Peavey Mace VT Series

[Perkinsman]

This amp is all original, including the six 6L6’s, which I tested on a tube tester & were strong. It hasn’t been used for years. I brought it up on dbt & the bulb dimmed so I brought it up slowly on variac to full line voltage & it stabilized drawing about .5A. The controls were cleaned and the amp ran fine for 10 minutes before the current just jumped & I shut it down. Nothing smoked & the fuse wasn’t open. Do these symptoms indicate anything? I’m going to recap it but I’d like to identify the problem before I start just replacing parts.

[Jon Snell]

More than likely just failed capacitors.
Check the bias after replacing the electrolytics and maybe the two coupling capacitors for the output stage.

[Enzo]

That or a failed tube.

[g1]

Did you check the screen resistors after the surge?

[Perkinsman]

I just received the caps from Mouser late today so I'll be working on it tomorrow, I'll check the screen resistors. Is it standard procedure to check the tubes again after a current spike?. The amp is labeled "Peavey Mace VT series" but the only schematic I could find is DUCEVT.pdf. It doesn't say Deuce anywhere on this amp but seems to be the only schematic I could find regarding a Mace.

[g1]

The schematic attached is labelled mace/deuce.
Tube testers usually won't find faults that occur due to higher voltages. They usually run them quite a bit lower than the average guitar amp. Checking them again may or may not show a fault. If you find a blown screen resistor, the associated tube is most likely shot.

[Perkinsman]

After checking all the 47k screen resistors & replacing all the Ecaps today, the amp came alive on the bench & my without any issues. As soon as I put it back into the cab I realized that it doesn’t have any connection cord to the speakers! Wtf? Is there a special connection cable from the 8 ohm out to the speakers?

[glebert]

Is there a special connection cable from the 8 ohm out to the speakers?

Just use a right angle 1/4" speaker plug and a bit of lamp cord.

[Perkinsman]

Do I just wrap the lamp cord ends around either one of the two speakers? Is there a polarity concern?

[g1]

After checking all the 47k screen resistors & replacing all the Ecaps today, the amp came alive on the bench & my without any issues.

The 47K are grid resistors. The screens have 100 ohm 5 watt, one for each tube. Check them all to ensure all power tubes are working.

Yes, your speaker cable can connect to either speaker. Polarity isn't critical, but in general, speaker (+) is connected to tip of plug, speaker (-) connects to sleeve.

[Perkinsman]

Thanks for that, they were all less than 10% out of spec so they stayed. Are screen resistors usually carrying the most current, hence their size difference?

[doombass]

Are screen resistors usually carrying the most current, hence their size difference?

The current through the screen resistors are low. The screen voltage OTOH is high at about 500V so your 100 ohm 5W resistor can take 10mA at that voltage Calculated using formula P = U x I (P=power W, U voltage V, I current A).

[Helmholtz]

The current through the screen resistors are low. The screen voltage OTOH is high at about 500V so your 100 ohm 5W resistor can take 10mA at that voltage Calculated using formula P = U x I (P=power W, U voltage V, I current A).

To clarify:

A resistor can't know what voltage(s) it's tied to. It just sees the voltage (drop) between its ends. So the U in the power formula corresponds to the voltage drop - not the screen voltage.

The screen current of a 6L6 is around 25mA at full output (further increasing with clipping). Even with an excessive screen current of 50mA, resistor dissipation will be only 0.25W.

High screen resistor power ratings are often used to protect the power supply and resistors in case of tube failure. But there are other philosophies as well.

[glebert]

Do I just wrap the lamp cord ends around either one of the two speakers? Is there a polarity concern?

Put quick connects on the speaker or lamp cord. What is the impedance of the speakers? If they are 16 ohm speakers then they should be in parallel, 4 ohm they should be in series. The speakers should be wired with the same polarity, you can use a 9v battery to test polarity, but Peavey speakers are usually consistent with the polarity. The amp itself doesn't really care about the polarity, it is just to prevent acoustic phase cancellation.

[Perkinsman]

As you can see in the pic, the 2 speakers are wired + to +, - to -. Is that series or parallel? The amp has two speaker outs, 4 & 8 ohm, I believe the 8 ohm out was mostly used. Even though I don’t see an ohm marking on either speaker, is it correct to assume they are 4 ohms in series, totaling 8 ohms?
I can’t use quick connects, since the speakers only have 2 each and they are already used by the +\-speaker to speaker connections.