Cathode-Bias AB-763?

[duke of earl]

My first build was a 2-6L6 Fixed-Bias AB-763 circuit. How hard would it be to build this in cathode bias?

[phsyconoodler]

Easy.
  But.....you need to have fixed bias to operate the trem circuit,as it takes afeed off the bias supply.If you were to implement a bias-vary trem like a vibro-champ or princeton reverb,then you would have no problems.
 

[FYL]

Lift the bias circuit, ground the 220K grid R's, insert a cathode R + C - the actual value depends on the actual amp, but 250 to 500R + 25 to 100µF @ 50V are OK with most builds with a sub 400V plate V - and you're done.

You may add a switch and some glue in order to be able to choose fixed or cathode bias, cf. for instance http://www.londonpower.com/cath-fix.htm
You may also play with the cathode bias circuit Da Geezer-style and get a switchable power amp, cf. http://www.el34world.com/Forum/index.php?topic=8858.msg79683

Lots of possibilities here...

[FYL]

  But.....you need to have fixed bias to operate the trem circuit,as it takes afeed off the bias supply.If you were to implement a bias-vary trem like a vibro-champ or princeton reverb,then you would have no problems.

Good point. You can also go Vox-style and reference the 220K's to the trem section while keeping the 6L6's in cathode bias.

[duke of earl]

Thanks guys. I like the versatility of both methods. I use Dougs Layout and boards so you know the tremolo method that I'm using.

[chocopower]

Thats the way i used:

[PRR]

> you need to have fixed bias to operate the trem circuit,as it takes afeed off the bias supply.

No.

The "bias supply" for a cathode-biased stage is just ground.

[phsyconoodler]

Yes indeed,but there is a wire that runs from the bias supply to the top of the trem circuit.A 2.2meg resistor.

[HotBluePlates]

Well, he did say he's using a Hoffman layout.

There is a wire, #12, that runs from the Intensity pot to the bias supply. Pretend the bias supply doesn't exist. Wire #12 goes to ground, perhaps at the buss on the back of the pot.

[phsyconoodler]

Ah yes,I missed the Hoffman layout part :toothy6: