Separate bias voltage

[überfuzz]

I have a homebrew under construction and an idea. Now I would like pointers... It will have a quad solid prestressed 6L6 tubes. Right now, I'm toying the idea of having a separate bias supply to each tube. Has anyone done this, with satisfactory results?

Thanks!

[Jazz P Bass]

Separate bias supply?????

In what way?

[Chuck H]

I don't really know what "solid prestressed" means. I can guess though. You didn't say these were matched tubes but I suppose for the effort of testing for solidity and then pre-stressing them that matching was the point. With this in mind I wouldn't think that separate bias supplies would be necessary. Tubes have a pretty wide range of acceptable bias condition so any bias within reason should have all the tubes in happy land. So it's not for the safety of the tubes.

Then there's the issue of finite balancing. Maybe for a sound reference, bass or hi fi amp, but IMHE balancing doesn't do anything for guitar amp tone. In fact, a little imbalance (or even a lot sometimes) can sound better (other circumstances depending). So you might have some fun with separate bias supplies by being able to intentionally IMBALANCE the circuit. And I guess another neat thing you could do would be to bias one tube on either side a little hot and the other tube on either side a little cool. That might make some interesting changes in the feel of the amp. Probably not big changes.

I'll be honest, I haven't done it. I guess I don't see any downside. I just don't see any big upside either.

[nosaj]

I don't really know what "solid prestressed" means. I can guess though. You didn't say these were matched tubes but I suppose for the effort of testing for solidity and then pre-stressing them that matching was the point. With this in mind I wouldn't think that separate bias supplies would be necessary. Tubes have a pretty wide range of acceptable bias condition so any bias within reason should have all the tubes in happy land. So it's not for the safety of the tubes.

Then there's the issue of finite balancing. Maybe for a sound reference, bass or hi fi amp, but IMHE balancing doesn't do anything for guitar amp tone. In fact, a little imbalance (or even a lot sometimes) can sound better (other circumstances depending). So you might have some fun with separate bias supplies by being able to intentionally IMBALANCE the circuit. And I guess another neat thing you could do would be to bias one tube on either side a little hot and the other tube on either side a little cool. That might make some interesting changes in the feel of the amp. Probably not big changes.

I'll be honest, I haven't done it. I guess I don't see any downside. I just don't see any big upside either.

Is it a guitar amp or a Hi-FI? If guitar I'd think you'd want a little nastyness to it.

nosaj

[überfuzz]

Thanks for the input!

I guess I can justify it with curiosity then, maybe not so much by functionality...

I'll have go and see where I end up with the bias circuit.

[Chuck H]

If you will actually enjoy the extra effort and the expense is NBD then I think you should do it for your own curiosities sake. It certainly wouldn't be a lesser circuit than a single bias supply.

[Gregg]

It can be done in several different ways. Basically it looks like the one on the picture below.



And this is a more sophisticated version (diode and cap before the common trimpot are not shown):



This is how I do the setup:
1/ Set all trimpots to the middle
2/ By tweaking the common trimpot adjust voltage for say 50% dissipation (this way you'll have room for more or less) or whatever you feel like
3/ Replace trimpot with fixed value resistor

After that you can adjust the bias individually.

[Enzo]

I hope you just mean four adjustments - one per tube - rather than four bias supplies. A single supply with four bias feeds.

Lots of people have done that. A bunch of commercial amps have dual bias, like the left pair and the right pair. All the Marshall amps with the three little pins sticking out come to mind. Guys take that a step further to adjust each tube individually. Many have done it, and I am not aware anyone reporting "OMG is this great or what?!!"

It is just a guitar amp, not precision laboratory gear. Bias is adjusting DC current at idle, which is no promise of balanced AC performance when amplifying a signal anyway. So just my opinion, but 10-turn pots for adjustment, and individual tube bias setups are just overkill.

[Justin Thomas]

Maybe for a sound reference, bass or hi fi amp...

Hey, why does BASS always get lumped in with Hi-Fi and sound reference? Sheesh, you'd think every bass player wants to sound like a sine wave! I just want to say, there's a difference between Bass and "rap bass." Bring me the unbalance, bring me the grit, and most importantly, bring me EFFICIENT SPEAKERS being driven by TUBES!!! Everything else is just clinical, sterile, and depressing.

Anyway, yeah... and, as always with my rants,

Justin

[Gregg]

It makes more sense to me to use separate feeds if your tubes are NOT matched or if you want to mix two different types like 6L6/EL34.

[Chuck H]

Hey, why does BASS always get lumped in with Hi-Fi and sound reference? Sheesh, you'd think every bass player wants to sound like a sine wave! I just want to say, there's a difference between Bass and "rap bass." Bring me the unbalance, bring me the grit, and most importantly, bring me EFFICIENT SPEAKERS being driven by TUBES!!! Everything else is just clinical, sterile, and depressing.

Anyway, yeah... and, as always with my rants,

Justin

OK! Ok.

FWIW I didn't actually want to lump bass in with reference and hifi, but that's the way most bass amp designs roll. LF is very demanding and things like imbalance can cause a lot more inefficiency for a bass amp than a guitar amp. (Not that efficiency is the only criteria, put the gun down )

One of my favorite "messy" bass tones is George Sheck on some songs from Edgar Winters White Trash. You can tell it's a clipping tube amp for certain.



Oh, sorry for the slight thread derail.

[Mike Sulzer]

IMO it is worth the effort to use separate bias pots if you are hand building your own amp. Then you get to have fun trying out lots of combinations, including all the same. All that matters is what you think of the results.

[Mike Sulzer]

Another thing, I think there are potentially interesting ways to use four tubes. Here is one that I have not tried, but probably is the first thing i would do if I ever build a parallel amp. As you know, in a two tube AB amp you have both tubes on at low level, and then one cuts off as the ac voltage increases while the other turns on more. With four tubes, suppose you bias two of tubes, one on each side, for AB with the other two cut off completely. Not consider one pair on one side of the transformer. Suppose you set up the bias on the one that is off at low level so that it starts to turn on (with increasing ac voltage) just as the one on the other side of the transformer is turning off. Do the same thing on the other side. Then you always have two tubes on and avoid an impedance loading change at the AB point. I do not know what this would sound like but it would be interesting to find out. The amp also would run cool since only one pair would be on at a idle. (OK, if you never stop playing that would not matter so much.)

[Enzo]

Sounds like you are just describing class B.


Just my opinion, but in bass, unless you are a one tone player, I think most bass rigs want to be ABLE to play a clean tone, even if they prefer overdriven gritty what-not. Unless you are maybe Chris Squire.

[Mike Sulzer]

Sounds like you are just describing class B.


Just my opinion, but in bass, unless you are a one tone player, I think most bass rigs want to be ABLE to play a clean tone, even if they prefer overdriven gritty what-not. Unless you are maybe Chris Squire.

Maybe ABB? Hard to see how it could be B with two of the tubes biased AB. As for linearity, maybe it would be more linear than AB since there are always two tubes on. Have to try it and see.