Auto biasing?

[Mike9]

I snagged an Ashdown Peacemaker 60 chassis last night. I was reading specs and it has 2 EL34s and is "Auto Biasing" - never heard that term before. I was just wondering if it will take any of the 6L6 family of tubes, KT-66, etc.?

I was intrigued because it has three foot switchable channels, reverb and an effects loop.

[UR12]

I'm not really familiar with that amp but I would think they mean self bias or cathode bias (both mean the same thing). Using cathode bias means the tube would automatically develope the bias needed using the current draw through the tube and cathode resistor. you may need to change the value of cathode resistor to run a different tube.

[AdmiralB]

It may be like the current Traynor amps, where there is a loop in the cathode circuit that constantly monitors voltage drop across a resistor and adjusts the bias to maintain that voltage. Pretty slick, as long as you stick with one tube type and like the factory-programmed setting.

[daveski]

The Crate Turbo Valve series and I think the Blue Voodoo series are self-biasing. I have had a couple of Turbo Valve amps and used both EL34 and 6L6 tubes with success.

[donnyjaguar]

I think most of these autobias configurations are a bit of a tradeoff. They'll place your tubes in a safe operating zone, but as you increase volume they drop more voltage across the sense resistor so the circuit compensates by making the bias a little colder. I guess this isn't a bad thing, but it does affect the tone.

[Blue Strat]

The Crate Turbo Valve series and I think the Blue Voodoo series are self-biasing. I have had a couple of Turbo Valve amps and used both EL34 and 6L6 tubes with success.

It's possible to have a cathode biased amp that will run both EL34s and 6L6GCs. It's likely that neither will be biased optimally or one will be biased correctly and the other type will run either way hot or way cold.

Self biasing often means "self biasing at the wrong bias point".

The only way to know is check with a bias probe or similar.

[Mike9]

I think most of these autobias configurations are a bit of a tradeoff. They'll place your tubes in a safe operating zone, but as you increase volume they drop more voltage across the sense resistor so the circuit compensates by making the bias a little colder. I guess this isn't a bad thing, but it does affect the tone.

Is there a work around for this?

[Blue Strat]

Is there a work around for this?

Yes, the cool circuit mentioned further up the thread, or a switch to select between EL34/6V6 and 6L6/6550s. It's very simple and requires one additional resistor and a switch.

[Mark C]

Mike,
The switch idea looks interesting. I've got an Electro single ended amp that I'm about to do some tweaks on - currently it runs a 6v6, but I'd like to set it up so it can run El34, 6v6, 6l6 tubes. So what I need is to use two different cathode resistors and hook em' up to a switch to change between them? I'm assuming that you've grouped 6v6's and El34's together for one resistor rating and 6l6's and 6550's with the other.

[donnyjaguar]

The only workaround is a more complicated circuit. Basically you need to determine the appropriate bias voltage level and then lock it in. Analogue circuits by their very nature don't like to stay in one place. :) If you are into digital circuits, you could make a PIC driven arrangement with a D-A convertor that would work this circuit alright. This is so far removed from vacuum tube electronics that it may be considered sacrilegious to talk about it here. An analogue workaround could be to apply some signal to the error amplifier from the "other" tube's cathode, thereby nullifying most, if not all, the excessive swing. If you go this route make sure you give me credit because I thought it up and is a DonnyJaguar(tm) idea. :)

[Blue Strat]

Mike,
The switch idea looks interesting. I've got an Electro single ended amp that I'm about to do some tweaks on - currently it runs a 6v6, but I'd like to set it up so it can run El34, 6v6, 6l6 tubes. So what I need is to use two different cathode resistors and hook em' up to a switch to change between them? I'm assuming that you've grouped 6v6's and El34's together for one resistor rating and 6l6's and 6550's with the other.

That's it. This will get you as close as any cathode biased amp will for the 2 different "average" bias settings for those 2 classes of tubes.

Don't forget that 6V6s will not hold up to the higher plate voltages that the 3 other tube types are capable of handling.

[Mark C]

Thanks Mike, I'm going to keep the stock power transformer, so plate voltages shouldn't be a problem.

[Old Tele man]

Mike,
The switch idea looks interesting. So what I need is to use two different cathode resistors and hook em' up to a switch to change between them?
...might be easier to use one BIG resistor and then selectively switch-in appropriately sized "shunting" resistors which "lower" the effective resistance down to the values needed with the other tubes.

...also don't forget: the "effective" plate voltage with cathode-bias is measured from plate-to-cathode...not plate-to-ground! Thus, if the cathode bias voltage is +40V and the plate voltage (to ground) is 440V, the actual "effective" plate voltage will be only 400V (Vp = 440V-40V). You'd use the 400V value when calculating idle power dissipation.

[Mark C]

Thanks for the tip Tele-Man!

[donnyjaguar]

You could use an op-amp and regulator circuit to switch in 2 or 3 additional shunt resistors via NPN transistors, or relays if you prefer. The down side to this is that the regulator will set the bias based on the idle current which you may want to change depending upon the tube types you have installed. Designing a circuit to identify the tube types wouldn't be impossible, but it would certainly be much more complicated.

The circuit I cited before would be fine for switching betwixt 6L6, EL34 and 6550, but once you add 6V6 to the mix it would present a problem. I suspect my work in power electronics is such that I may be talking over a lot of heads here. I don't mind drafting up a schematic if someone is genuinely interested.