No bias tap

[gui_tarzan]

So how does one come up with a bias circuit with a transformer that doesn't have a bias tap? I saw a schematic somewhere that had a connection to the B+ (pre-rectifier tube) and that just seems out there to me given the 300+ volts that come off that lead.

Never mind, I found the threads. But this brings up another question: how does one terminate the bias circuit if they don't want a trem on the amp?

[Chuck H]

Can you link the circuit you're proposing?

[Mick Bailey]

I've built amps using transformers with no bias winding. The technique is to use a low-value cap as a reactive coupling from one leg of the HT AC (B+ refers to the rectified HT). The smaller the cap, the lower the voltage. There are other ways of using resistive dividers, but this one works really well.

[Pedro Vecino]

In this case I use the Vox AC50 bias system. It is very easy to adapt to different tubes/requirements.
http://www.drtube.com/schematics/vox/ac501965.gif

[Mick Bailey]

The Pignose setup (see post #2) also works well;

http://music-electronics-forum.com/t35814/

[gui_tarzan]

I would like to build the Princeton 6G2 without the trem circuit.

[g1]

Well, as you see, the 6G2 does not have a bias tap. Connect point Y from the bias supply directly to the junction of the two 220K resistors. Ignore the rest of the trem. circuit. Where the 30K resistor is at the bias diode, you could instead put a 10K resistor in series with a 50K pot for bias adjustment.

[gui_tarzan]

Now that's what I'm talking about. Thanks G!

[gui_tarzan]

So it would look like this?

[g1]

No, like this

If anyone notices any errors, please comment.

[gui_tarzan]

Is that optional presence control correct? Minus the 56K resistor obviously.

[Chuck H]

g-one has it right. Do that.

That's actually a variable feedback control, not a presence control. And yes, you could omit it by connecting the output directly to the 56k resistor.

[gui_tarzan]

What I actually meant was can I use the 50k pot as a variable negative feedback control there or won't it make much of a difference? I guess I'm not understanding the NFB like I thought I was.

[g1]

I don't think that feedback pot should go to ground, just use 2 legs shown and leave the one shown as grounded disconnected. Keep the 56K resistor in series with the pot. Then you have stock feedback (56K) variable up to 106K (less negative feedback).

[Chuck H]

Ah, but with that leg of the pot grounded the NFB loop can go from stock to zero nfb. And I see no harm in it's implementation.