adding a bias balance pot?

[iknowjohnny]

I'm not sure how to do this, but i figured you probably add about a 25k pot between the bias resistors, no? I want to be able to adjust the bias of the 2 tubes seperately w/o having to have 2 bias circuits. I know fender does this on some newer amps but i can't find an amp schematic with it. am i correct in how to do this, and if not, how?

[Structo]

Sure, I did that on my D'lite.

On my amp I had 10K pot and a 10K tail resistor to ground.

So that is like 20K.

So with two bias pots being fed from the same supply you want to have a bout the same resistance.
So I used two 25K pots each with a 15K tail to ground.

So that is 40K each which works out to 20K because they are in parallel.

The top schematic is the way it originally was, the bottom is what I changed it to.

[iknowjohnny]

Ok, i must admit i'm not following that. I don't understand why theres a ground point there for one thing. Fo you have a full schematic of the amp?

[ampdoc1]

Remove the two bias resistors, then replace them with a linear pot ~= to the sum of those two resistors. Connect the legs of the pot to the power tubes, and the center tap to your bias supply.

adoc

[iknowjohnny]

Thats almost what i was thinking, tho i thought to replace the 220k's with a couple 180k's then put each one to a 25k pot with the wiper to the bias supply. Or would that give enough range in that place?

[ampdoc1]

I believe that will work fine and with better precision than using one pot like I suggested.

adoc

[iknowjohnny]

Well, i think we both have a lot to learn about this ! First i tried my way, then yours. Even with a 250k pot in place of the 2 resistors i couldn't make any change at all. one side was at 40 Ma and i turned the pot both ways and no change. So much for that idea. I'll just do the dual supply.

[Structo]

You have to separate the two 220K resistors since each feeds a power tube.

I did my amp exactly as the drawing shows, I have a very good range for 6L6 tubes in my amp.

What amp are you working on?

Most bias circuits have the CW lug to ground.

This is for a Brown Note D'lite.

[Structo]

Sorry, I think I misunderstood your question because of the way you worded it.

I see you asked for a balanced bias.

Well some old Fenders had what was called a hum balance pot.

You simply adjusted it for the lowest audible hum.
It does nothing for adjusting the bias current.

Lots of guys convert those amps to adjustable fixed bias.

What I tried to show was how you would have two bias pots from a single supply, I really like this method and it works good.

This schematic shows a hum balance pot.
It is the AB165 Bassman.
http://www.schematicheaven.com/fenderam ... _schem.pdf

A earlier version, the AA165 had adjustable bias.
http://www.schematicheaven.com/fenderam ... _schem.pdf

[iknowjohnny]

Thanks. But there are fenders with a bias balance pot. I know because i have adjusted a few before, but i can't recall the models. they are newer amps tho. I think they are in fenders "custom tube series" if i recall. Anyways, on the back they have insert points for meter probes where you can set bias, then set the balance between the tubes so they read the same. I will try and find which amps have that and post the schematic. If it's as much trouble and parts and space as adding another bias circuit i'll probably just follow your design. Thanks.

[ampdoc1]

Bandmaster reverb aa1069.

[Structo]

Interesting, I don't think I had seen that type before.

I see how it would balance the voltage but how would you adjust the bias current?
And since it has the vibrato on the bias it is a bit different there as well.

Personally, I would rather go the way I showed.
You can dial in the tubes as close as you want to each other or off balance them by 5ma if you feel it sounds better.

[Colossal]

Personally, I would rather go the way I showed.
You can dial in the tubes as close as you want to each other or off balance them by 5ma if you feel it sounds better.

+1. Separate bias pots work <i>very</i> well and it's the only way I'll build an amp henceforth! There is no need to buy so-called "matched" tubes and you can match them and then unbalance them to your ear's taste. It is simply a matter of paralleling another trim pot (or 2W precision locking pot if externally mounted) and tying the ground into the same tail resistor (which sets the voltage range for your bias circuit) as the other trim pot.

I've attached a drawing of a potential circuit, similar to Structo's, just drawn another way. It's adapted for use in a Marshall circuit and allows switching between EL34s and 6L6s. The 68k resistors across the pot wiper and leg is to keep the tube from running away if the pot fails; just a safety precaution.

Hope this helps.

[Structo]

Thanks, I do like the idea of a safety resistor.
I may have to implement that into mine.
Thanks!

Do you feel the 10uF + 10uF is adequate filtering for the bias?

For instance my D'lite has a 100uF 100v cap on the bias board, same as ODS #124.

What range of adjustment does the 56K give you on a 6L6?

On the D'lite there is a 45v bias tap, it has a 470R on the board then I use the dual 25K pot and 15K tail.

I bias my 6L6's fairly cool at 36ma @ 440Vp

[Colossal]

Do you feel the 10uF + 10uF is adequate filtering for the bias?

You know, I think it would probably be better to use more...I haven't calculated what might really be needed but will do that. These old, traditional values like 8uF or 10uF are probably just remnants of the classic designs where caps were more costly and weren't readily available in high values.

For instance my D'lite has a 100uF 100v cap on the bias board, same as ODS #124.

I don't think it could hurt to have a stiffer supply and 100uF would certainly provide the brute force. I thought it might be interesting sometime to put in a regulated supply but probably <i>complete</i> overkill

What range of adjustment does the 56K give you on a 6L6?

Again, sorry, I don't know yet although it should bump up the voltage by at least -8 to -10V. This is for a build that I'm doing and haven't tested it yet. I'm showing -36 to -42V for EL34s and -48 to -55V for 6L6s. This amp will be two-tube power amp.

On the D'lite there is a 45v bias tap, it has a 470R on the board then I use the dual 25K pot and 15K tail.

Yeah, that 220k resistor on the front end is just a placeholder on the schematic and would assume that the voltage for the bias would be tapped off one of the HV secondaries. It turns out that the PT that I'm using has a bias tap so that 220k will likely turn into 470R. I'll need to do a little math to sort it out.

I bias my 6L6's fairly cool at 36ma @ 440Vp

Yep, that's what I have in my notes for the lower end for a pair of 6L6s. This Marshall-eque build will be running the plates at 460V.