12ax7 that can handle 400v on the plate!

[django]

hiya i'm looking for an alternative to 12ax7's that can handle a voltage of roughly around 400v constantly and put out roughly the same gain, has anyone got any ideas?

[Enzo]

In what application?

Is there truly going to be 400v right on the tube, or will the plpate load be connected to 400v B+? In which case the question arise, what voltage will be on the plate. And that question begs another, what is on the cathode? The voltage specs on tubes are for the most part the voltage across the tube itself, not necessarily the voltage to ground. That is to say if a 12AX7 has 400v on the plate and 300v on the cathode, the tube only sees 100v.

And that brings up another factor - heater to cathode voltage. That one is important. If you have a 12AX7 in a cathode follower where the B+ is 300v and the cathode is at maybe 150v, if the heaters are left near ground, there is 150v between cathode and heater, and that could be too much. In those cases a simple DC elevation of the heaters solves the problem.

[django]

ahh thanks! i recently built myself a 70s hiwatt style amp without thinking they used to run there valves alot hotter than modern valves can handle, The plate voltage is roughly 380v which seemed a bit high to me. But with taking other readings it just seems like its running on its limits which should be okay

[MWJB]

380volts from plate to CATHODE on a 12AX7 sounds wrong to me (just looked at a few HiWatt schems), BUT as Enzo says, a 12AX7 may well have 400v+ measured from plate to GROUND and still be working well within its perameters (usually 300v max on the data sheets - some SF Fenders ran their preamp tubes at 300v and failures are no more common than with other amps).

The EH12AX7LPS spiral filament tubes have been highlighted as a potential weak link in typical cathode follower applications, but that's the only one I'm aware of?

[GainFreak]

The EH12AX7LPS spiral filament tubes have been highlighted as a potential weak link in typical cathode follower applications, but that's the only one I'm aware of?

Not only EH12AX7LPS. Check out this thread:

12AX7EH CF Failure