Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on December 06, 2025, 09:14:08 am

Title: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: kagliostro on December 06, 2025, 09:14:08 am

Previously I was thinking to use a pair of SF using two mosfet, one for each brench of the PI but this thing seems to be way simpler

the difficult thing was to find a 4 ganged pot with the correct resistance value, but it is on the market

the idea is an L-PAD/PPIMV

(https://i.imgur.com/wSt1ldM.jpeg)

Franco

Title: Re: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: acheld on December 06, 2025, 10:22:18 am

I like the idea but am skeptical that the pots would be so precise.   But maybe they are.  I guess linear pots would be close enough.

Interesting!  Let us know how that works . . .

Title: Re: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: kagliostro on December 06, 2025, 11:01:30 am

For that use the pot must be Linear, not Logaritmic

Franco

Title: Re: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: kagliostro on December 10, 2025, 01:14:35 am

Not an optional douable choice, the safe 2M2 resistor introduce too much non linearity and alter the load seen by the PI

After investigation NOT a good idea

Franco

Title: Re: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: CascoSieg on December 15, 2025, 11:33:07 am

Thanks for doing and posting this test! Good info for everyone.
Can you elaborate on why the PI load changed through the rotation? Looks like it should remain constant, yes?

Title: Re: I think I had an idea for a "new PPIMV" that represent a Constant Load for PI
Post by: kagliostro on December 15, 2025, 11:56:13 am

The problem is the 2M2 resistor that is there to maintain a ground reference to G1 if the wiper (those of the pot connected to ground) loss connection

Rotating the shaft the presente of that resistor modify the taper of the pot that don't result linear and didn't maintain a constant load for the PI

Franco