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FDP Forum / Amp Mods, Repairs, and Projects / USA blues Deluxe adjustable bias

quindecima

U.S A.

Apr 27th, 2017 10:38 PM   Edit   Profile   Print Topic   Search Topic

Does anybody here have a schematic or layout for an adjustable bias or know of one?

guitarcapo

U.S.A.

Apr 28th, 2017 08:28 AM   Edit   Profile   Print Topic   Search Topic

All I can mention is that I purchased a Torres kit to do this and the "sweep" was very minimal....like maybe up or down just a 2 or 3 mA when using a bias tester. I don't recommend it.

quindecima

U.S A.

Apr 28th, 2017 02:55 PM   Edit   Profile   Print Topic   Search Topic

That could be changed by different resistors/caps correct?

guitarcapo

U.S.A.

May 1st, 2017 08:27 AM   Edit   Profile   Print Topic   Search Topic

I ended up selling the amp after that. I think the problem might have been that the supplied pot didn't have a high value or much sweep. Torres might have been protecting himself from lawsuits making the kit not able to bias the amp too extreme.

quindecima

U.S A.

May 1st, 2017 07:55 PM   Edit   Profile   Print Topic   Search Topic

I believe for that pot a 50K is needed.

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
May 2nd, 2017 07:26 AM   Edit   Profile   Print Topic   Search Topic

I used to do a lot of these mods. (Last one was 12 years ago or more.)

Here is some info I had in my files. I added a bias adjust for each power tube, and added a current sense resistor (one ohm precision metal film resistor) between pin 8 of each power tube and ground. (trace cutting needed) I mounted these on a terminal strip attached to the circuit board by a screw on the lower board.

BD bias mod

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
May 2nd, 2017 07:26 AM   Edit   Profile   Print Topic   Search Topic

I used to do a lot of these mods. (Last one was 12 years ago or more.)

Here is some info I had in my files. I added a bias adjust for each power tube, and added a current sense resistor (one ohm precision metal film resistor) between pin 8 of each power tube and ground. (trace cutting needed) I mounted these on a terminal strip attached to the circuit board by a screw on the lower board.

BD bias mod

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
May 2nd, 2017 07:27 AM   Edit   Profile   Print Topic   Search Topic

Here's the board where I put the parts. I siliconed them in.

BD Bias Layout.

quindecima

U.S A.

May 3rd, 2017 08:15 PM   Edit   Profile   Print Topic   Search Topic

Looking at the schematic the caps are non-polarized correct?

Now I see it on another schematic, they are polarized

(This message was last edited by quindecima at 11:25 AM, May 4th, 2017)

FDP Forum / Amp Mods, Repairs, and Projects / USA blues Deluxe adjustable bias




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