Well fellas, here we go.....
So I stopped last night with the board loose and ready to flip. But I decided to go after the easy shit first. This amp has two speaker jacks. I only need one since I link my cabs together instead of having a bunch of cables plugged in behind he head. So the secondary speaker jack got yanked out and in it's place goes the master volume pot. The volume in the back is cool because the front still looks stock. And it doesn't require another hole to be drilled. That black piece with the long bent wires hanging off is the primary speaker jack. I left those wires long and just bent them out of the way so I could easily put the other jack back in if I want.
That was all for the easy shit so it was time to get down and dirty. I tilted up the board and gently de-soldered the two resistors that had to go. R25 and R26 now empty and ready to have some wires penetrate their delicate holes. GN5 and OR are where the Green and Orange wires ran to pin 5 of the power tubes. Those got removed as well.
With the resistors out cleanly, in goes what I think is the negative bias voltage wire. Without this wire the power tubes immediately go kaboom. I chose a blue wire because I had it laying around. This also goes up to two lugs of the volume pot. On the board side I looped it under to make contact to both negative voltage spots just like the resistors did. That little titty sticking up through R26 is the end of the blue wire. The resistors on the pot are part of the bias circuit and they're just mocked in there for this pic.
Next was to solder the the swamp resistors to the leads going to the pot and solder the assembly to Pin 5 of power tubes V4 and V5. This part was pretty harrowing because the connections have to be very good and very clean and very sturdy and very insulated and there aint much room down there to fuck around with a boiling hot soldering iron. These pin 5 spots are where the green and orange wires went..
I added the other bias supply wires to the board and this is the underside. Pretty clean! I didn't melt anything or lift any traces.
Now it's time to connect everything at the pot. This was kind of a bitch because everything is pretty cramped and the shielding needed to be grounded. I trimmed everything to just fit nice and tidy and stuck it all together. Good clean solders and nothing is touching.
The finished PPIMV mod.
I used a chicken head knob because it's easy to grab and tell where it is when reaching behind an amp. So now this amp has a master volume!
And the verdict...........SUCCESS!
Bias is perfect. No redplating. No death. No noises. No explosions. No tube meltdowns. Perfectly functional master volume control on an amp that would normally blow your face off. I've taken it from cranked to whisper quiet and it works perfectly. Sounds great, works great.
So now with that done, it's time to get into the cascaded gain mod. Stay tuned.............
I assume there are protocols for discharging them safely before you start any work?
