Dual Bias Question

[Structo]

I'm going to install dual bias pots in my 100w ODS that I am building.

I did this on my D'lite and like it a lot.

You can match as close as you want or offset it for more harmonic overtones.

When I did my D'lite I took the pot value and tail resistor value and doubled it since the two bias pots are in parallel you end up with the same range of bias voltage as the single setup.

Just trying to confirm what I'm doing here.

So the stock circuit is for a 10K pot and a 27K tail resistor which is 37K.
Double that you get 74K.

I plan on using the PEC locking pots at 25K and a 50K tail resistor.

That gives me 75K total. With the two in parallel that gives me 37.5K for each side.

Does that sound right?

[sergio]

Hi , maybe this can help you!

[Structo]

Interesting.
I did not change the filter cap on my D'lite when I went to dual bias.
Only changed the pot and tail resistor.

How important is it to maintain the same R/C in the bias circuit?

What would the effect be if I don't change the filter cap?

[sergio]

I never tried the dual biasing mod , I don't know the effect of the R/C time constant , here there is just a little note.
In my D-clone amp I use a single bias pot.[/code]

[FUCHSAUDIO]

Interesting.
I did not change the filter cap on my D'lite when I went to dual bias.
Only changed the pot and tail resistor.

How important is it to maintain the same R/C in the bias circuit?

What would the effect be if I don't change the filter cap?

No need to change the bia filter cap when making this change.

[FYL]

I'm going to install dual bias pots in my 100w ODS that I am building.

A voltage + balance system is IMO much better than separate voltage pots. Easier to set-up - way less interaction between pots, easier to dial-in - get the right bias, then balance as you wish.

Here's a clever approach from a French amp builder, totally fail safe - max bias is applied if a pot fails, thus protecting the tubes and the OT.

Text is in French, but the schemo and formulas are self-explanatory. Just in case: AlimNeg is raw DC bias, Vg target bias voltage, R1 and R7 are grid leak/bias mixing resistors, Equilibrage means balance and Total is bias voltage.

[FUCHSAUDIO]

I don't see the two circuits being any better or worse than the other, but they really give the same end result. In any case, I have never seen a modern (sealed high quality) trimmer fail, although I think Leo's original intent with the original Fender method was to allow for a potential bias pot failure not killing his tubes...which made sense in the 40's...

[Structo]

Interesting take on the bias and balance.

On the cap values, it lists it as ,8uF

Is that .8uF?

I wouldn't mind fitting a fail safe resistor, not quite sure about that yet.


Martin Manning posted this before.

Could it be as simple as bridging the bias pot with a 1M resistor and still use the Dumble circuit?

[FYL]

I have never seen a modern (sealed high quality) trimmer fail,

I've seen wipers fail on some carbon trimmers because they were crappy cheap or because the "user" had previously used a metal blade as a chisel when trying to set the bias while under the influence of sniffed glue or whatever was available. Quality Cermets are a much safer bet, but still not idiot-proof.

[FYL]

Here's a pdf copy of the original Fender Custom Shop web page presenting the Two Tone and the Bass Breaker.

[FYL]

On the cap values, it lists it as ,8uF
Is that .8uF?

Yes, 0.8 µF. Europeans use the comma as a decimal separator. A small cap won't dominate the time constants. If your bias circuit uses 10 µF caps in the filtering section, up to 2.2 µF will be fine; you can use 10 µF with larger caps.