Dual bias vs single

[daz]

I simulated a bias circuit once that showed what might be going on here. Daz's circuit does not use a bias tap, it uses a big resistor (220K) from the HV winding. In my simulation the cap (might have been 100uF, I don't remember) hadn't charged up to it's final value after 60 seconds.

I'm not sure what you mean, but mine is exactly the same as a JCM800, and i imagine the same as most marshalls. In fact i copied the bias circuit from the 800 i believe. I only made it dual after a few years after i built it. Just like the 800 it taps from the HT before the rectifier thru a 220k. What do you mean by "bias tap"? You mean a dedicated winding? I don't see that in any marshall schematics. No disrespect, just asking because im unsure what you mean.

That said, just tested it at volume and dual or single all sounds the same when turned up. Still fighting that hard tone.

[loudthud]

There are some Marshall amps from the G&W Unicord days (~1970) that have bias windings.

When SGM mentioned 100uF, he was talking about the typical Fender bias supply which has 1K or 470 ohms in series with the rectifier. The RC time constant is .07 seconds (1K 70uF). With 220K and 10uF the time constant is 2.2 seconds, 22 seconds if you use 100uF. It takes five time constants for a capacitor to charge to 99% of final value. But the point of my post was that the bias voltage will still be moving around a minute after you flip the amp on if you are using 220K and 100uF. As to SGM's assertion that the amount of capacitance makes a difference that is clearly audible, I can't comment because I have never experimented with that but I can see where it might have an influence on the Feel of a guitar amp. There are things that happen in tube guitar amps at sub-sonic frequencies that give them a feel that you just don't get with opamps and clipping diodes.

[GainFreak]

When talking about individual/dual bias I didn't mean any vintage bias circuits but a case when a dedicated/separate bias winding is available. Then I use circuit/s like these:



What I do is set the bias trimpot to coldest desirable bias let's say 20mA tweak R1, R2, R19 to achieve it, then set the bias trimpot to the hottest bias say 40mA and tweak R3, R4 (dual pot) and R7 . Sometimes I would add C4, C5 and C10. I usually end up with the values from the schematic.

[daz]

Well, since I determined the difference is not what I 1st thought, at least not at volume, I no longer have a reason to go with single. But I do have another question. I tried losing one of the 220k resistors and diode and used a single supply line for both sides. I now see what the benefit is to doing it the way i did, with supply lines. The two sides don't affect each other near as much. With one supply you adjust one side and it changes the other, so theres a lot of going back and fourth. But it's really not a problem, so a single supply would be fine with me. In case anyone isn't clear on what i'm saying, i am NOT talking about single bias supply vs dual. Look at my schematic....i'm talking about removing one of the 220k/diode supply lines and using only one to supply both sides of the dual bias. So the question is, does anyone see a reason to change to a single supply?

[Dave H]

...With one supply you adjust one side and it changes the other, so theres a lot of going back and fourth.

That is because in your circuit the bias is set with a variable resistor which will change the loading on the bias supply and affect the other side. If the bias control is a potentiometer as in the two circuits GainFreak posted it won’t interact with the other side.

...i'm talking about removing one of the 220k/diode supply lines and using only one to supply both sides of the dual bias. So the question is, does anyone see a reason to change to a single supply?

If it ain't broke, don't fix it.

[daz]

That is because in your circuit the bias is set with a variable resistor which will change the loading on the bias supply and affect the other side. If the bias control is a potentiometer as in the two circuits GainFreak posted it won’t interact with the other side.



If it ain't broke, don't fix it.

I didn't say mine does that. It's the SINGLE supply line that i found one bias pot affects the other side. When i removed one and used a single supply, thats when the sides were affected by each other.

[Dave H]

I didn't say mine does that. It's the SINGLE supply line that i found one bias pot affects the other side. When i removed one and used a single supply, thats when the sides were affected by each other.

Sorry, I understand that. I just didn't word it very well. Try this.

"That is because in your circuit the bias is set with a variable resistor which will change the loading on a SINGLE bias supply and affect the other side."

[daz]

Sorry, I understand that. I just didn't word it very well. Try this.

"That is because in your circuit the bias is set with a variable resistor which will change the loading on a SINGLE bias supply and affect the other side."

Ok, then the question is, if mine is more stable in that regard, why do people use the single supply for dual bias? The manufacturers mostly use a single bias supply, it's really only (or at least mostly) the builders that build for themselves like us who make dual supplies. So the old "to save $.50" rule doesn't apply.

[Enzo]

Because in the real world, bias is merely a "close enough" adjustment. Like tire pressure in your car. Does anyone measure tire pressure down to 1/100 of a pound pressure? Do we adjust tire pressure when we load extra weight into the car? Do we monitor it while we drive? No, we fill the four tires to more or less the same pressure and drive on. Some people decide they have to have ultra precise bias settings, some want to adjust individual tubes. But most don;t. Other than when a circuit is defective, I don;t recall many bias adjustments that were not "stable." The old Fender balance pot we convert to bias level on a zillion amps. Well like it or hate it, it usually sat right there unchanging once it was set. If you hear a benefit, then great. But for many it would be more like "oh, so now i have to make TWO adjustments instead of just one." And as to interaction, every bias adjustment affects B+ - you increase the tube current and it pulls down the B+, so you have to continually go back and watch that. And like the three-pin Marshall adjustment for the two sides individually, well, you adjust one side it affects the other for just that reason. SO instead of one little twiddle, I have to make a couple, then check the results then a couple more fine tube twiddles.

[Dave H]

Ok, then the question is, if mine is more stable in that regard, why do people use the single supply for dual bias?

Probably because they uses similar circuits to the ones GainFreak posted. Your circuit is more stable on a dual supply but GainFreak's would be equally stable on either a single or dual supply (or more likely it's as Enzo said).

[g1]

Daz, I think there may be a little confusion with the terminology here.
To rephrase your question, I think you're asking
"why use a single rectifier dual bias set-up rather than a dual rectifier dual bias set-up".

But I think DaveH mentioned your circuit is shown with the trimmers used as rheostats rather than potentiometers, so you would have more loading effect.
Or are you using Gainfreak's bias circuit for your experiment?

[daz]

Daz, I think there may be a little confusion with the terminology here.
To rephrase your question, I think you're asking
"why use a single rectifier dual bias set-up rather than a dual rectifier dual bias set-up".

But I think DaveH mentioned your circuit is shown with the trimmers used as rheostats rather than potentiometers, so you would have more loading effect.
Or are you using Gainfreak's bias circuit for your experiment?

Mine is as in my schematic, i haven't changed that at all except to experiment with 1-removing one of the rectifiers and supplying both sides of the dual bias with one instead of 2. and 2-ialso tried single bias. None of it matters, as i went back to the schematic's circuit exactly. Mine is exactly the same as a 2204 X2.