I'm about to buy a used 1987xl 50w plexi that has a master volume mod on it. I can't try it out because it's pretty far away from me. What exactly does the master mod allow me to do? Does it just mean i can crank the loudness knobs and play at a lower volume or will I still lose gain when I turn down the master volume? Sorry if this seems like a stupid question but I don't really have any experience with single channel amps.
Hello Babu,
In normal circumstances a master volume mod would give you a volume control for the pre amp and a master volume control.
Kind Regards
Marshall Support
I did this to a Black Flag plexi, and it's very useful.
The MV control goes at the output of the Phase splitter (both sides of the last ECC83) . Normally the phase splitter's anodes drive the grids of the output tubes, via DC-blocking capacitors (100nF 400V?). Now the anodes go to a dual-gang pot, again via (the same) DC-blocking caps. The wipers of the dual-gang pot go to the grids via two new DC blocking caps (same values), because the output-tube grids are biased at a reasonable negative voltage, and the pot is at ground. I think I used a 100K log pot, though this would slightly reduce the gain of the phase spiltter because it's in parallel (for signals) with the plate (anode) resistance of the ECC83 (and with the grid bias resistors of the EL34's). As is common practice, I replaced one of the output jacks with the MV control. I added some turret posts to the holey circuit board too, and it all fitted where the original DC-blocking caps were (I put the new turrets in the middle of the board.)
Now don't do this mod if youare not trained and experienced with about high-voltage electronics, as there are fatal high voltages around even when the amp is off.
Anyhow, soundwise, a lot of the distortion comes from the phase spiltter which is fed from the tone-stack. When overdriven, the phase splitter does all sorts of crazy things, I've read that it's akin to a Schmidt Trigger (a circuit with hysteresis, so it locks positive or negative until the input has swung widely the other way, cutting out some of the faster, variations but leaving the fundamentals). A scope shows the interesting waveforms. Thus the MV control allows the Schmidt trigger to do its thing, but then attenuates the drive to the output tubes (valves), so you can do it quietly. Anyhow, you can get nice tones like the Zappa/Beefheart live album, without annoying people too much (except with the statistical density of the music!).
Arthur
