I still wanted gain JCM800 style and I wanted something resembling a second “clean” channel. To try a more authentic Marshall topology I decided to add another preamp tube and change to a LTP phase inverter. This solves the NFB/Conjuntive filter issues and produces a sound more like the real thing.
The overall circuit is a little simplified in a few places compared to a real JCM800 but this is a Valve Junior chassis after all…This circuit fits in there pretty well actually, I even have a few eyelets on the board not being used. I think the guts might look tidier as well.
The basic deal is three gain stages and a cathode follower/tone stack before the phase inverter. After the PI there are a pair of 6V6 output tubes in fixed bias. I added a switch that bypasses the second gainstage and changes the biasing on the first triode. This is my “clean” channel so it’s a little more JTM45 than JCM800. Also converted the amp to use 6N2P preamp tubes.
I’m using the same transformers I had in it before. The PT is a 275.2 from MPS. I’m using the 115v tap to bring the voltages on the 6V6s up a little and a couple diodes on the heaters to bring the heater voltage back down. I’m also using the bias tap and converted the amp to fixed bias. I’m using a very simple bias supply that is similar to old Fenders. The OT is still the big Vibrolux one running at 8k/8 ohm. It’s pretty loud for a little amp.
It sounds really good and I haven’t had any problems with hiss or other noises. I think the Marshall tone is most definitely there. Someday I’ll try to get some clips done.
Here is the schematic: