Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on May 31, 2017, 09:16:58 am

Title: Adjustable Negative feedback
Post by: pbman1953 on May 31, 2017, 09:16:58 am

From the help of Ed, I'm using a 250K pot for an adjustable Negative feedback instead of R39 (100K). When maxed the sound is very hard (harsh)  but focused. Should I decrease the pot value?

Title: Re: Adjustable Negative feedback
Post by: Ed_Chambley on May 31, 2017, 09:23:38 am

Ed always uses a 100K linear with a locking nut in this type of circuit and I will bet you just used what you had on hand.  So you can either change the pot, don't turn it up high enough to make it harsh or hang a resistor across lug 1 and 3 to achieve the maximum adjustment you wish.  Your choice.

Title: Re: Adjustable Negative feedback
Post by: MakerDP on June 02, 2017, 01:54:26 pm

I do mine on a push-pull switching pot so it can be defeated altogether as well.

Title: Re: Adjustable Negative feedback
Post by: DummyLoad on June 02, 2017, 02:00:16 pm

Quote from: pbman1953 on May 31, 2017, 09:16:58 am

From the help of Ed, I'm using a 250K pot for an adjustable Negative feedback instead of R39 (100K). When maxed the sound is very hard (harsh)  but focused. Should I decrease the pot value?

work around: put a 180K resistor in parallel with the 250k pot. equiv. R will be ~ 104K - close enough.

solder the 180k R from pin 1 to pin 3 of the pot.


--pete