He brought it to me initially because someone else had put in a MV that was improperly done and he wanted it fixed. Basically what had been done was the dude tried to do the Dual 250K MV post PI type of MV but had done it all wrong with a single gang 250K pot and it was causing him all sorts of grief. He had done what I had seen 100s of times and put the MV pot in place of one of the channel I input jacks. My buddy had a new set of Mullards for it, so I removed the faulty MV wiring and pot, took it back to stock, reset the bias for the new Mullards and tested it out all with him there watching. No problems at all to this point.
My friend wanted a working MV in the amp and had a 1 Meg single gang pot so I proceeded to do what I had done many times before and put in a 2203 style MV pot after the Treble control and before the .022 mfd cap just as in a 2203. Treble control center wiper to the pot input lug, center MV pot lug to the cap connection, other connection of the MV pot to the shield, using shielded wire with the shield connected to the preamp ground only at the treble pot side. I have done it many many times before without issue. I fire up the amp, recheck the bias, start bringing it up on a test load watching the scope and at about 2/3 the way up on the bright channel with the MV on full I get parasitic oscillations from hell on the scope. Deep channel works just peachy all the way to clipping putting out about 120 watts. I start checking wiring, solder connections, lead dress, all the usual things and nothing, still oscillations from the bright channel.
At this point, as a part of what I was going to do anyway to the amp, I started doing a little reworking. I put shielded wire on the inputs and moved the 68k resistors to the tube socket, reworked the preamp, presence control, and power amp grounding per the "Larry" style "star" grounding technique, updated the grid resistors' wiring to using 5.6k resistors on each OP tube, triple checked all of my connections and made another "close eyed" once over the entire amp again. I STILL wound up with the same oscillation problems yet again. Now, I pull the MV pot wiring completely loose and direct wire the connection again just like stock from the treble pot to the cap and "woila", no oscillation whatsoever.
I am really stumped with this one. I have done tons of MV mods like this over the years and not once have I run into this issue. Anyone have an idea?
~Avguy49
