M-1000 has an appetite for preamp tubes, help?

[tommy d]

I recently traded for a Metaltronix M-1000. It needed a little work and I have it running but it does eat a 12AX7 in V2 every once in a while. I have ordered new 9-pin sockets because I noticed the sockets were mangling pins. I tested all the rails and they match what Lee Jackson said they should be on his maintenance video on youtube. (Cool of Lee to give that away for the m-1000) Sounds killer when it runs, I'm just trying to work out the bugs...

On 6V PS rail after the rectifier, the zener was burned so I replaced it. That "fixed" the amp but not completely. Beyond a shorting socket, what should I look for? It seems to me that the zener blew up because there was a short between the rails and one of the higher voltage connections either through the socket itself or because the mangled pins are pushing the socket tabs into each other somewhere.

Schematics here: Metaltronix Schematics

zener photo: Picasa Web Albums - Thomas

[Enzo]

I have no idea what eating a 12AX7 means. The heater burns out? the glass cracks? What specifically goes wrong with these tubes?


A lot of times a loose socket can't make good contact with tube pins, you try a new tube in the socket and it works, then later that tube stops working, so you try a third, and you are back in business. What usually does not get tried is to put the original tube back in. SOmetime just the act of shoving a tube into the socket refreshes the contacts. The tube may have been good all along.

[tommy d]

I placed the suspect tubes in known good amps at the v1 position and no sound is made. On the first tube I witnessed it arcing inside the tube and the gettering was stained as a result. The second tube I did not witness failing but I went to play the amp and no sound. Replace V2 in m-1000, sound. Place bad tube in good amp, no sound, good tube in good amp, sound, QED bad tube. The filament in both bad tubes still has resistance and I can't test beyond that with my DVM. Physically the tubes look fine exept the gettering...

Thanks for the reply.

[Enzo]

OK, that is unusual.

It is pretty hard to blow up a 12AX7, after all there is usually a 100k or larger resistor between it and any high voltage. There does not seem to be any heater elevation here, so we can rule out the heaters sitting at B+ or some such. And V2 is not a cathode follower.

Trying hard to imagine somthing.

ANy chance of a solder blob or loose wire strand shorting pin 5 and 6? Or are they REAL close?

[tommy d]

Well pins 5,6 are pretty close. I moved them out of the way and I just played the amp for two hours without a problem. Thanks for the tip, I'll post if I find a gothcha! Thanks alot for the help.

Tom

[WholeToneMusic]

Try shutting the lights out and look for arcing on the socket when you turn it on.

[Enzo]

Good idea. To blow up a little tube like that you pretty much have to have voltage getting somewhere it doesn;t belong, and from some place not suggested by the schematic. Like an arc on a socket.

[tommy d]

Great idea guys

[WholeToneMusic]

If you test for arcing at night when it's darker, then it's easier to see.