Small-signal (or small power) pentode that can handle 400V (or more)?

[Rod Coleman]

Tom, Tweeker - THANKS. I have learnt something, and know now that spalling is right.

Tom, is the spalling effect the only reason why (for instance) EL84 is rated for 300V in dc service, but EL86 is rated for 250V? Both have Vao = 550V.

Are there other limits on continuous anode voltage?

[Tweeker]

Yes, voltage isolation- leakage, arcs.

(external and internal) isolation considerations

That said some tubes may in fact be just as strong but rated more conservatively by their makers, or possibly because of intended duty. TV service often being particularly brutal. Whats ok with a new hard tube in a clean room might not be so great after thousands of hours in a hot dusty box.

The EL86/6CW5 was intended for higher current at lower voltage than the EL84. It even saw use in some OTL apps with 800 ohm speakers.

[Tweeker]

That is why bright emitters (uncoated directly heated pure tungsten cathodes) are preferred for high tension and high power duty, like with transmitting tubes running typically at anode voltages starting with, say 1kV and above.

Hi Tom, I dont think pure tungsten filaments (not thoriated), are preferred until above several kV, as they use a lot of power, though the point still applies to all direct heated tubes. On the bright side, pure tungsten filaments have very long lives.