PPIMV in my supersonic

[GVC66]

So, I installed a ppimv in my supersonic because I love it in my 1987x. But, it hums like crazy when the master gets past 1??? Any ideas?

[martin manning]

Any ideas?

Looks like you have some errors in your wiring. I suggest starting over and using the layout below.

[GVC66]

Wow. I guess so. Thanks Martin! It sounds great but hummmmmmmmmms like crazy. I will try this and report back. Thanks again!

PS... I did move the grid stoppers off board to pin 5 connecting the second gang of the pot in series. but it persisted. I left the 1.5k on the board unconnected. I am going to start over with your directions and see how it goes.

[martin manning]

I'm interested in hearing how it works out. Note the shields in the two-conductor shielded cables are carrying bias voltage, so keep them insulated from contact with ground or other parts.

[GVC66]

Please excuse my stupid questions, Looking into this a bit more the only differences between the way I have the wiring now (aside from lead dress being off board) , that I can see, is that the pot CW lugs are going directly to the 1k5 grid stoppers, I left the original 1k5 on the board, and I did not connect the shields to the board but rather connected a 22 gauge stranded per the attached diagram... Sorry for the follow up.

[martin manning]

It's hard for me to see exactly what you have in the photo, but the MV pot CW lugs should not go to the 1K5 stoppers, they go only to the output side of the coupling caps.

[GVC66]

Ok... Followed your diagram and spent, probably too much, time on my heat shrink and lead dress. The hum is still present. When the master is cranked it is gone so I think it is still wiring. One thing I noticed was that your diagram did not connect lug1 of pot1 to lug1 of pot2 like the original diagram. Could this be the source of the issue? Ground loop introduced when the master goes ccw?...

I do like your approach much better since all three leads go to one pot which makes for neater wiring...

[martin manning]

Looks nice and tidy. Since both shields are connected to the bias supply voltage, there is no need to connect the CCW lugs together. Indeed that might cause a ground loop hum since there would be two paths to AC ground. You could short them together to find out what happens. Does the hum diminish gradually as the MV is turned up from 0 to max?

[GVC66]

Right now they are connected so I might clip the connection. It is not gradual it just disappears around 8.5 and then their is normal white noise from a 60 watter being dimed.

[martin manning]

Definitely try clipping that connection- you have a big loop now.

[GVC66]

Bingo!, can't thank you enough! Love this mod on this amp.

[martin manning]

Cool! You're welcome, it was an interesting exercise devising the mod, and you've proved it out for others.

[GVC66]

I don't mind being the canary in the coal mine if it means a chance at better tone and saving my hearing. Now on to my two blackstars...

This really is a great mod and I think we proved that when the pot is fully clockwise it is out of the circuit because the hum cut out when the loop was still on the lugs.