Calculations and the bias supply circuit

[Gibsonman63]

I am working on adding individual external bias pots to my Express for each power tube. I understand how to calculate voltage drop across series resistance, but the half wave rectifier is throwing me off.

For example, refering the the Express Schematic, there is a 220K resistor between the transformer and the diode. Lowering this resistance gives the 25k pot downstream more range to adjust the bias. I know that they interact much as they would if this was purely an AC or DC circuit, but since I am working with AC upstream of the diode and DC downstream, I am not clear on how to calculate it.

Anyone willing to share some of the math behind this?

[Gibsonman63]

I read Merlin Blencowe's chapter on Valve Bias Supplies and I am still not figuring it out. He highly recommends a full wave rectifier instead of half wave. Has anybody done this in an express?

"Full-wave rectification is easier to filter, allows faster charging time and avoids transformer saturation" - pg. 182

Maybe it's time to invest in a breadboard and play around until I get the right values. Most of his examples do not have the resistor between the transformer and the diode. If it tie both bias adjust pots to ground through the same resistor, wouldn't that serve the same purpose? Then all of my resistors would be on the same side of the rectifier and I can calculate the voltage drops.

[tictac]

What I do is put a 1 meg resistor in series with the bias pot and ground.

Then I clip an ajustable resistor load across the 1 meg and bring the amp up to voltage with a variac while monitoring the bias current of each tube.

While slowly increasing the voltage I monitor the current of the tube with the highest current draw and ajust the resistor load for the correct bias current with the bias pot in the middle of it's rotation.

When the mains voltage is on full and the bias current is correct then simply replace the adjustable resistor load with a resistor of the same value and you're done.

This works whether you are using a resistor for each bias pot or just one and you can alway change the resistor if you want to accomidate different tubes...

TT

[leadfootdriver]

ttt

[thejaf]

I don't think you can full-wave rectify a single AC bias tap (there's only one leg to rectify). Also, I may be an ignoramus here, but I cry BS on transformer saturation from bias voltage feed...there's very little current draw from the bias circuit.

[Alexo]

+1 on everything.

You can use both PT legs and full-wave rectify the bias supply if you want to, but you are solving a problem that is not there and using extra parts, and you know what they say: "mo' parts, mo' problems."

I always get the maths wrong when a half-wave rectifier meets a resistive divider that spans both the AC and DC portions of the circuit.

So I second the notion of making a ballpark guess that errs towards a cold bias and then tweaking with a pot to find the right value once the amp is mostly built.

Even if I could get that math right, the bias voltage that works in the real world is usually a bit off from what the figures predict.

[Gibsonman63]

You may be right about solving problems that don't exist. I am starting to rethink the need to have individually adjustable bias adjustment as well as the full wave rectifier. It makes things much simpler without it.

Back when I was in electronics school (25 years ago!), we would spend two hours on theory and math and two hours breadboarding and testing, five days a week. I am finding it much harder to get that deep into the math now that my education is self directed. The 21 year gap in my electronics experience isn't helping either.

[thejaf]

Exactly. My dad has 40 years of professional electical experience, and his #1 advice to me with guitar amps was "these tube circuits are VERY forgiving" to trial and error. Just keep it in the general ballpark, don't try anything too wacky with the high voltage stuff, and you probably won't end up with a ball of smoke. Translate: trial and error is usually much quicker than theory, and you're not likely to fry anything if you are careful.

[gearhead]

Yeah, I was in basic stages of EE program 30 years ago myself. Spent the first quarter on semiconductor calculations out to the third digit ( .XXX). No credit if not exact. When we finally hit the labs the next quarter, I was pretty furious. Variance in manufacturing gets you +/- a lot. Crickee.

[tubeswell]

Figure on the peak DCV from the bias rectifier diode and the bias filter cap to be 1.4142 x what the bias supply VAC is. (e.g. if you are starting with 50VAC, then a 1/2-wave 1N4007 and a 10uF filter cap get you to about -70VDC (-69)). There is almost no current draw on the bias supply (maybe a milliamp or two tops), so this is one instance where a 1/2-wave rectifier is efficient enough to do the job without noticeable ripple current.

Then you can design your bias supply voltage divider to get you in the ballpark and tweak it with a bias pot.