Any reason this wouldn't work?

[12AX7]

Instead if building a 2nd bias circuit into a amp so i can match the bias current in a 2x8L8 amp, split the 220k resistors and feed the voltage to the wiper and each side to one of the 220k's. A bias balance control if you will.  I would think there are amps with this if theres no reason it wouldn't be a good idea. Of course i realize i'd have to use a small value trimmer then swap the bias set resistor so the bias pot is still in range. Any thoughts on this and what size pot would be good ? assuming of course i'm not missing something that makes this a no go.

[Willabe]

......in a 2x8L8 amp, .....

8L8?

Do you mean a 6L6?

[12AX7]

No, typo. 6V6.

[12AX7]

Well i think i either answered my own question or it takes a much bigger pot. I tried it with a 10k and it made no difference. I could check either tube while turning it fully in each direction and the voltage at the tubes didn't change at all. The set resistor in front of the bias pot is 15k so i would think a 10k balance pot would be enough. So apparently for reasons i don't understand and i'm sure someone will soon chime in to explain, this was a flawed idea.

[sluckey]

Show us a schematic for your idea.

Something to consider... You must have current flow somewhere if you want to change the voltage sent to the grids. IOW, even if you have 220K to one grid and 1M to the other grid and feed the bias voltage to the junction of the two resistors, the voltage that arrives at each grid will still be the same because there is no current flow into the grid. No current flow means no voltage drop.

[12AX7]

Normally with a single bias you have a bias pot that sends a voltage to the junction of the two 220k resistors. I was suggesting separating those two 220k's and instead of connecting to each other and the bias supply, each resistor goes to one side of a balance pot and the supply to the wiper. I just saw the same thing in the Rivera fender concert II here ... http://schematicheaven.net/fenderamps/concert_ii_83.pdf

He has a 33k load at the wiper too, i guess thats what you were saying and why it didn't work when i tried it?

[Willabe]

Fender (CBS) switch to bias balance with their silver face amps.

You can have/wire up both bias balance and/with adjustable fixed bias.

http://el34world.com/charts/Schematics/files/fender/Fender_bandmaster_reverb_aa768_sch.jpeg

[TIMBO]

 

[kagliostro]

Franco

[sluckey]

Normally with a single bias you have a bias pot that sends a voltage to the junction of the two 220k resistors. I was suggesting separating those two 220k's and instead of connecting to each other and the bias supply, each resistor goes to one side of a balance pot and the supply to the wiper.

I thought that was probably what you had done. As you found out, that will not work for the reason I mentioned.

I just saw the same thing in the Rivera fender concert II here ... http://schematicheaven.net/fenderamps/concert_ii_83.pdf

He has a 33k load at the wiper too, i guess thats what you were saying and why it didn't work when i tried it?

That's quite different from what you did. That's a special pot with 4 lugs. It's a regular pot but also has a fixed center tap lug. The bias voltage is connected to that lug. The wiper is connected to ground thru a 33K resistor. These two connections form a voltage divider. If you use that same arrangement on your amp, it will work as you expected.

[HotBluePlates]

Normally with a single bias you have a bias pot that sends a voltage to the junction of the two 220k resistors. I was suggesting separating those two 220k's and instead of connecting to each other and the bias supply, each resistor goes to one side of a balance pot and the supply to the wiper.

I just saw the same thing in the Rivera fender concert II here ... http://schematicheaven.net/fenderamps/concert_ii_83.pdf

... That's a special pot with 4 lugs. It's a regular pot but also has a fixed center tap lug. The bias voltage is connected to that lug. The wiper is connected to ground thru a 33K resistor. ...

To beat a dead horse (maybe), the connection from the 33kΩ to ground is a path through which d.c. flows from the filtered bias supply voltage to ground.  There is current through that path.  That's why voltage changes when the 33kΩ is moved by the wiper, because the current is flowing through a different resistance in each path to the 33kΩ.

But from the filtered bias voltage to the tube grid, there is no connection through which direct current can flow; the grid is an open-circuit to ground.  So no voltage drop across resistance along that path, and no change of voltage when you change the resistance.

[12AX7]

That's quite different from what you did. That's a special pot with 4 lugs. It's a regular pot but also has a fixed center tap lug. The bias voltage is connected to that lug. The wiper is connected to ground thru a 33K resistor. These two connections form a voltage divider. If you use that same arrangement on your amp, it will work as you expected.

Ahh, i wondered why it was drawn like that. I will see if i can find a 10k pot like that and try it. Thanks.

Franco

I saw that circuit (emulation) at some forum during a google search and tried it but the voltage shot thru the roof so i just abandoned it rather than try and figure out what values might work and whether to adjust the 2 grounded resistors or the bias set resistor.

[sluckey]

...tried it but the voltage shot thru the roof...

It would be a good idea to kill the B+ with the STBY switch or even remove the power tubes while experimenting with the bias circuit.

[12AX7]

Yes, I will, and i think i will try it again because thinking about it that should be the better option considering the fact i won't find that pot locally and online i'd likely have very little choice as to where to buy one and it would cost me probably $15 with shipping. Free sounds a lot better.

[HotBluePlates]

Ahh, i wondered why it was drawn like that. I will see if i can find a 10k pot like that and try it.

I saw that circuit (emulation) at some forum during a google search and tried it but the voltage shot thru the roof ...

Yes, the "emulation circuit" is not for actual use but only for fooling a simple sim program into showing you how the circuit will behave. Within the sim, one resistor to ground is made 33kΩ and the other is made ∞Ω, and then the sim will show the resulting output voltages to each grid.

[kagliostro]

Ciao HotBluePlates

The "emulation circuit" idea was borrowed from Merlin B., and also if useful for an emulation program, is the proposed solution to the problem to get the adequate pot with the 50% intake

Here the values that Merlin suggests to be used



Franco

[sluckey]

I'm betting that once you work out a nice balance circuit that you will still want to be able to set the overall bias voltage. And that brings me back to the top of the discussion and a question I wanted to ask but decided to tackle your balance problem first...

Instead if building a 2nd bias circuit into a amp so i can match the bias current... ...A bias balance control if you will.

Why get a headache figuring out a bias adjust and bias balance circuit when a dual bias adjust is so much simpler and works just as well?

[12AX7]

I'm betting that once you work out a nice balance circuit that you will still want to be able to set the overall bias voltage. And that brings me back to the top of the discussion and a question I wanted to ask but decided to tackle your balance problem first...

I already have a adjustable bias, i just wanted to balance the voltage after that.

Instead if building a 2nd bias circuit into a amp so i can match the bias current... ...A bias balance control if you will. Why get a headache figuring out a bias adjust and bias balance circuit when a dual bias adjust is so much simpler and works just as well?

Because theres no room. This amp has been thru a number of incarnations and was first a very simple circuit with a very small board and little room in the chassis. I did a dual bias in my last and and thats what i wanted to do, but i just have no room.

I just finished it but the issues i have are 1-i'm not sure if i need a bigger pot or bigger resistors than the two10k's as shown in kagliostro's last post, but the balance only changes by 3v at extremes of the pot. and 2-voltage is down from 35 to 6 and i'm not sure where to adjust for that, but adjusting the bias set R in front of the bias pot has no effect. The feed R from HV is only a 100k as is. I just have to look at this and see what i'm missing because it's not immediately obvious to me as to you guys. I have to stare at it a bit and try and get a picture of whats going on in my head.

[Willabe]

There's a few ways to do bias balance/dual bias. What Sluckey's suggested works great and it will work with power tube bias vary tremolo.

I've done the 1 on the right, dual bias, in the drawing below, from K. O'Connor TUT books. (They both really do the same thing.)

 

[12AX7]

I wanna keep it as it is rather than pull it apart to try the above. So i'm still working on the "emulation" diagram kagliosro posted.  I found if i insert a 220k from what in the diagram is the ground point between the two 10k's to ground i have the correct range and upon inserting the tube sure enough i get the proper mV at the one ohm resistors. So i will remove the 220k use two 120k resistors in place of the two 10k's. But i still think the pot needs to be 25k to give me enough range between the tubes so i'll see if i have something usable.

[sluckey]

And then next month when you decide you want to try 6L6s rather than 6V6s you can repeat this headache. 

[12AX7]

And then next month when you decide you want to try 6L6s rather than 6V6s you can repeat this headache. 

  Naa, i don't like 6L6 as much and the trannys wouldn't work right with them anyways. So this is what i ended up with and it works perfectly, tho i don't understand why i had to do some of what i did. The 220k for example. At first i had 10k's in place of the 120k's you see in the diagram below and by putting a 220k between them and ground i was able to get the voltage in proper range. So i figured i'd dump the 220k and up the two 10k's to 120k each and i figured that would keep the same resistance to the ground point. But it didn't work. Even with the 120k's i needed that 220k between them and ground. So i'm going to assume it will work the same as it does now if i replace those 120k's with 10k. None of that makes much sense to me due to my lack of theory. But it works so what the heck. I'll try the 10k's again and if it works i'm done. If not, back to the 120k's and i'm done. That is unless you guys can tell me why i should do it different due to non subjective reasons.

EDIT:50k pot and 10k's where the 120's are in the diagram and it works perfectly. One question tho....why is it i had to use that 220k between the two resistors and ground, whereas in the emulation diagram about they are just grounded?

[sluckey]

We need to see the entire bias circuit and know the voltage on the dc side of the diode. Then we can tell what's going on.

Your balance circuit will have a different range of adjustment for different bias supplies.

[12AX7]

here ya go. Sorry for the homer simpson like drawing.  I had to change the bias set resistor in front of the bias pot from 15k which worked before i put this balance circuit in to the 22k you see in the drawing.

EDIT: Well, i think while you'll all tell me it's placebo effect, and maybe it is, but i can't help but feel it changes the tone or feel and i restored it back to single bias without balance and i could swear it has a different tone and/or feel. Maybe I'll try it again if i come to realize i did something wrong that may be responsible for the change i seemed to hear but for now i just selected my 2 closest 6V6's.
I did however realize one obvious mistake, tho i think it only affects balance range. That being i should have selected the the 2 resistors that were 10k by halving the pot's valve....50k pot, two 22k resistors. Didn't hit me till i thought about it they should mimic a center tapped pot. So two 22k's would make the 50k pot essentially about a 25k center tapped. But for now i'm not gonna bother with it barring any replies that can confirm there could be a tonal/feel change due to the way i had it configured.

 

[Pwurso]

Resurrecting this from the dead…

Apologies if this was mentioned, but there is a difference between individual bias pots and a single bias pot with bias balance.

The “bias balance” option has better voltage stability. The “balance” pot always has the same shared value, 25k, and the summed resistance with the “bias” pot (and overall voltage) is the same & shared with both tubes. When one tube voltage fluctuates, it changes the other in unison, keeping the balance you have set.

In the “individual/dual bias” option, when one tube changes bias, it gives or takes voltage from the bias supply without changing a shared line with the other tube, causing a mismatch. This will change the sound and feel of the amp. Also, it takes more time to bias with individual bias pots, as when one tube is biased, the other loses/gains voltage, and you have to dance back and forth much more setting them the way you want.

Leo Fender knew what he was doing.