Peavey Classic 30 Help

[Shawnobi]

Hi there, I was wondering if I could get some help with my Peavey?

First some background. The amp was due to be thrown in a skip. It had had a foot through the grill and speaker. Pots were snapped off, the tolex was moldy and the metal work rusty. I've stripped the whole thing and cleaned & polished it up. Replaced the pots, fitted a new speaker and reverb tank. I inspected the PCB and noticed a burned out CR6 diode so replaced that. Also sprayed electrolube into the tube sockets.

So it's now been fired up and I have sound (and sounds good). However the power tubes start glowing very red very quickly so I shut it straight off. I noticed tubes 1 & 4 red plate (?) quickest. So I swapped them with 3 & 4 and they continue to redden quickest in the new position. However all tubes glow very red.

Can anybody advise why this is? I guess they are drawing rather too much current for some reason.
Note, whilst the pre-amp tubes are new, the power tubes are some old ones of dubious history. I intend to replace them at the end of the month, but am worried of replacing while the old ones look like they will burn out.

Thanks in advance for any advice you can give.

[Enzo]

But all it takes is one shorted power tube to kill the bias to the others, and they all start to melt. You need good tubes inthere. You also should check the bias voltage to see if it is present at all power tube sockets. If you pull any power tube, they all go dark, but for testing voltages, that doesn't matter. Remove the power tubes and check for bias at the grid pin of each socket.

[Shawnobi]

Hi Enzo, thanks for the response. I did wonder that about the tubes.
How do I know which pin is the grid? Tubes are EL84.

And I would connect the meter between this pin and the chassis, yes?

[Enzo]

You would look on the schematic and find it to be pin 2.


And, yes.

[Shawnobi]

Learning all the time!

Voltages on pin 2 are all between 0.6 VDC and 0.8VDC

[Enzo]

The bias ought to be about -14vDC, more or less. Either yours is missing, which would surely cause red plating, or you are measuring pin 8 by mistake.

[Shawnobi]

Looking at the empty tube socket i counted clockwise from the first pin hole to the left of the gap.

I measured with all PT's removed but Pre's still installed. Amp switched on.

- I've just measured the ones on the opposite side (which are maybe the correct pin 2). These all measure -16VDC

[nashvillebill]

(hope this isn't a double post)

Unless I'm having a brain fart--always possible-- it's clockwise from the back side of the socket (solder side)--just like looking at the bottom of the tube itself, and pin 1 is the first one clockwise from the gap.

[MWJB]

+1 on what NashvilleBill says.

[Shawnobi]

In that case pin 2 is reading -16vdc across all POwer tubes. Is that close enough?

[MWJB]

It's a start, it shows that negative voltage is preset at all the tube grids, however you still need to determine the plate current to ascertain whether your tubes are underbiased (too much current). Or, buy some tubes from a vendor that pre-selects sets to drop in to a Peavey C30.

[Shawnobi]

So I should really note the voltage at all pins on each socket?
Cool. I shall when I get home and update.

[Shawnobi]

OK. So here are my results from the tests.

Tube / Pin
1/ 2/ 3/ 4/ 5/ 6/ 7/ 8/ 9
EL84 - 403/ -16/ 403/ 0.2/ 0.2/ 0.2/ 0/ -16/ 0.5/
EL84 - 402/ -16/ 403/ 0.2/ 0.2/ -18 / 0/ -16/ 0.3
EL84 - 402/ -16/ 403/ 0.2/ -17 / 0.2/ 0/ -16/ 0.3/
EL84 - 401/ -16/ 403/ 0.1/ 0.2/ 0.2/ 0/ -16/ 0.2

A couple of values that stand out like a sore thumb.

[MWJB]

You still appear to be reading the pin no's back to front, 7 is the plate an will have the highest dc voltage, pin 2 is the grid and has the negative voltage...pins 4 & 5 are the heaters/filaments and should have hi current AC voltage not dc. Check this carefully.

Pins 1 & 6 are not usually used, they might serve as tie points for components in some amps.

Your plate voltage is way high, should be <350v-ish, not 403. The middle 2 tubes could be shot & not drawing current, all of them may be biased cold, pushing up the voltage.

You need bias probes to check the bias, a pair of probes will do, just do 2 tubes, say 1 & 3, then 2 & 4.

Some bias probes & some known good, pre selected, tubes are your first port of call, still have problems? Go to a tech.

[Shawnobi]

Your plate voltage is way high, should be <350v-ish, not 403. The middle 2 tubes could be shot & not drawing current, all of them may be biased cold, pushing up the voltage.

.

I measured with power tubes removed so may explain this. Right I'll sort myself out with fresh tubes as soon as poss. I believe the C30 is fixed bias btw.
Thanks for the advice.