Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cage771 on February 10, 2019, 09:54:18 pm

Title: Adding a PPIMV to a Silverface Bassman 100
Post by: cage771 on February 10, 2019, 09:54:18 pm

All -

Does anyone have a schematic or layout on adding a Ken Fisher PPIMV II LarMar control to a Silverface Bassman 100? I've installed these on all sorts of old Marshalls and Fender Tweeds, but after sorting out this basket case, my mind is shot. Looking at the schematic, I have some thoughts. But when I look at the amp itself and how it's wired stock, I can't connect the dots. It's as if this repair has gotten the best of me....grrrr.

Any help, links, diagrams, and etc. would be appreciated. The amp is pretty much stock working off of the schematic. 

Thanks!!!

Brandon

Title: Re: Adding a PPIMV to a Silverface Bassman 100
Post by: 2deaf on February 10, 2019, 11:21:16 pm

Not sure which one is LarMar, but here is a PPIMV.  It doesn't quite turn clear off, but isn't that how life is?  It can be made to turn clear off with more parts.

 

Title: Re: Adding a PPIMV to a Silverface Bassman 100
Post by: cage771 on February 11, 2019, 02:42:26 pm

Thanks 2deaf for the schematic! It what I was on the hunt for.

The Lar-Mar addition to the KFII PPIMV is really designed for older Marshalls and Blackface Fenders. Insert in the same point, install a 250K pot, remove the 2 220K R, and parallel 2.2M R with the pot. At wide open, the pot is pretty much removed from the circuit.

I have a 250K dual pot. If I were to move the 68K Rs from their current location and parallel them across the pot, I should wind up with about 53.5K at wide open. That will be too low. But in theory, I can make up for the change due to adding the control of the 50K pot in place of the 15K resistor. Or am I losing my mind......Im at work troubleshooting something completely different at the moment.

Brandon

Title: Re: Adding a PPIMV to a Silverface Bassman 100
Post by: 2deaf on February 11, 2019, 03:48:23 pm

My take on the balance pot is that it was just something used by the Fender builders to get their amps to idle with lower hum even with mismatched output tubes.  It was never meant to be adjusted by the end user.  I don't think the balance pot is necessary with matched output tubes, so I suggest you use that pot for bias level instead.

250K must be fine with 6L6's because zillions of people do that.  Well, maybe half a zillion.

Attached is a modified PPIMV circuit.

Title: Re: Adding a PPIMV to a Silverface Bassman 100
Post by: cage771 on February 16, 2019, 03:45:48 pm

2deaf -

Thanks for the schematics but I ended up not implementing the PPIMV. It came down to a matter of tonal taste and how the owner uses his amp. I demoed it for him on a 50 watt Marshall 1986 amp that I had built. While he like it in that amp, he didn't see the benefit of it in the Bassman.

On the plus side, that Bassman is roaring now. He picked it up last night and played for a good hour. Thanks for all of your help on it!

Brandon

Title: Re: Adding a PPIMV to a Silverface Bassman 100
Post by: TWANG on April 23, 2020, 12:54:13 pm

I had the same problem.. what I saw online didn't seem to match what I had in front of me.
these schematics did the trick.
Does the bias circuit change have to be done?
Guess I'd better go read some more.

TWANG