Bias Mod For Peavey Mace VT

[TMOD_121]

Hi all,

I'm considering a bias mod on my Peavey Mace VT and have failed to find any solid info on this. I've read about 4 suggested ways to do this mod and all are totally different. Most of the mods do reference IR9 and sometimes IR10. Instead of controlling the voltage upstream of all 6 tubes I would like the capability of adjusting bias at each individual tube. Any ideas?

Thanks

[Enzo]

Totally different? Or just not identical? You didn;t provide links to the methods you found. There can be various ways of doing thing, doesn't make competing ways wrong.

Adjusting the bias is nothing more than changing the bias voltage. The bias supply starts out at ABOUT 76 volts across C11. IR8 and IR9 form a voltage divider across that. The bias voltage is taken from their junction. IR10 is just part of the filter for the supply and doesn't affect voltage. To make any voltage divider variable (adjustable), you need to be able to change the ratio of the two resistors. The simplest way to do that would be to make IR9 variable.

IR9 is now 68k. I could replace it with a 68k pot wired as a variable resistor if I wanted. Not sure I ever saw a 68k pot. The thing is, if I just replace IR9 with a pot like that, it is then possible to dial the bias down to zero volts. That would burn up your tubes pretty quick. So instead of just a pot, we wire a pot plus resistor in series, and wire THAT in place of IR9. Howe about a 50k pot and an 18k resistor. Those are common values, AND they add up to 68k. Now with the pot all the way up, we have 68k. With the pot all the way down, we still have the 18k, so can;t turn it to zero. Me, I;d probably use a larger resistor, so that the pot itself has a wider useful range. 47k resistor and a 20k pot, or maybe a 39k resistor and a 25k pot. It really is not critical at all.


ALmost no one wants to run these cooler than stock, so you never nbeed higher voltage than stock, so the total we replace IR9 with can be somewhat smaller than 68k and still be just fine.

Now the other ways. COuld we do about the same thing by making IR8 variable instead? Sure. But I think the range will be tons better my first way. COUld we replace BOTH IR8 and IR9 with a pot and have the wiper be the bias feed? Yes, that would work. 100k pot maybe. Or smaller pot plus resistor for reasons I explained earlier. But that leaves us dependent upon the wiper remaining in contact with the pot resistance. If the wiper fails, you lose bias. Tubes burn. I ALWAYS wire bias pots as variable resistors, so if the pot wiper fails, the bias just reverts to the max voltage (coldest setting).

All six tubes individually? It can be done of course, but my personal view is that would border on the silly. These amps are designed with a loud clear strong power amp. Can't imagine you;'d be going for power tube distortion. There are six tubes, they tend to average out anyway.

But you want it? OK. Start with that -76v on C11. Now duplicate the rest of the supply to the right of that point five more times. That is make up identical circuits of IR8,9,10, and C12,13. All six of those circuits are now running off C11. It can handle it because ther is no current demand upon the bias supply. Each C13 is now an individual bias supply. You need to modify that to include the variable element we discussed earlier.

Now that you have six bias circuits, you need to rewire the power tube board for them. All those 47k grid stopper resistors have to become independent. Each bias C13 will need a 220k resistor running to the 47k for its tube. And finally, the IC22 and IC23 need two more of each, so the phase inverter can send three separate signals to the three tubes on each side.

Really not worth the effort - just in my opinion of course.

[TMOD_121]

Totally different? Or just not identical? You didn;t provide links to the methods you found. There can be various ways of doing thing, doesn't make competing ways wrong.

Adjusting the bias is nothing more than changing the bias voltage. The bias supply starts out at ABOUT 76 volts across C11. IR8 and IR9 form a voltage divider across that. The bias voltage is taken from their junction. IR10 is just part of the filter for the supply and doesn't affect voltage. To make any voltage divider variable (adjustable), you need to be able to change the ratio of the two resistors. The simplest way to do that would be to make IR9 variable.

IR9 is now 68k. I could replace it with a 68k pot wired as a variable resistor if I wanted. Not sure I ever saw a 68k pot. The thing is, if I just replace IR9 with a pot like that, it is then possible to dial the bias down to zero volts. That would burn up your tubes pretty quick. So instead of just a pot, we wire a pot plus resistor in series, and wire THAT in place of IR9. Howe about a 50k pot and an 18k resistor. Those are common values, AND they add up to 68k. Now with the pot all the way up, we have 68k. With the pot all the way down, we still have the 18k, so can;t turn it to zero. Me, I;d probably use a larger resistor, so that the pot itself has a wider useful range. 47k resistor and a 20k pot, or maybe a 39k resistor and a 25k pot. It really is not critical at all.


ALmost no one wants to run these cooler than stock, so you never nbeed higher voltage than stock, so the total we replace IR9 with can be somewhat smaller than 68k and still be just fine.

Now the other ways. COuld we do about the same thing by making IR8 variable instead? Sure. But I think the range will be tons better my first way. COUld we replace BOTH IR8 and IR9 with a pot and have the wiper be the bias feed? Yes, that would work. 100k pot maybe. Or smaller pot plus resistor for reasons I explained earlier. But that leaves us dependent upon the wiper remaining in contact with the pot resistance. If the wiper fails, you lose bias. Tubes burn. I ALWAYS wire bias pots as variable resistors, so if the pot wiper fails, the bias just reverts to the max voltage (coldest setting).

All six tubes individually? It can be done of course, but my personal view is that would border on the silly. These amps are designed with a loud clear strong power amp. Can't imagine you;'d be going for power tube distortion. There are six tubes, they tend to average out anyway.

But you want it? OK. Start with that -76v on C11. Now duplicate the rest of the supply to the right of that point five more times. That is make up identical circuits of IR8,9,10, and C12,13. All six of those circuits are now running off C11. It can handle it because ther is no current demand upon the bias supply. Each C13 is now an individual bias supply. You need to modify that to include the variable element we discussed earlier.

Now that you have six bias circuits, you need to rewire the power tube board for them. All those 47k grid stopper resistors have to become independent. Each bias C13 will need a 220k resistor running to the 47k for its tube. And finally, the IC22 and IC23 need two more of each, so the phase inverter can send three separate signals to the three tubes on each side.

Really not worth the effort - just in my opinion of course.

Thanks Envo for the reply! A very clear and thorough explanation. For now I will do the single bias adjustment. My only concerns with that initially were what if the current was out of range on several tubes after replacement. I realize that is why you always should buy a MATCHED set to prevent that. But if that issue did arise there wouldn't be a way to bias the odd balls into range. I will replace all tubes with a matched set and measure and record current for all 6. Once I have my current values for each tube which tube/value would you recommend referencing while making bias adjustments? I'm assuming I would need to re-connect the bias probe to the tube that had the highest current reading? Also, who would you recommend contacting for 6 matched tubes.

Thanks again for the very informative explanation.

[Enzo]

Keep in mind that while we all like to use matched sets of tubes, Peavey and all the other brands do not use matched tubes. They grab 6 tubes from the big box 'o tubes, and plug them in.