Bias Blues

[sepulchre]

This amp was meant to use 5881 power tubes. The PT is a Classictone 40-18015. The mains are 300v @ 200ma, 6.3 @ 3.75a and 39v @ 3.5ma for bias.
I wired it up exactly as I have several others with minor tweaks for the power tubes, etc. The bias circuit looks like this. But there are 0 volts coming from it - nothing at all. I've checked every thing many times. The only way I can get any voltage (negative, of course) is by completely disconnecting the ground from the pot. But doing so makes the pot not adjust it at all.
Am I missing something? Has the universe changed?


Sorry about the dual pictures. Can't seem to fix it.
Activax Amplification

[xtian]

That leaves wiring error. Send photos!

[bal704]

You are using a bridge rectifier, so the bias circuitry is different. I had a similar problem with a build like this recently. I posted it here, and somebody chimed in with a working schematic for the bridge rectifier based circuit.

https://ampgarage.com/forum/viewtopic.p ... 9&start=15

[martin manning]

Do you really have a 220k in series with the bias rectifier? I think 220 ohm might be closer to what you want.

[martin manning]

You are using a bridge rectifier, so the bias circuitry is different. I had a similar problem with a build like this recently. I posted it here, and somebody chimed in with a working schematic for the bridge rectifier based circuit.

https://ampgarage.com/forum/viewtopic.p ... 9&start=15

This PT has a separate bias winding.

[pompeiisneaks]

Sorry about the dual pictures. Can't seem to fix it.
Activax Amplification

You attached the image twice. Delete one of them and it should resolve?

~Phil

[sluckey]

Do you really have a 220k in series with the bias rectifier? I think 220 ohm might be closer to what you want.

+1

[R.G.]

I wired it up exactly as I have several others with minor tweaks for the power tubes, etc. The bias circuit looks like this. But there are 0 volts coming from it - nothing at all. I've checked every thing many times. The only way I can get any voltage (negative, of course) is by completely disconnecting the ground from the pot. But doing so makes the pot not adjust it at all.
Am I missing something? Has the universe changed?

Let's play circuit swaps.

This thing is a capacitor-input rectifier/filter circuit, yes? That is, the voltage on that first cap will be the peak of the DC voltage sent to it by the rectifier, modified by any loading/dividing by the resistors. So let's "simplify" the circuit as one of my old professors used to say, to understand it.

Any series string of electrical components can be swapped into any order as long as you don't look at the voltages in the middle of the string. In particular, we can exchange the positions of the 220K and diode and make no difference at the first filter cap. So we do that.

Now the first filter cap is fed by a peak voltage from the diode through the 220K. The first filter cap is loaded by a 15K, a 25K, and a 47K to ground. That's 87K, total. There is that second cap, but in theory it eats no DC current. Right?

So we have a peak voltage through the diode into a 220K, filtered by a cap, then with an 87K to ground. The cap picks off the peak of the incoming AC, and holds it. However, the incoming peak of the AC is "divided" by the 220K and 87K to V = 87/(220+87) = 0.283 times the incoming AC peak.

The incoming AC peak is 1.414 times the AC rms, so you should get out the RMS voltage of the bias winding times 1.414 * 0.283 for a voltage on the first filter cap.

If you don't get this voltage, there are only thre possibilities. One is that the circuit is not wired up the way you think it is; two is that some component is not the value/type you thing it is, and three is that a component (including the "components" loading the bias pot) is not functioning properly.

Checking your wiring is easy enough to do with a DMM set to measure continuity from part to part. You might also want to measure resistance to ground at each node to be sure you don't have a solder blob or some such confounding you.

Obviously, the caps might be too leaky. I always confuse polarity in negative voltage supplies, so you might check and be super sure the polarity is correct. In fact, if you remove the caps, temporarily disconnect them, and temporarily feed DC into the circuit instead of the bias winding, the voltage ratios still ought to come out the same. This might be a handy way to debug. A 9V battery instead of the bias winding, or some such will make this into a DC debugging problem, and much easier.

If it were me wiring this up, I'd have gotten the polarity of one of the caps wrong, damaged it, the "corrected" the mistake and made one of them permanently leaky. But that's just my cross to bear.

[Ten Over]

39Vrms for the bias winding is 55Vpeak and multiplying by 87K/307K is 15.6V. But we only get maybe 6V. What up?

I'm going to explain this for a positive power supply. To apply it to a negative bias supply, stand on your head.

The first capacitor after the rectifier only charges for a fraction of each positive pulse because the capacitor doesn't start charging until the voltage of the positive pulse is greater than the voltage on the capacitor. If we could supply infinite current with that positive pulse, it would charge the capacitor up to the peak voltage of the pulse. If we start restricting the charging current, there will not be enough time to fully charge the capacitor before the pulse starts to decline and the average DC voltage on the capacitor will, thus, become smaller. As the resistor that restricts charging current gets larger, the DC voltage on the first capacitor gets smaller.

How fast the first capacitor discharges after the pulse stops charging it also affects the average DC voltage. A faster discharge not only decreases the average mathematically, it also means that the pulse starts charging the capacitor at a lower voltage and stops charging it at a lower voltage. Therefore, the DC voltage on the first capacitor is a function of the current-limiting resistor, the capacitor, and the discharge circuit.

[sluckey]

Do you really have a 220k in series with the bias rectifier? I think 220 ohm might be closer to what you want.

+++++++++++++1

[pompeiisneaks]

Do you really have a 220k in series with the bias rectifier? I think 220 ohm might be closer to what you want.

+++++++++++++1

OVER 9000 +

[R.G.]

Along the same lines, what happens if you actually DO swap the 220K and rectifier, then move the first filter cap to the diode?

The "any order for things in series" no longer applies to the rectifier and 220k resistor because now the first filter cap is mucking with the center connection between them. The bias cap is charged much harder, and now the 220K/87K really does do only a DC potential divider.

I think the difference between the two is probably that in the original setup, the cap is being charged by the voltage divider of 87K/307K, but also through the Thevenin equivalent series resistance of 87K || 220K = 62K, so the charging pulse is spread out too wide and the average voltage goes down, as well as the charging voltage being divided down.

[sluckey]

The point is that 220K is wayyyyy too big when the input AC is only 39V. That resistor needs to be on the order of 220Ω or 470Ω, or something under 1000Ω.

[R.G.]

Probably. I'm just speaking from my own way of looking at things.

My own personal quirk is that I think that if I understand what the resistors (and diodes and caps) are doing, I can adjust their values to get them to do whatever I want, within the bounds of real components. To me, every single one of those components comes with a value which is variable from 0 on up.

As I say, it's a personal quirk.

[Tony Bones]

I agree that 220K is way to big. Just because I can, I mocked it up in LTspice and found that he should still be getting between -3V and -5V bias, though it takes ~30 seconds to get there. If sepulchre is really getting ZERO volts then there must be a wiring error.