Bias circuit resistor value question

[Brad@166]

Hello everyone!!
Long time lurker, first time poster. Smitty told me to come check out the forum and boy was he right. This site is a treasure trove of info and friendly community.
Thanks for having me.
Well my question is this bias circuit in the picture, its ment to tap off a high voltage supply. In using this circuit but Im going to have a 50v bias tap on the transformer Im using. What would be an appropriet value resistor to start with to get to correct voltage using only 50v?

It will be used for a SE 6L6GC single power tube.

Thanks everyone!!!!

[sluckey]

Change the 220k to 470Ω or 1k. Adjust value up/down if needed to fine tune.

[Brad@166]

Thank you sir for giving me a good starting point. Wasnt sure how low I should have gone.

[Helmholtz]

Single ended power amps run in class A and are typically cathode biased.
Are you sure you need a negative grid bias?

[Brad@166]

Yes sir, I know its not normal. Putting something together that goes against the grain I guess. But when Im done I have a feeling it will be a KILLER sounding 10 watt SE amp. Thats my hopes anyway.......

[Helmholtz]

What power tube?

[Brad@166]

6L6GC

[Helmholtz]

So you might need something like -18V.

[Ten Over]

What plate voltage are you planing to use?

[Ten Over]

Preliminary Circuit

Bias Two Cap Tap Tinnopher.png

[Brad@166]

What plate voltage are you planing to use?

Im not 100% sure, Iv not built a SE 6L6 amp before. I figure Ill start with about 415 and see how it goes. My 6V6 Vibro Champ sounded better at 375 then it did at 415 (no worries, it was a JJ 6V6S). But Iv not tried to voice a 6L6 before. It will be fun.

[Brad@166]

Preliminary Circuit
Bias Two Cap Tap Tinnopher.png

Oh now what have we got here?
This is diffrent then Iv seen before?

I like the use of the 10K bias pot so a "regular" bias pot can be used.

[Ten Over]

Tweaked a little for 415V:

Bias Two Cap Tap Timmopher 2.png

The DC bias voltage that is developed is fairly sensitive to the AC voltage at the transformer bias tap and that AC voltage never seems to be certain until you fire it up and measure it. Different 6L6GC's require different bias voltages in order to idle at 90% plate dissipation or 100% or whatever you are going to idle them at. The upshot here is that you will likely have to tweak the bias circuit once the amp is actually built.

[martin manning]

You need to take the idle screen voltage into account too. This is the bias circuit I have been recommending for some time. It uses the standard 10k pot and is fail safe since it reverts to minimum bias voltage (most negative) in the event of a pot failure.

[Brad@166]

Tweaked a little for 415V:
Bias Two Cap Tap Timmopher 2.png
The DC bias voltage that is developed is fairly sensitive to the AC voltage at the transformer bias tap and that AC voltage never seems to be certain until you fire it up and measure it. Different 6L6GC's require different bias voltages in order to idle at 90% plate dissipation or 100% or whatever you are going to idle them at. The upshot here is that you will likely have to tweak the bias circuit once the amp is actually built.

This is really great. Thank you so much. I noticed how much the bias range changed when changing the plate voltage in my 6V6. Now Iv got a great starting point for the 6L6 project. Big thanks!!!!! You Rock!!!!!