Help preamp tubes keep blowing in Twin Reverb!

Hi Guys,
I have a mid 60's BF Twin Reverb that keeps blowing preamp tubes in the normal ch. The tubes don not glow at all.
I've used JJ 12ax7's, EH, etc and they all seem to last anywhere from 3 weeks to 6 weeks.
Any suggestions?
Oh yeah the voltages on pins 1+6 read 270v which seem to be right on spec +/- 20%.

Are they dead after they're blown ? Why don't the "glow at all" ?

I would check the surrounding circuitry, also check the filament voltage. When cold, the filaments are practically a short and draw a lot of current, this could overstress a weak point of the filament wire and burn it. Also wrong cathode resistor values and leaking coupling caps could damage the tubes.

More details, please!

They burn out. I put in a new tube and it glows okay and within a matter of weeks that goes too.
I did notice that the preamp tube in this socket glows a lot brighter than all the others.

Over voltage or over current are the only things that immediately pop into my mind as the means of burnout. As to the cause, you will have to locate the source with a voltmeter.