Bias Circuit Help

[Jaymz77]

Hi all,
I'm hoping someone can help me with some issues I'm having with a build i have just completed.
I have built myself a BE50, based on photos and a schematic i found online.
The issue i have with the bias circuit is that the original BE100 uses a PT with a dedicated bias tap of 100v (according to Phil at Heyboer).
The 50w PT i am using does not have a dedicated bias tap so i need to take the bias from the B+.

So basically i need some help with the maths so i can work out the bias output to the tubes with an input voltage of 100v to the bias circuit.
Then i need to work out how to get that same output with an input voltage of 350v.

Im sure when someone shows me i will feel like an idiot, but i have not had to do this kind of maths for years... since trade school really.

Any assistance is much appreciated.

[danman]

You could look at the bias circuit for a 50 watter, like a 2204 or 1987 and use the same. It taps AC voltage from one side of the secondary and drops it down to appropriate levels. Some fine tuning of resistor values may be necessary but it should get you really close.

[Jaymz77]

Thanks for the reply.
You are right, but I need to know what bias voltage is actually going to the tubes when 100v is applied to the bias circuit before I can know if that will work.
In fact, I did what you suggested using a 2550 bias circuit and it didn't work. So I need to do the maths.
If you look at the circuit attached, I need to know what comes out at the point between R56 and R57 when 100v is applied to R58.

Cheers

[Jaymz77]

Damn forum won't let me attach a pic

[neikeel]

The 100w amps use a dedicated 95vac feed via a 27k dropper pre diode. The 50w amps use 350vac feed and a 180k to 220k dropper is normal for a 50w (both assuming EL34s.
You are aiming for around -40 or so volts dc at the grids of the outputs.
Of course different tubes and different values of bias resistor and pot will alter things.

[danman]

Like Neikeel mentioned, the bias voltage to the tubes will vary quite a bit depending on what set of tubes you are running, plate voltage and what dissipation level you are aiming for. If your amp has a 350-0-350 PT and you want to tap voltage for the bias circuit, the Marshall 50 watt circuit will get you very close. You will have to adjust the dropper values at the beginning of the circuit to fine tune. You will probably want to aim for a bias voltage of around -40 to -45 volts or so with some range on either side of this with your pot. It will take some experimenting with the resistor values.