Pre amp valve fail??

[keeponehandloose]

My Mesa Walkabout seems to have a problem where both 12AX7 valves have failed at the same time.
They no longer glow and even though there is speaker hiss when the amp is on,there is no sound from any input source.
A search on the net led me to try plugging into the FX return which bypasses ALL controls including the master volume, this works fine and,and using an external
pre amp would get me through a gig .
Trouble is ,I have 2 gigs this weekend so my question is ,would it be safe to do this ?
Also ,why would both vavles fail at once,is it a sign of a bigger problem.
Any help appreciated.

[Mr. Foxen]

If they've lost the glow, but haven't turned white, maybe they are fine, but they aren't getting power any more, maybe a fuse somewhere. Easy enough to swap them out to check.

[escholl]

The two 6.3 volt heater elements might be in series across 12-ish volts, so if the heating element in one went open circuit it would take both of them out. Should be easy enough to test by just sticking two new ones in there. Could always try just wiggling them a bit too, to see if it's just a bad connection?

[keeponehandloose]

2 new Groove Tubes seems to have solved the problem.
I know nothing about electronics,so excuse my stupidity,but if something has gone "open circuit" is that cause for concern??
ie could it happen again at any time .

[escholl]

[quote name='keeponehandloose' post='1273679' date='Jun 18 2011, 12:38 PM']2 new Groove Tubes seems to have solved the problem.
I know nothing about electronics,so excuse my stupidity,but if something has gone "open circuit" is that cause for concern??
ie could it happen again at any time .[/quote]
It just means the heater element has burnt out and cut the connection electrically, which is what might have happened given that replacing them both fixed it. No cause for concern, no permanent damage, and my guess would be one of the original valves you took out is probably still good.

Could technically happen at any time but is very rare, usually the valves themselves stop working before the heater elements -- I wouldn't expect it to happen again unless there is some underlying fault with the amp.

[keeponehandloose]

Thanks for explaining in laymans terms .