Peavey Classic 30 Output Tube blows

[Hawk]

Hi,

So I thought I had a microphonic power tube next to the phase inverter--it squealed if moved slightly in its socket but not always and then it would sometimes squeal on its own but not always. I swapped tubes to see if that made a difference and when I turned the amp back on that tube blew in the suspect socket and now none of them work (fuse is still good and amp turns on). The tube may not have been sitting in properly as  the tube holder was a little loose. I had cleaned the tube socket earlier with De-Oxit and let it dry for a good half hour to remove any gunk from the tube socket.

These amps are a pain to take apart but I'm wondering 1. Loose solder joints?  Or 2. Can a tube that doesn't sit tightly in its socket cause arcing and possibly create a microphonic situation?

The tube  furthest away from the phase inverter seemed a little microphonic when I moved it a little as well. But I can't test that now as these tubes are in series so no dice. I'll have to pick up some more tubes. I should probably check to make sure all voltages are correct as well.

Thoughts on how to proceed without  blowing more tubes and spending more money?

Thanks,

Hawk

[g1]

Yes, bad connection to the socket could cause squealing.
The power tube heaters are all in series, so you need to verify all the socket solder joints, and verify the socket pins are contacting the tube pins.
Your description of the tube holder being loose sounds like the socket pins need to be retensioned.

[Hawk]

Thanks g1. So can a loose tube arc and destroy itself?

[Hawk]

With Output Tubes Removed I measured Heater voltages starting with the tube closest to the phase inverter and then went to tube 2,3,4.

Measure voltage across pins 4 and 5
Tube 1. .5v  2. 1.72v 3. 1.89 v 4. 1.25 v

Pin 4 to ground

Tube 1..5 V  2. 21.91 v 3..44v 4. 0v

Pin 5 to Ground

1. 0 v  2. .52v 3. 20.78v 4. .46 v

I know that the heaters are connected in series but I'm trying to understand that concept in light of these voltage readings. Those high AC readings especially have me confused. Anyone want to take a stab at it? Where is the 6.3VAC in all of this??? Thanks!

[Enzo]

Generally a tube won't be harmed by a loose socket, but possibilities exist.  If the grid pin loses contact, then the tube loses bias and can red plate etc.  I guess it is conceivable if the plate pin loses contact then the screen might try to be a plate and melt, but I can't say I ever saw that.

But it is also possible you just had some iffy tubes, and moving them around was enough to push one into failure mechanically.

If none of the power tubes light, have you checked the INTERNAL fuses?  That the amp lights up only tells us the mains fuse is OK, but there is a heater fuse and a high voltage fuse inside on the circuit board.  And if it is a ceramic type, you cannot see if it is blown.  You will need to pull the fuse from its clip to measure it with an ohm meter.

[Hawk]

Thanks Enzo will check over weekend. What do you think of those high AC voltages I mentioned? Also its my understanding is that if one if these output tubes blow they all blow as they're connected in series. Thanks!

[Hawk]

I also replaced the dead transformer with a Peavey Universal Transformer and the voltages on the plate and screen are approx  353 v plate, 351 v screen volts. I believe these tungsols are rated at 300 volts although the original voltages from the old transf. were 332 plate, 328 screen.

Also found, on grid pin of output tubes, -21v.

Any thoughts on this? Wouldn't these high voltages overload the plates? If that is true then why would the amp be originally engineered with voltages that are over spec for the plates and screen? How much voltage is too much on the plates and screens of output tubes?

Should also be noted that amp played for two hours without blowing tubes, and only one of the tubes blew.

Will check the fuses later this weekend when I return home.

[Enzo]

WHAT high Ac voltages?   You mention that a couple times, but then show voltage readings of 4v and less when you are looking for 6v..

Edit:  Oh, I see a 20v in the middle.  No, that is an artifact.  Set your meter to DC and see if anything is there.  About the only bad thing I can think of is that something has shorted the bias from pin 2 over to pin 4 in that socket.  Seems unlikely.

There is no 6.3v heater circuit.  The heaters of the four power tubes are in series.  That adds up to 24v or so, and the tubes are wired across a 24v source.  With the tubes out, there is no series circuit, and any voltages you read are just your meter picking up stray fields.  Set your meter to AC, then grasp the point of the red probe with your fingers, and leave teh black probe on the bench not touching anything.  You should still see some AC voltage reading because you act like an antenna.

When one power tube HEATER goes open, then all the tube heaters go dark.  That is not the same as one tube "blowing" the others. 

You say the tube is rated for 300v, yet the factory voltage is 332.  from there 350v is less than 10% high.  That is more like the Classic 50 voltage anyway.  But those few extra volts are not what would kill a tube, it is excess dissipation that does, in other words, too much current.

[Hawk]

Thanks Enzo, I appreciate your heater circuit explanation. And also the info about the tube heaters. And the "meter picking up stray fields."
I will examine the bias currents and see what I can find.

:tu:

[Hawk]

One more question: does -21 grid voltage seem reasonable? Shows the same on all tubes, tubes not inserted. The old schematic shows -14 volts...

[Enzo]

It is more than it should be, however it won't hurt the amp, it will just run the tube cold.  We need to get a full set of working tubes in it to put proper load on the power supply and to see how the amp operates.  One grid voltage by itself doesn't tell me much.  For example, the -14 is made by a voltage divider off the -36, so is the -36 close to 36v?  or is it closer to -54?

[Hawk]

Thanks Enzo. :cheesy:

[Hawk]

Anyone have any experience checking current through these ouput tubes? Considering the awkwardness of this pcb layout and difficulty accessing the tube socket pins, can anyone suggest a method that has worked for them?

[Enzo]

Voltage across the DC resistance of the output transformer winding, center tap to plate.  Note the resistances of the two halves are not equal.

[Hawk]

Thanks Enzo. I have the pcb out of the amp and need to replace a bad cap, 22uf, 500V. But it is glued to the board. I've unsoldered one end but I'm afraid to pull on the cap in case I crack the pcb. All the other caps are glued as well. What is the best way to remove these large glued-to-the-board caps? Thanks.