It's strongly risky to apply power to the screen first.
The screen current will rise much higher than normal operation, and if the product of Vg2 x Ig2 exceeds the rating for Pg2 in the data sheet, you risk burning out the screen.
Sometimes a high enough value of resistor in the screen will prevent damage, but that may degrade the operation of the pentode/beam tube.
applying anode voltage with zero screen voltage during warm-up, OTOH, is safe, - even kind to the valve.
The screen current will rise much higher than normal operation, and if the product of Vg2 x Ig2 exceeds the rating for Pg2 in the data sheet, you risk burning out the screen.
Sometimes a high enough value of resistor in the screen will prevent damage, but that may degrade the operation of the pentode/beam tube.
applying anode voltage with zero screen voltage during warm-up, OTOH, is safe, - even kind to the valve.
I figured it was risky to power the screen first. Nevertheless, I have seen amplifier designs where that can happen easily.
I am asking because the easiest way to switch the screen current in and out of the circuit is with a PMOS. Relays that can handle those voltages are expensive. Are there any techniques I am missing here? I can switch B+ with a NMOS in the cathode easily, but cannot "go inside the tube" to switch the screen. The problem with PMOS is I cannot find any 1000V devices.
I am asking because the easiest way to switch the screen current in and out of the circuit is with a PMOS. Relays that can handle those voltages are expensive. Are there any techniques I am missing here? I can switch B+ with a NMOS in the cathode easily, but cannot "go inside the tube" to switch the screen. The problem with PMOS is I cannot find any 1000V devices.
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A solid-state relay would do (e.g. photo transistor, phototriac etc.). You only need to limit the screen current during warm up, and an SS relay that is "off" will do that.
a Mosfet source follower with time delay is what i plan on using...RC time constant / voltage calcultor
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