bias question

[C Moore]

Did I build this correctly.?
The one change to the drawn schem is the resistor to ground from each pot. I believe it shows a 10k from each pot. What I actually built was a 15k (to ground) that is common to both pots.
What I end up with is about a range of -2.5 to -5.0 VDC at each pot. But adjusting one pot effects the grid of BOTH power tubes.
I forgot to show the other schems..... sorry.
What I have going on is a bias-vary trem, like a Fender Princeton. So from my bias circuit there is a 220k resistor from each bias pot wiper.
That goes to a dual 250k tremolo pot.
And that goes to a 220k resistor on the grid of each 6K6 power tube.
The amp uses octal preamp tubes and 2x 6K6 power tubes.
I guess my biggest problems are trying to figure out why my bias pots do not control just ONE tube. Each bias pot effects both tubes.
And my bias voltage is pretty low and without much range.
Guess I can adjust that with the resistors.
So why do the pots effect BOTH power tubes.?
Very Sorry for the long post.
Thank You
btw. the first cap in my bias circuit is not 50/100. it is 22/100.

http://i111.photobucket.com/albums/n143 ... uit002.jpg
http://i111.photobucket.com/albums/n143 ... uit001.jpg
http://i111.photobucket.com/albums/n143 ... uit003.jpg

http://i111.photobucket.com/albums/n143 ... ype001.jpg
http://i111.photobucket.com/albums/n143 ... ype002.jpg

[martin manning]

Can you show the whole bias circuit?

[Firestorm]

If you have your pots wired as variable resistors, then each will affect the other. You have to take the bias off the wiper and wire a 220k from the wiper to the raw bias input terminal to guard against the wiper opening.

[C Moore]

Hey Martin -
Not trying to sound dense, but I am not quite sure what is missing.
The first diode is connected to one side of the PT secondary.
At the other end, each pot wiper go to a 220k resistor, and then onto a dual 250k trem pot.

Firestorm -
I thought i have the pots connected as potentiometers. Input at one end, other end to ground (via a resistor) and the wiper is the output.
Thank You Both

[martin manning]

If it was that simple it would peobably be working ;^) Something is loading it down and causing the interaction, so I'm wondering what the whole thing from the PT bias tap to the LFO to the power tube grid resistors looks like.

[Dr-Joned]

Your bias is nothing more than 2 strings of parallel resistors. They need isolation from each other. Try the attached setup. If my mind is clear enough tonight, it should work. (The bias, that is!!!!)

[C Moore]

Martin -
OK, gotcha. I will post the other schems (hand drawn also... sorry) so you can see what I have done.

Doctor -
I will try your suggestion. If I am reading your addendum correctly, it is "just" the addition of those 2 diodes.?
Thanks Again of course

[martin manning]

If the bias voltage is taken from the wipers then the current in each branch is fixed since there is essentially no current drawn by the grids and only AC through the coupling caps. Accordingly, the extra diodes are not necessary.

[C Moore]

If it was that simple it would peobably be working ;^) Something is loading it down and causing the interaction, so I'm wondering what the whole thing from the PT bias tap to the LFO to the power tube grid resistors looks like.

Hey Martin -
Added schem links to my OP.
I believe I have these drawn correctly. I have been battling this on and off for several months. But I think these schems are still accurate.
I guess I do not show the power supply do I.?
OK, will get to work on that.
A computer geek I am not. Trying to teach myself J-Schem or something Similar/Easier
Thanks For Looking

[Tillydog]

Added schem links to my OP.

Just double check that the 10-100 caps in this drawing have their +ve ends to ground. I can't see what else would be drawing current.

Edit: Looks like they're OK from the photo

I think Martin has it.

[paddy]

Is that 4K7 resistor on the bias board replacing the two 10K's as shown on the schematic? If so, I think you need to follow the schematic and use two separate resistors here.
Also, I agree with Martin-No need for the isolation diodes as the grid circuit draws next to no current.
The wiring on your boards looks o.k. (apart from the 4K7) but I can't tell if you have wired the pots correctly. I would double check the pot wiring as I think this may explain the very low voltage.

Cheers,
Paddy

[martin manning]

The low voltage is due to the too-large resistor before the diode. The circuit below should do it, based on a 300 VAC input and a requirement for Princeton-like bias voltage (~34V). The resistor with a ? value will have to be adjusted for the LFO signal that you have. I'd find a value that makes the p-p voltage swing about the same magnitude as Vbias. The first filter is reduced in size to get the bias voltage established quickly. The following 22u's could probably be 10u, as that's what you have in there now. Finally, the 0.05u caps to ground are copied from the fender circuits that use bias vary trem.

[C Moore]

Jesus Martin.....
You did that fast.
I REALLY appreciate the help.
Yeah, the other Elytics after that first 22/100 are 10/100.
Will give this a shot ASAIC.
Thank You So Much

[C Moore]

Making progress. But I wanted to ask a few questions.
I have -45.0 VDC on one trim pot, and -23.0 VDC on the other. They do not play at all. No matter how much I turn the trim pots, those voltages stay right where they are.
My question(s) is this.
My bias circuit is below. I robbed it from an Ampeg Mercury because they had a Trem circuit for an Octal pre tube. It was made for a single Trem Pot. There is a 0.10 cap, into a 470k resistor, and them into one side of the trem pot. Do I need to do anything different there.? Do I need 2 caps and 2 resistors for some type of isolation.?
I also do not, as yet, have the caps that Martin shows in his schem. They are 0.05uf between the trem pot wiper and 220k grid resistors. Are those likely to be crucial at this point.? Are they some type of high frequency attenuation for the trem signal to the power tube grids.?
Thank You

http://i111.photobucket.com/albums/n143 ... ype002.jpg

[martin manning]

The 470k in your schematic is the ? resistor in my schematic, and I suspect it is ~5x too big. No, the 0.05u's to ground are not critical for the bias voltage, but the two 0.1u must be there to isolate the two branches. I believe you could delete the 0.1u coming off the plate of your oscillator since the two 0.1u's in my schematic will block the DC.