12AX7 Burned Out

[TomCarlos]

Mates...

I have a Peavey Encore 65. I rebuilt this amp in the last year but hardly used it. Today, no output.

I narrowed the problem to the output section. After pulling the metal covering on the preamp tubes, I found the Phase Inverter tube (V4) was burned up - see photo.

I replaced the tube and here are the voltage measurements (with the new tube in place).

Pin 1 - 323 volts
Pin 2 - 74 volts
Pint 3 - 106 volts
Pins 4 and 5 to 9, 5.9 volts (heater)
Pin 6 327 volts
Pin 7 96 volts
Pin 8 106 volts

It all looks ok compared to the schematic.

So was it just a bad tube perhaps? I have never seen this happen to a preamp tube.

Thanks, Tom

[Enzo]

That is not a "burned up", that is an "air got in the tube". As in the thing has a crack somewhere. it isn't anything electrical.

[TomCarlos]

Ok .... thanks Enzo !!

[Jon Snell]

Judging by the bent pins, I would guess it was not aligned in the socket properly and the bent pins allowed air to enter.
Always ensure the pins are inserted into the small recepticles, not between the recepticle and the edge of the valve base. They look splayed out in your photo.

[Leo_Gnardo]

I have never seen this happen to a preamp tube

What Enzo said. ^^^

No longer a vacuum tube, now it's an air tube. And air tubes look just like that. All the "silvery" parts (getter on the glass) turn chalk white. Keep working on tube gear, you'll see it from time to time. You'll know at a glance that tube is kaput, N.G., non-functional. Pitch it in the bin & replace it after checking the circuit for damage that might have been caused if it leaked while powered up - usually toasted resistors.

[J M Fahey]

Out of the 4 pins visible, 3 look straight and silvery, fist one at the left looks very twisted, brownish and shorter.
Bet pin got very bent, was "straightened" and pushed in.

Metal is metal and glass is glass, I am certain there are cracks in the glass around the abused pin.

Even if tiny, air will get in.

[TomCarlos]

Thanks again for the replies....

I appreciate the comments about the bent pins. I don't know how that would have happened. I checked the tube socket and it looks fine. I ran the amp yesterday and pulled the new V4 out. The pins are straight.

At least I now know how the white cloud was created in the 12AX7.

Much appreciated... Tom

[Mick Bailey]

Sometimes the vacuum loss can be very gradual - I had a set of 6550 tubes where one had just the slightest signs of white on the very edge of the getter. Over time it slowly crept until after something like three years it was completely white.

B9A tube pins are remarkably resilient on account of the glass/oxide/metal interface and can take some fairly serious abuse and pin straightening - it's unusual to see a white tube.