Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Justa on October 08, 2016, 06:17:41 pm

Title: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: Justa on October 08, 2016, 06:17:41 pm

I am considering a 5E3x2 build with a quad of 6V6's that I would to have the option of running fixed biased in addition to cathode biased.  To bias the amp in fixed biased mode I would like to be able to balance the sides in addition to having a bias level pot.  I'm not having luck finding a bias balance pot like Fender used, 10KL with a center tap.  Anyone know where I might be able to locate one, Doug doesn't carry them?

Thanks

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: sluckey on October 08, 2016, 08:17:58 pm

Can't help with the tapped pot but I can suggest a much better alternative...

Just build a bias supply with two pots, one for each side.

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: Justa on October 08, 2016, 08:35:05 pm

Wow, thanks Sluckey,
Never would have thought of that.  That should work just fine!

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: sluckey on October 08, 2016, 09:56:30 pm

Here's the dual bias mod I did to my Sunn...

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf (http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf)

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: kagliostro on October 09, 2016, 02:05:06 am

I recently posted a possible solution on a TIMBO thread

(http://i.imgur.com/AHk9c5G.jpg)

it uses a standard pot and two resistor to achieve the same result of a center tapped pot

(the faint drawing is part of the circuit)

Franco

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: Justa on October 09, 2016, 07:20:04 am

Thanks Franco & Sluckey!
I can see now that there is more than one way to skin a cat!

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: Alistar on October 11, 2016, 12:04:38 pm

Sluckey - what's the deal with that "MidBoost" switch on the Sunn schematic - either way, doesn't the signal just ground-out there?
 :dontknow:
Pretty dang cool circuit there, though!

Title: Re: Fender Style 10KL Bias Balance Pot with Center Tap
Post by: sluckey on October 11, 2016, 12:52:52 pm

As shown, the switch enables a local feedback circuit from plate to grid of V3A and also switches in a cathode bypass cap on V3A. Flip to the left and the local feedback loop is disabled and V3A cathode bypass cap is removed ***AND*** a cathode bypass cap is enabled for V1A.