adding a bias balance pot?

[Jana]

If you're tapping off the HV supply for the bias and have a large resistor (220K) then the 10uf + 10uf is plenty. Going too large on these caps is not wise--the time constant for charging up those caps gets too long. Besides, any AC ripple, which is quite small, is cancelled out by the push-pull nature of the tubes.

[iknowjohnny]

is this correct? Don't mind anything missing elsewhere in the schematic, as i just quickly erases anything in the way of copying the bias circuit over to make two of them. As you can see there are 2 complete bias circuits now. Is this the way to do it or are there components that can be shared?

http://i24.photobucket.com/albums/c39/dazco/bias.jpg

[Jana]

That is the way I would do it. You can delete one of the first 10uf caps so you only have three of them total. You will probably have to increase the value of the 33K to get the voltage back in range. Start with a 68K. That will narrow the range of the bias adjustment pot though.

Another way would be to drop the value of the 220k by the diode. Try a 150K to start with.

Main this is measure voltages before you flip it off of standby to make sure you are in range with the bias.

[Structo]

Yeah, you don't need to be so redundant with the parts.


This is how I did it on my D'lite.

[Jana]

I like Johnny's way better--there is filtering at each 220k bias feed. This will keep the AC signal that will be present on the 220K bias resistors referenced to an equal impedance to ground. And it is also a fail-safe arrangement, if the bias pot fails or loses contact the bias voltage will go cold (the safe way to go).

[iknowjohnny]

Darn....i wish my way were NOT the better solution, as his is much easier ! guess i will do that tho, thanks.

[Jana]

Well, don't take my advice as gospel--I'm are an English major!

[iknowjohnny]

i knowd that already.

[iknowjohnny]

So i did the two completely discreet bias circuits and now when i try and balance them, adjusting one throws the other off so i'm doing this back and fourth tap dance. I have been brainstorming on a way to add a balance pot instead of two discreet circuits but before i start trying my lame ideas, what are yours ?

[martin manning]

So i did the two completely discreet bias circuits and now when i try and balance them, adjusting one throws the other off so i'm doing this back and fourth tap dance. I have been brainstorming on a way to add a balance pot instead of two discreet circuits but before i start trying my lame ideas, what are yours ?

Are both sides drawing current from the diode through a single 220K resistor? If so try putting a second 220K in, one for each side.

MPM

[iknowjohnny]

yes, and i thought about that but wouldn't it still happen, just to a lesser degree?

[martin manning]

yes, and i thought about that but wouldn't it still happen, just to a lesser degree?

Should be negligible unless there is another big resistor upstream of the diode. You'll need to have duplicate caps after each 220K resistor too.

[Structo]

Hmmm, maybe that way isn't the best way after all.

I have no problem dialing each tubes bias in at all.

I realize my way does not have the safety resistors but that is why I went with the high quality PEC pots for the bias adjustment.

[iknowjohnny]

Ok, i got it. The reason it was affecting the opposite side that i was adjusting is because of my bad eyesight. Sometimes i may see brown as red. Twas a 120k in front of the diode, not a 220k I didn't even know i had any 3w metal oxides at 120k, so i suppose a quick look was all i thought i needed.

[martin manning]

I don't think that will solve your problem.