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RE: Extend tube life options..

The problem with HV on the plate is ion bombardment (cathode poisoning). This is controlled in operation by the electron cloud, which exists only when the filament is hot.

Traditional best practice is to remove the high DC voltage from the plate, and operate the filament at a lower temperature which balances heat damage against temperature cycling - for long periods of shutoff, it's probably best to shut then down completely, which shorter periods might work best at half voltage.

Note that hot-switching high DC voltage damages switches quickly; the only reliable switch is a vacuum relay. A time delay arrangement might work - switch off the HV supply long enough to discharge the caps, switch off (or down) the filament voltage, disconnect the HV from the tube, then re-activate the B+ disconnect, then re-start the HV supply. This will increase the cap voltage since there is no current drain, of course...

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