Peavey Mace 320T

[kkatarn327]

I have recently acquired a Peavey Mace 320T 2x12" 160W combo, solid state preamp, 6x6L6 poweramp. Minimal research shows that it is a somewhat sought after amp, also being Gary Rossington's choice for so long, so I decided I'd stick with it.

The amp was DOA. The power lamp came on and the tubes heated up but that was about it. I soon noticed that the 10W 400R screen resistor was burned out, and upon replacing it with the same type and value, the amp works again... however not without some problems.

First of all, the amp produces a very loud, present and consistent hum that is unaffected by the controls or whether or not an input is plugged in. Secondly, the three tube sockets on the OT side redplate pretty bad, and it seems to be affected by volume/playing intensity.

My plan begins with buying a matched 6L6GC sextet, but I have a feeling these issues go beyond just getting new tubes, especially since the redplating stays with the socket. I already cleaned all the sockets with DeoxIT and reflowed solder on all the PCB pins, as well as checking connectivity across the entire power tube PCB.

I also noticed that the power switch is an ON-OFF-ON/polarity switch type even though it has a three-prong power cable (which may be aftermarket) and I did notice the two death caps in there. I plan on converting this to the non-lethal power switch assembly just as soon as I get the other issues resolved... the amp might be so messed up that I might not even get to that point and I don't want to waste any money or time.

Any ideas what might be going on here? I'm crossing my fingers that mismatched power tubes are causing both problems but I've never had that kind of luck.

P.S. For those interested I have attached the Peavey owner's manual for this amp which includes a full schematic.peaveymace320t.pdf

[52 Bill]

The three way switch and the 3 prong power cable are factory original. What you choose to do it is up to you, as it's your amp.

Have you read the bias voltage on the grids of the outputs? Check all pin 5 voltages and see if they are different one side to the other. They should all be around -55vdc. If not investigate the bias supply resistors, coupling caps, the driver stage transistors, etc.

[Amp Kat]

Not being nit-picky but when you said 400ohm that is actually the dropping resistor that takes place of the choke that does go to the screen supply but are not screen resistors. If that fried tells me either a tube short or possible filter cap shorts as well as diodes. So check the incoming diodes very good for shorts and take note of plate voltages and power supply voltages with the tubes out. A light bulb limiter would weigh in bigtime here as you want to make sure all is well before inserting tubes that could cause even more damage if the supply isn't right. The filter caps in that thing if not changed are getting old and new caps would make a big difference in tone and being that it is humming is a sign possibly of that . Some leave them in only to face other problems in the near future. The bias supply I believe has two seperate leads a brown and blue going to each 3 side of tubes so make sure they are both reading the -55 as Bill suggested. You don't have to run all 6 tubes to test but you do need at least 1 and 1 on each side would be better.

[Enzo]

I agree. All it takes is one bad tube to shunt off the bias for a whole side. Pull all the power tubes and see if the bias voltage is present on all 6 sockets.

And if you eyeball the AC mains wiring you should find it matches your schematic. The three wire system prevents "death-cap injuries. and though technically it no longer matters, it never hurts to have a polarity reverse swich.

[kkatarn327]

The plate voltage on both sides with the amp on standby is 510VDC. With the amp live I get 404VDC on the "good" side and 380VDC on the "bad" side (the side that redplates). From the bias supply I get -46VDC on the good side and the bad side reads nothing. The 220K bias supply resistor to the good side of the amp checks out fine but when I check the other one the multimeter starts at around 20K then keeps rising until it stops at 50K which leads me to think that there is a capacitor problem. The six individual 47K resistors from that point to the tubes all test fine though. All the screen grid resistors (6x 100R/5W ceramic-type) test out fine as well.

I suppose this points to the coupling capacitors. How can I identify/test the faulty capacitor?

[Enzo]

Unsolder one end and lift it from the circuit. DO DC levels restore?

[kkatarn327]

I guess I'm still unsure which caps would be the coupling caps. Are they they 0.1uF/600V pair between the bias supply and the driver transistors, or the 10uF/150V pair by the bias pot? Or am I looking at the big 350V cans?

[52 Bill]

I guess I'm still unsure which caps would be the coupling caps. Are they they 0.1uF/600V pair between the bias supply and the driver transistors, or the 10uF/150V pair by the bias pot? Or am I looking at the big 350V cans?

The name "coupling cap" should lead you to the answer. It is a capacitor that couples one stage to the next.

If either of the 0.1uF caps leaks any positive voltage from the driver stage, that positive voltage will reduce or eliminate the negative bias voltage from the power supply and cause problems.

So the answer is the 0.1uf caps.

[kkatarn327]

The name "coupling cap" should lead you to the answer. It is a capacitor that couples one stage to the next.

If either of the 0.1uF caps leaks any positive voltage from the driver stage, that positive voltage will reduce or eliminate the negative bias voltage from the power supply and cause problems.

So the answer is the 0.1uf caps.

Ok that makes sense. If you could forgive my ignorance a little further, what type of capacitor should I use to replace these? The schematic shows polarized caps but the actual caps on the board look like normal film caps, albeit a little bigger.

[52 Bill]

I don't think you are being ignorant, I'm hoping that you are learning about this stuff from these posts and not just blindly following someone's directives.

Before you replace the caps, test them for leakage. Do as Enzo suggested, on the side with no bias voltage, unsolder the cap from the 220K ohm resistor. Does the bias voltage come back to pin 5?

[kkatarn327]

That seems to do the trick. With the cap removed I see -48VDC on both sides. I think just to be safe I'll replace both of them and even look into how much it will cost to recap the whole board. What type capacitor should I use for these coupling caps?

[kkatarn327]

So I ordered a pair of .1uF/600V Sprague caps online a week ago but they still haven't shipped yet (not to mention that shipping cost an arm and a leg) so I'm beginning to consider cancelling that order and just buying the parts locally. Unfortunately the local shops don't have the .1uF/600V value, the closest ones they have are 1uF/600V and .01uF/600V mylar caps. Would the 1uF cap be a close enough value to work in this application?

[52 Bill]

I'd say no. The amp will work, but there probably will not be enough room to fit them in the amp. And the frequency response of the amp will be greatly altered.

[kkatarn327]

And I'm guessing smaller (.01uF) is a no go too huh?