Aiken's back biasing circuit....has anyone tried it?

[reddart]

The one at the end of this article: What is Back-Biasing?

Any comments about function, tone, feel when using this method?

It appeals to me as I'd like to drop a little voltage in an amp I have.

[Dave H]

It appeals to me as I'd like to drop a little voltage in an amp I have.

It would be simpler to use cathode bias. It would drop the same voltage and only requires two components.

[Chuck H]

What he said^^^^^^^^^^^^^^^^^^^^^^^^^^

It was noted that this method is useful if you need to drop some plate voltage. Well, if you cathode bias the plate to cathode voltage is still the "working voltage" so it becomes apples to apples. Sort of. I suppose that at some grossly high plate voltage the actual voltage relative to the load can become the issue. But at that point I think if 15V is the difference between an operational amp and blowing up power tubes then you've already made a design error that should be corrected at another level.

As to bias tracking... It's noted in the article that this is good. And it's noted that it's bad.?. It's a little unclear, but wither way, it is what it is and many amps have been built with unregulated bias supplies that track and regulated bias supplies that don't. The article demonstrates methods for achieving either anyway. I personally think that for a reference amplifier it's probably better to regulate and for a guitar amp it's better to track. But that's JMHO. Either can be done with any sort of bias supply without much fuss.

So the question is, I think, is there any advantage to back biasing over the more typical bias systems we see in guitar amps now? I don't think so. With the possible exception that can behave much like cathode bias (ie:reduces working voltage and self adjust some relative to current and voltage changes) but is fully adjustable without the need for something like an expensive power rheostat such as might be needed for an actually adjustable cathode bias. So that's pretty neat. But IMHO still just novel and I think the bias systems that are typically used are usually easier to implement.

I think the article was written more as an exercise in understanding and design and not at all a suggestion of superiority for back biasing.

JM2C

[g1]

I got the impression that reddart wanted to drop the voltage for the entire amp, not just for the power tube.

[Chuck H]

Ah. Ok. Yeah, if you want another 15V off your screens without adding series resistance to the rail then it would seem like a smart solution.

[Dave H]

So the question is, I think, is there any advantage to back biasing over the more typical bias systems we see in guitar amps now? I don't think so. With the possible exception that can behave much like cathode bias (ie:reduces working voltage and self adjust some relative to current and voltage changes) but is fully adjustable without the need for something like an expensive power rheostat such as might be needed for an actually adjustable cathode bias.

You don't need a power rheostat for adjustable cathode bias. It can be done as below. It has zeners for fixed bias but it will work just the same if the zeners are replaced by an R/C for cathode bias.

[Mike Sulzer]

In addition to lowering the HV, it makes it less stiff at dc. Maybe that is what you want for a guitar amp. Bypassing the resistor with a cap means that you do do not get the benefit of the extra ac filtering it would provide. Maybe that is OK, too. But I would rather keep the flexibility for the hv supply, and make a very low current negative HV supply and drop the voltage for bias.

[Chuck H]

In addition to lowering the HV, it makes it less stiff at dc.

This is probably a better reason to put a resistor on a full wave CT. I've done it both at the CT and after the rectifier. Both are effective.

Then, if you're using the zener at the FW CT you won't get the sag. So it still depends on design specifics.

[Chuck H]

You don't need a power rheostat for adjustable cathode bias. It can be done as below. It has zeners for fixed bias but it will work just the same if the zeners are replaced by an R/C for cathode bias.

[ATTACH=CONFIG]42085[/ATTACH]

That would appear to partly bypass a "fixed" cathode bias. I wouldn't expect much of the same self biasing that one expects from a typical cathode bias. To do that you'd need something like an expensive power rheostat

I saw another zener based adjustable cathode bias scheme once. IIRC it routed voltage from the top of the slightly oversized cathode resistor through a divider and the adjusted voltage could be applied to the grids. So the cathode resistor still pumps with additional current but a fixed voltage at the grids determined the actual starting bias point. Clever.

[Dave H]

That would appear to partly bypass a "fixed" cathode bias. I wouldn't expect much of the same self biasing that one expects from a typical cathode bias. To do that you'd need something like an expensive power rheostat

I could only find the fixed bias schematic yesterday and was too lazy to mod it. It's done now. Here's the cathode bias version, no rheostat required.

[reddart]

Thanks for the comments.

I want to avoid conventional cathode bias, as I built the amp to be fixed bias because I aready have a similar amp that is cathode bias, and want something different.

In my "mind", the back bias might be in between the stifness of a cathode bias and standard fixed bias?

The amp is using 6BM8's, and the voltage as it stands right now is 309V plate voltage and 305 screens. Was shooting for <300 ish when I designed it, but went higher than I wanted (and per specs of the 6BM8's, I think 300V screen max, but plates can be higher?)

The "easiest" solution I guess is just plug it into a varaic and lower it to <300V ....the heater voltage will still likely be in spec.

The back bias also appeals to me as I often like to do different things just for the knowledged gained. I was mainly curious if anyone had tried it and if they had any opinions about its performance...so maybe I'll just have to try and see myself.

[Chuck H]

Cool on being unconventional just for it's own sake. I wouldn't sweat an extra 5V though. That's not even 2% difference! It's not like the spec is a hard line after which a catastrophic explosion occurs

A bias source that uses voltage dropped with current across a resistor will rise in voltage as current increases. Just like a cathode bias does. So if you want it to be distinctly different in behavior from cathode bias you'll probably want to use one of the zener based circuits.

[reddart]

By the way, this is the same amp as the other thread "Blackface-ish...", I wanted to decouple the topics so as not to have too much on one thread.

[Chuck H]

[QUOTE=reddart;445093]By the way, this is the same amp as the other thread "Blackface

Figured

[Shadrock2]

I thought I'd throw my 2 cents in here since I built a back biased amp. I designed the circuit around tubes and iron that I already had and had to scrub a lot of voltage cause my power trans was
370 - 0 -370. The first schematic is the original version and the amp evolved into the second version which I finally got up enough courage to play through at a gig last month.

The main difference in the bias circuit is exchanging the 220 ohm resistor for a 75 ohm one and adding another zener to make up the lost voltage drop. The first version would drop the bias voltage more negative by more than 10V and this resulted in a noticeable loss of volume on a sustained note after about a second. The circuit change completely eliminated that problem and now it can do a pretty good imitation of Santana like sustain. Measured voltage drop is now only 2 - 3V.