Crate VC30 keeps killing same tube

[DonMoose]

This is a little odd - have a Crate VC30 that keeps killing the EL84 in one particular socket. It apparently has to be in the cabinet and running loud for a while to make it happen, but the heater in that tube burns out and the HT fuse goes (250mA SB).

With the chassis out, everything measures fine - identical to the other 3 sockets. I've retouched some iffy-looking joints, but all of those were on the OTHER EL84 sockets.

About to get out a blow dryer, but can't afford another toasted tube. Ideas?

[martin manning]

Maybe there was arcing from the HT to the filament circuit on the socket? Might have to remove/change the socket to find out, as it could be between the board and the socket.

[DonMoose]

Ugh. The socket's a through-board affair, with no visible carbon on top or bottom.

Guess I'll have to pull it anyway.

Are there any common failure modes in this line? (you know, like Fenders HR series' low-voltage supply self-desoldering resistor)

Thanks!

[xtian]

Does this model have ribbon cables?

[DonMoose]

Does this model have ribbon cables?

There's one cable - not technically a ribbon - that jumps the FX loop from where the jack lands to an inch deeper into the board.

It's got one board with pots and tube sockets on it. No speaker jack as such - the OT secondary hooks straight to the speaker, and the reverb cables go through the chassis with a strain relief.