I have an early 70s pro reverb that came to me not working, and I willingly accepted it for the asking price (free).
I dumped the trem and reverb, and converted channel 2 into a tweed bassman, since there were plenty of tube spaces for it.
Channel 1 is standard AB-763.
I would like to add a master volume, but only on one channel. The two channels are summed together with 220K resistors, like the original blackface circuit.
The phase inverter and output stages are mostly stock, save for changing the feedback values to 56K and a 5k pot to facilitate a presence control.
The amp sounds pretty good, until you add a master volume. The master volume I chose was like a standard Marshall, following the treble control. The wiper of the treble control is fed to the top of the Master volume, 1M, and the wiper from the master volume feeds to the phase inverter. If I go directly to the phase inverter, and don't use a mixer resistor, it works great. If I feed the MV control to the 220K mixer resistor, it sounds absolutely terrible. The bass is farty, thin, and it just basically does not work.
The problem with going directly into the phase inverter with no mixer resistor, of course, is that the MV control interacts significantly with the other channel, shunting it to ground.
Is there any way to do this? I thought about feeding the two channels separately into each side of the PI like a Vox, but then I'm going to lose the feedback loop, and this amp is a bit too obnoxious for that. It needs some NFB.
+ on that. Depending on how you have it wired, the PI needs to be isolated by a coupling cap to bias correctly. If you have an additional "load" at the PI input without a cap to block DC the bias of your PI is going to be thrown off. This is the first thing that comes to mind.
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There should be no DC, as the tone stack itself is the last thing before the MV control. The two mixer resistors feed the PI through a .001 cap. So there is no DC at the input of the PI.
The problem has been resolved though, and the amp sounds quite good. Fenders make better Marshalls than Marshalls, you know...
Somehow, I got a really crappy spool of wire. I've since junked it, because on two occasions now, I've had broken wires at solder joints. I didn't know copper could be so brittle. Anyway, the problem was a poor connection at the regular volume (gain) control after going over the amp several times. Once I get the layout ironed out, I'm going to redo it (again!) with the new wire.
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