Power-On Delay Circuit

The gs35b and other metal-ceramic tubes need some time to warm up before they can be driven.  If driven before their cathodes are up to temperature, tube destruction can result. While there's a circuit in Orr's radio handbook using two cascaded transistors, that circuit is finicky about component values.

In this circuit, I'm using a 555 timer to drive a FET, which in turn switches the power to the amplifier keying relay.  It could also be used to drive a contactor on the Plate transformer primary, so there's no HV until the tube has warmed up.

R3 and C2 determine the power-on delay. If one uses the CMOS version of the 555, C2 can be a lower value than for the non-CMOS parts. Also, the CMOS version of the 555's 'hi' output voltage is closer to V+ than the non-CMOS version -- something to think about when driving transistors.