When you switch your amp off, the heater supply goes down quickly but if you use a
large high voltage supply, this high voltage will remain a long time after the tubes have
finished to conduct. That would reduce the tube's life.
The solution is to add a discharging circuit that would download this high voltage to ground when there is no more heater supply.
Another advantage is that such a system could prevent accidents to the DIYer ...
A relay is maintained open by the heater supply. When you switch off the amp, the relay closes and discharges the high voltage to ground through a power resistor.
The relay should be chosen according to your heater supply, the high voltage to switch
and a minimal primary power comsumption.
It should have a closed contact whithout power.
If your filaments are supplied by DC, you can omit the diodes bridge and following cap.
The small cap in parallel with the relay contacts avoids noisy clics when the relay is disactivated.
The power resistor could be undersized as the discharging time is short.
A power and current calculation example:
B+ = 600V, R=5K => max instantaneous power of 72W and 0.12A.
A 25W resistor in an aluminium case is enough.
A discharging time calculation example:
C = 1000uF, B+=600V, R=5K => to obtain a residual voltage Uc of 10V, we need 20.5 seconds.
A small example installed in an existing amp:
From left to right: the power resistor and its small radiator, the cap, the relay (green), the rectifier bridge (black) and the supply cap (blue, partly visible). It's a small circuit that could be inserted nearly everywhere.