Not Enough Bias Voltage?

[PepeSilvia]

Hey guys, I've posted about this amp a time or two, but I've got an old Peavey Butcher and I'm in the process of adding a bias circuit to it. I think the way I set it up is of little consequence to what I'm seeing, but I'll describe it anyways. I used the same process this gentleman did Half Nrrdy, Half Music, Half Absurd – Always poor at Math » The Peavey Butcher There's a 47kOhm and 470Ohm resistor creating a voltage divider that supplies the bias voltage. I replaced the 470Ohm resistor with a 1KOhm pot. The problem I'm having is that I'm just not getting enough current. When I completely peg the pot, so it's a short, the most current I get is ~125mA for the four tubes. The voltage at the grid resistor is ~-56V. Can weak tubes cause this sort of problem?

Thanks!

[Amp Kat]

Are you saying that you can't make the bias voltage any lower than -56 or should I say less negative going toward zero ? What type of range do you have ? If it won't go less negative you may not have it wired right and may be getting the set bias voltage. Peavey biases that amp right at -55 volts which is a tad on the cold side.

[PepeSilvia]

I am unable to make the bias voltage get further from zero.

[Amp Kat]

You don't want it to go further if you're trying to increase the current you want it to go towards zero somewhere around probably -48 to -50 should do it. Increasing it to -60 would decrease the current even more.

[Enzo]

You are making the wrong resistor variable. Replace the 470 ohm upper resistor with something like 3-4k. 3300 or 3900 ohms maybe. I use 3300. Now make the 47k variable.

Look at the schematic. You start with -57v, and you have two resistors to divide it down. If you leave the bottom one 47k, then a 1k variable top resistor varies the bias from -57 down to -56. No range at all. Just making the 470 into 3300 drops the bias to -53v.

Replace the 47k resistor with a 22k resistor and 25k pot in series. Now we have a variable resistor instead of the 47k. We can now vary from 22k with the pot at zero up to 47k with the pot at max. Turning the pot to zero leaves the 22k resistor as a minimum. And at that 22k, the bias drops to 22/25.3 x -57 = -49v. Want even hotter? Then drop that 22k to 15k.

[Jazz P Bass]

125 ma for (4) tubes.
That is 31 ma per tube.
How hot do you want it?
Please keep in mind we are talking "idle current".

[Enzo]

If he is going for the "70% rule" then at 500v or so on the plate, he is looking for about 42ma. Personally I am a lot happier a few amperes lower than that.

[PepeSilvia]

The 70% rule is what I'm going for. I think I'm starting to get this a bit better. Per the graphs here: The 6L6 / 5881 Tube in triode mode, which is what this amp is connected in, the closer grid voltage is to zero, the higher the current flowing through the cathode, correct?

[Amp Kat]

I don't think the Butcher is wired in Triode Mode. It is wired in Pentode Mode.

[Enzo]

The Butcher is not remotely wired as triodes. It is a plain old ordinary Class AB1 push pull pentode amp. Loud.

Yes, the lower the bias voltage, the more current the tube conducts. That will be the case in triode or pentode arrangements. If that is what you want to do, then target your idle dissipatiion at about 20 watts.

[Albert Kreuzer]

The 70% rule is what I'm going for.

That's not a rule, that's the recommended maximum. Most amps sound good at 50-60%, some need even less.