PPIMV Silverface Twin

[shiner555]

Hey Guys,

Quick question. I recently tried installing a PPIMV in a sillverface twin reverb, the one with that push/pull MV(I'm in the middle of putting it back to blackface spec's) and I tried installing a PPIMV according to the "Rich Mod" as shown here
http://marshallampforum.com/forum/vi...light=rich+mod

However, it works fine at low volumes, but when I crank it the amp just screams/squeals...almost like I'm making it oscillate.

Does anyone have any ideas I could try??

Thanks!

[pdf64]

Most probably you've swapped the wires over, so that the phase splitter sections are driving the opposite set of output tubes to what they were before. That will turn the negative feedback (as was) into positive feedback, as the MV is turned up. Either trace that through, or just swap over the wires going to the MV pot wipers to see if it cures it. Sorry this sounds a little confusing, but it's tricky to word it simply! Peter.

[shiner555]

Hey PD,

Thanks for the reply, I was thinking that myself...almost like I flipped the phase. However when I traced the wires they seemed correct. I'll try flipping them and see what happens.

Also, should I disconnect the negative feedback?? There was no mention of that in the RICH MOD, actually it still showed it connected, that was on a marshall however.

Thanks!

[boyt_bpc]

I've got a '74 Twin with the Push/Pull Master Volume. Did you notice a big difference with the PPIMV? Did you remove the mid-boost circut?