Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kerobit on May 11, 2022, 09:52:28 pm

Title: Larmar PPIMV on amp with 4 power tubes w/ individual bias
Post by: kerobit on May 11, 2022, 09:52:28 pm

I've added a few larmar volume mods to some of my amps but I'm a bit stumped on how to approach this one. I know the 390k resistors (R23, R24, R18, R20) would need to come out to be replaced with the dual gang pot, but since there is four of them I'm not sure how I'd approach this without removing the adjustable bias for each tube?

Any suggestions?

Title: Re: Larmar PPIMV on amp with 4 power tubes w/ individual bias
Post by: d95err on May 12, 2022, 02:53:19 am

I’d recommend separating the PPIMV from the bias feed, by an additional coupling cap on each side. Keep the original bias feed resistors and reference the now isolated PPIMV to ground.

This is the style of PPIMV recommended by, for example, Kevin O’Connor.

Title: Re: Larmar PPIMV on amp with 4 power tubes w/ individual bias
Post by: sluckey on May 12, 2022, 05:46:34 am

... a pic...

Title: Re: Larmar PPIMV on amp with 4 power tubes w/ individual bias
Post by: kerobit on May 12, 2022, 01:21:26 pm

Ah cool thanks both of you! I sorta had the right idea of where to put it but didnt realize I could add the coupling caps in front and tie it in like that, should be an easy install! Thanks again:)