signal-path and the other a very high-gain boogied variation on the first channel that produces decent distortion on its own ... of course, with a good overdrive pedal the tonal potential increases even further ... btw, this mod will fit in most guitar tube amps, Traynor CustomSpecial amps are good candidates ...