Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on April 01, 2022, 09:16:23 am

Title: Dual bias mod question
Post by: joesatch on April 01, 2022, 09:16:23 am: Can someone tell me why they built two complete bias circuits in this layout instead of separating the grid leaks and feeding with separate pots from a single negative bias source? Nik does this with all his dual bias amps. Why?
Title: Re: Dual bias mod question
Post by: sluckey on April 01, 2022, 10:26:27 am: Builder's choice.
Title: Re: Dual bias mod question
Post by: joesatch on April 01, 2022, 10:34:28 am: So i'm good going this route?
Title: Re: Dual bias mod question
Post by: sluckey on April 01, 2022, 10:59:33 am: I like this...
Title: Re: Dual bias mod question
Post by: tubeswell on April 01, 2022, 11:27:05 am: Quote from: sluckey on April 01, 2022, 10:59:33 am
I like this...

Or maybe this?
Title: Re: Dual bias mod question
Post by: Willabe on April 01, 2022, 11:47:27 am: Quote from: tubeswell on April 01, 2022, 11:27:05 am
Quote from: sluckey on April 01, 2022, 10:59:33 am
I like this...

Or maybe this?

Why the 2 extra diodes?

For isolation of the 2 -bias adjust circuits?
Title: Re: Dual bias mod question
Post by: tubeswell on April 01, 2022, 11:51:33 am: Quote from: Willabe on April 01, 2022, 11:47:27 am
Quote from: tubeswell on April 01, 2022, 11:27:05 am
Quote from: sluckey on April 01, 2022, 10:59:33 am
I like this...

Or maybe this?

Why the 2 extra diodes?

For isolation of the 2 -bias adjust circuits?

Yes I probably should have finished that more with another cap to ground at the 1k
Title: Re: Dual bias mod question
Post by: sluckey on April 01, 2022, 12:16:59 pm: No need for isolation diodes. There is no interaction between the pots.
Title: Re: Dual bias mod question
Post by: astronomicum on April 01, 2022, 12:27:05 pm: How about adding some resistors to protect in case of wiper failure (across terminals 1 and 2 on each of the pots). 680K perhaps.