Individual Bias Pots

[patlaw]

Can someone suggest an amp schematic that has individual bias controls for each of two 6L6GCs? I'd like to add it to my Vibrolux Reverb.

VIBROLUX_REVERB_AA270_cropped.pdf

[g1]

2 tube or 4 tube shouldn't matter, they still only use single or dual bias set-up.
Fender red knob 'The Twin', DSL and TSL Marshalls all used dual bias set-up I think.

[Leo_Gnardo]

This isn't the solution you asked for BUT I'll pitch it anyway: take the ground leg of the bias voltage divider, typically 15K, and substitute a 10K in series with a 10K trim pot. With this you can adjust overall bias, and use the existing pot to set balance. Yes, it's a little fiddly but it works easily enough. FWIW I use a Bourns "blue cube" trimpot and park it atop the existing bias pot.

[TomCarlos]

Patlaw....

Here is what Hiwatt has published. It shows the two control. I guess you can make a four control but I have never tried it.

[Enzo]

I am with Leo here. The existing arrangement allows balancing the two tubes. Just a simple modification and that all becomes adjustable. You can adjust the bias, and the balance control becomes a fine adjustment for each tube.

[Leo_Gnardo]

Here is what Hiwatt has published. It shows the two control. I guess you can make a four control but I have never tried it.

Thanks for posting this Tom. I like Hiwatt's "pull up" resistor from pot wiper to higher (negative) voltage. In case the wiper contact gets funky or fails, bias voltage will drift up to that value, which will cause crossover distortion, but save the tube from losing bias and going out of control.

I worked on a Matamp copy a couple years ago that had 4 bias pots. Just extend two more onto the Hiwatt circuit and there you are. Some adjustment of the overall bias voltage would have to be made with double the number of bias pots, no big deal.

[catalin gramada]

The balance pot rules. You stick the probes in both rails and fit the offset to null (or as close can get) no matter how many tubes there are.

[Helmholtz]

And don't forget a balance pot allows to minimize power stage hum.

[Leo_Gnardo]

And don't forget a balance pot allows to minimize power stage hum.

Easy enough to dial balance in by ear then with the overall bias "in the ballpark." Then recheck after balancing and trim overall bias a bit if necessary. I knew there was a reason I liked my preferred method.

[Helmholtz]

Easy enough to dial balance in by ear then with the overall bias "in the ballpark." Then recheck after balancing and trim overall bias a bit if necessary. I knew there was a reason I liked my preferred method.

Yes, I support your proposal from post 3.
Adjusting overall bias may not have been important in the old days where tubes conformed to their datasheets but certainly is now with all these out-of-spec tubes.

[Enzo]

Let us be fair, even in the good old days, tubes met the data sheet, but the data sheet was a range. If I bought 25 6L6 new in the box, I would not have expected them all to settle in at some exact figure current one after another. I used to go to my local parts store and buy factory matched pairs of 7027 for my AMpeg. Two boxes glued together with 7027MP stamped on the boxes. SO RCA knew their tubes were not all matched.

[TomCarlos]

At the risk of being voted off the island for a dumb question.... Has there been an amp, any amp that had both a balance and bias adjustment pots? Or is that not feasible to do?

[Enzo]

Or course it is feasible, we even described how to do it up above. I don't know of any offhand, but I don't think about it much. But really, just having dual controls is about the same. In the case here, the balance is already there. If you build from scratch you can have two pots as dual trims. It is two pots either way. So like any of the Marshalls with the three pins sticking out and two trim pots is equivalent in my view. And how about things like the SVT with dual bias controls plus the red and green lights to help. The balance control reduces hum, but setting dual bias even does the same.

A lot of OEMs now include bias systems because consumers want it. It really isn;t necessary. Fender gave us that balance control to satisfy an actual need - hum reduction. They left out the bias level adjustment because it wasn't very important. The amp will be just fine at a wide range of idle currents.

[g1]

At the risk of being voted off the island for a dumb question.... Has there been an amp, any amp that had both a balance and bias adjustment pots? Or is that not feasible to do?

I'm not sure exactly what you mean. Do you mean strictly labelled as such? If you have 2 separate bias controls then you have bias and balance. Or a means to achieve both.
The 3 examples I gave in post #2 will do both.
The 'The Twin' was labelled as bias & balance on the controls, but circuit wise it was just 2 duplicate bias controls like the DSL, TSL, etc.

[TomCarlos]

Enzo and G1..... I meant to ask, has there been an amp that had a (1) Balance control and (2) a single bias pot for two. I take it the only time you would find a balance control is an amp that has only two output tubes and you would never see it on an amp that had four tubes (to balance out the push and pull pair).