Smokin' tubes

[OldFarb]

Having a problem with the preamp section all of a sudden on a Peavey Delta Blues. Twice, the overdrive preamp tube has 'smoked' on me. The tube is cloudy inside at the top, and the glass is cracked at the bottom.
I wouldn't think anything about one tube going bad, but two has me thinking. When I bought it used it appeared to have 12AT7s instead of AX7s, and it acted like it with reduced gain. I put 12AX7s back in and have smoked two tubes in short order, with little playing time. Clean channel is fine, drive channel is affected.
Does this sound like a voltage issue in the preamp? I have not had it open to see if any mods have been done.
Any help is appreciated.

[Tom Phillips]

...The tube is cloudy inside at the top, and the glass is cracked at the bottom. I wouldn't think anything about one tube going bad, but two has me thinking.

I think it’s either coincidence or someting common to both tubes such as being subjected to too much stress around the pins which cracked the glass.

...When I bought it used it appeared to have 12AT7s instead of AX7s, and it acted like it with reduced gain. I put 12AX7s back in and have smoked two tubes in short order, with little playing time. Does this sound like a voltage issue in the preamp?…

I would say no. IMHO mechanical stress, thermal stress, manufacturing defects or bad luck of the draw would cause the cracking and subsequent loss of vacuum that you have experienced but not over voltage.

I’d try a third known good tune in that position as the next step. Also, it never hurts to inspect the inside of a newly acquired amp.

Cheers,
Tom

[Enzo]

The only way the tube will crack open is from mechanical stress - like whacking it when you toss a cord into the rear opening. VOltages won't do it.

SOmeone put a 12AT7 in V2 to reduce the gain of the dirt channel. Many people do it. Has nothing to do with tube failures.