unstable cathode voltage help, please

[rfgordon]

I've built this amp, which has pretty much a textbook BF reverb circuit, which I've done many times with no problems. This one is different. The reverb driver won't seem to operate correctly. When I put the meter on the top of the cathode resistor, the voltage falls to near zero. What I hear when I play is that, initially, there is some verb signal, but it dies out so that a rapid single note run ends up sounding completely dry.

I've gone over every connection. The tube, tank, cables, xfmr, rca jacks are all known good.

Could the previous stage cap be leaking DC and cause this? Wouldn't my verb coupling cap also have to leak DC for it to affect the driver tube?

I'm stumped. The entire amp is dead nuts perfect, except that stinking cathode voltage, which means the driver ain't working properly.

Help!!

[jjman]

I would watch it's plate voltages when this occurs. If it goes down maybe the transformer or it's connections could be the culprit. If it goes up it could be the cathode resistor or connections.

[rfgordon]

OK, I fixed the amp. First, I replaced the cathode lead, and that stabilized its operation. So I don't know if it was a micro crack in the wire or the joints on the pins, but that helped. That revealed that the particular 12AT7 in there motorboated--I've seen that before where, for some reason, particular dual-triodes will motorboat with shared cathode resistor. I suspect it has to do with grossly mismatched triodes, but that's just a guess.

Any ideas on that one?

Anyway, now the amp is just lovely. If it is well-behaved over the next week, I'll deliver it.