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RE: Max Tube voltage during warm up

The datasheets nrmally have a maximum open voltage for when the tube is not heated up. E.g. you'll find something like 550V for the 6BQ5. (Vao)

However it still shortens tube life.

tubebooks.org - the oxide coated cathode Herrmann & Wagener.

Some smaller tubes may have their filaments light up like a flashbulb - put some NTC's inseries with the primary of your transformer and that stops that.

I have a NTC on the primary, let the filaments of the audio tubes warm up 30 ~ 60 seconds and then use a 60 second time delay relay (e.g. the Amperit 6NO60) to switch in the filament for a single 6CL3 damper diode in order to let B+ slowly ramp up to the audio tubes. Even there I use some NTC in order to reduce the stress on the cathode while the 6CL3 heats up.
(the design uses solid state rectification but the B+ is too high so it can nicely be reduced in this way)


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