Just wanted to add to this thread... Somewhere else on this forum I posted that I bought a MESA GZ34 from a local musical instrument supplier. I stated in that post that it seems to work fine in my ST-70.
Well, yesterday I finished the second of a pair of Dynakit Mark III amps, bought from Dynakitparts, and where the first amp fired up and biased beautifully first time, the second one would just splutter a few times, arc for a split second and then blow the fuse... Nothing wrong with the wiring that I could see... On a whim I plugged in the Ruby GZ34 that I tested the other Mark III with, and that cured it - works perfectly fine. Refusing to believe that the MESA rectifier is bad, I tried it again - and it just blew the fuse again.
The bizarre thing is that this MESA GZ34 seems to work fine in my ST-70 - but flatly refuses to work in any of my Mark III amps, and perhaps it would probably cause damage to the other tubes (thank you, fuse!). The Ruby just takes it all in its stride - works perfectly fine no matter what it is plugged into.
Is the MESA a bad tube? As far as I know, a GZ34 is a GZ34 - it's not like you get different grades of GZ34 tubes, do you?
-- 'Jan in BC, Canada