Peavey Mace power amp modification

[adam]

I have an old Peavey Mace that I'm looking modify. Stock, the power amp runs 6 6L6's which is what you could call over kill. Currently I've been running the amp with just one 6L6 into an 8ohm load with no problems. It makes for a good recording amp. But what I'd like to do is modify the power amp to run a pair of 6L6's. I was under the impression that in a fixed bias amp you could easily do this by pulling power tubes in a symmetrical manner. Which would give me either 1, 2, 4, or 6 tubes in the power amp. However this doesn't work. As soon as the power switch is flipped I blow the fuse. So I assume that this makes this amp cathode biased which would not allow this method of power reduction. Is this correct? Regardless, I was hoping to get some advice as to the best route to reach my goal. thanks adam

[Enzo]

As soon as the power switch is flipped I blow the fuse. So I assume that this makes this amp cathode biased which would not allow this method of power reduction. Is this correct?

I thought you were running ONE tube and it worked.

When you say power switch do you mean the main power or the standby switch? If the main power switch, and hte mains fuse blows, first thing I ask is are all the 6L6s in the sockets the right way? As in are any lacking their center posts?

Make sure the flyback diodes are not shorted. Make sure none of the 6L6s are shorted. The plate voltage is present even in standby.

I don't follow your cathode bias theory. I can't imagine an amp of this power runnning cathode bias, but even so, pulling tubes wouldn't pop the fuse. This amp is fixed bias, the cathodes are all grounded. You should be able to install anywhere from zero to six power tubes and no fuses blow.

I suspect a bad tube in your set.

A pair of 6L6s would be preferred to just one. At low levels it might sound OK< but any level over small will run into lopsided distortion.

[adam]

The amp was running fine with just one tube. So that means that it is definitely fixed bias because I could only do that in a fixed bias configuration? By power switch I meant the main power. standby never mabe it on. All 6L6's were in correctly. If I had a short some where would I still be able to get this to run with only the one tube installed or would a short over one socket affect all sockets? I like the bad tube theory. That seems to make the most sense. Maybe I'll give this try with another set of tubes.

[Amp Kat]

There are also two bias legs on that amp although they are from the same voltage source one brown wire and one blue. I agree that if one tube works it's biased to 100% dissipation which will fry it very quickly at that high of a plate voltage. Can't see any reason why two tubes couldn't be used if they are both good. Check pin 5 of each side and make sure your around -50 volts are so. If biased right using only one tube should sound very bad.

[Enzo]

OK, so the fuse blows without turning the standby on, which means it does NOT work fine with one tube. Don't care what it "used to do."

it is fixed bias because that is what the circuit is. If it were cathode bias, the bias might wind up all wrong, but it would not blow fuses just because there was only one tube in it.

You have a shorted power tube, a shorted flyback diode, a shorted B+ rectifier, a shorted main B+ filter cap, a short to frame on the output tranny. In about that order.

Pull all the power tubes, and power up the amp. If teh fuse no longer blows, that means you have a bad power tube. If the fuse still blows, the amp circuits have an issue. That is when I start looking at flyback diodes etc.

[loudthud]

Mace amps are notorious for developing bad connections between the power tube sockets and the PC board. The best thing is to suck off as much solder as possible and use a No-Lead solder like you can find at Radio Shack. It melts at a higher temperature and is stronger. Also re-tention the sockets.

[adam]

Thanks for everyones advice. I suppose I will give this puppy the once over twice this weekend and see what I find.