|
It's always beneficial to apply the B+ (HT) supply of the tubes after the heater has had sufficient time to heat up the cathode. This minimises the cathode poisoning of the tube. So for an ordinary EL84 tube the heat up time is somewhere around 15s, so a time constant of 16.5s is sufficient to allow the cathode to start its emission. The circuit is a simple comparator driving a transistor driven relay. The delay is brought by the 330k/100uF RC network and the reference voltage for the relay from the 4.7V zener diode. The discharge is made to be abrupt, by the PNP transistor, so that there isn't any delay between powering it off from the mains and the relay actually breaking contact. To bring the voltage to 12V from a 6.3V heater AC supply, just use a voltage doubler and an LM7812 regulator. Also use the relay to break the circuit before the rectifier, relays handle AC easier, because arcing is easier to quench, and the relay will last longer. Also the valves won't be directly attacked by high voltage, but will be brought to it smoothly.
The unconnected PNP transistor base goes to the second comparator output of my Tube Heater Cycling circuit.
|
