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

[frus]

Why do you need the 75 ohm resistor at all? You could have gone all zener, according to the article

[Shadrock2]

Two reasons... The main reason is that I guess I just don't completely trust silicon devices. So if for some reason the zeners short out there will still be "some" negative bias on the grids. The second reason is just my opinion and probably a little over the top. Feel free to criticize. But the Aiken article talks about compensating for line voltage variations as well as developing cathode bias behavior in the amp. So with the resister I have at least a small amount of both of those characteristics.

[reddart]

Thanks for the info Shadrock.

It sounds like there might be a practical limit to how much current/voltage you can back bias before you get over compensation and cutting of the power due to the bias voltage rise (with just a resistor).

After thinking about everything a few times, I think the way I will proceed is:
1-do nothing/or variac it a little while I am debuging the other issues.

2-If being over the max V of the screens still keeps me up at night after fixing the other problems (ringing/motarboating/layout), since I seem to be OK with going in between cathode bias and fixed bias, that would probably be the easiest solution...partial cathode partial fixed bias. Since I already have the (adjustable) fixed bias circuit, I can just add smalish resistor and cap to cut the screen/plate : cathode voltage a bit.

[Chuck H]

You can read around the forum and there's some threads where I advocate using zener across the cathode resistor in cathode biased amps to limit the voltage rise. I set mine up to stop rising a little past where the power tubes start clipping. That way I have cathode bias "effect" for clean tones but it transitions into fixed bias for overdrive. It also greatly reduces crossover distortion because the bias doesn't continue to get colder and colder with current. Works a treat if you think you might like that sort of thing.

[Justin Thomas]

I may be in the minority opinion here, but I <REALLY> liked the sound of my '69 Dual Showman Reverb, that had the 'mixed bias," supposedly the circuit that made the silverface amps so awful. I think that amp sounded awesome, and I'd be more than willing to try that circuit out in a future build.

Just thought I'd mention it, since you were talking "a little bit cathode bias, a little bit fixed bias..." And no silicon devices involved, except the one to derive the fixed bias supply...

Justin

[Chuck H]

Turns out there are a few amps that are well respected that use the combination bias. I can't remember what any of them were, but it was discussed here once. IIRC the conclusion was that most of those amps ended up biased too cold from the factory and from techs and that's actually what players didn't like. YMMV

[reddart]

You can read around the forum and there's some threads where I advocate using zener across the cathode resistor in cathode biased amps to limit the voltage rise. I set mine up to stop rising a little past where the power tubes start clipping. That way I have cathode bias "effect" for clean tones but it transitions into fixed bias for overdrive. It also greatly reduces crossover distortion because the bias doesn't continue to get colder and colder with current. Works a treat if you think you might like that sort of thing.

That sounds like something I'd like to try.

[reddart]

BTW, bypassing the resistor in the back bias on Shadrock's amp might also work as well (for the original problem of it over biasing)?

[Shadrock2]

It was suggested before that I should increase the size of the bypass cap.
100 uF is on the small side. The Aiken circuit used 470 uF I if I remember right.

[reddart]

It was suggested before that I should increase the size of the bypass cap.
100 uF is on the small side. The Aiken circuit used 470 uF I if I remember right.

I was thinking with a zener....