Mike KC7NOA writes:
I'm changing my 2@3-500Z amp to a single GS-35B and in the spec's for the
tube it states that the warm-up time is Aprox~ 2 minute. So instead of using
a power SCR in the mains to switch on the HV bank( and probably darn slow to
disengage if a fault occurs) I would like to use a Vac relay - probably
close to the Plate choke since i have room in that compartment and a delay
circuit is darn easy to DIY.
Mike
KC7NOA
I'm afraid ordinary vacuum relays can't be used for your intended
application, Mike. Relays such as the RF1D, RJ1A, RB2, RF3A, etc. aren't
intended for hot-switching high voltage. If you try to use one in that
application (I'm speaking from experience, now!), an arc will form inside
the relay when the relay opens. This destroys the relay and doesn't
interrupt the circuit. There are vacuum relays intended for hot-switching
HV, but they're rather uncommon. If a vacuum relay's rating is, e.g., 12
kV, that means that an open contact inside the relay can withstand that much
voltage. It does NOT mean the relay can interrupt a high voltage short
circuit!
In my amplifiers, I normally protect against a flashover by using the
tried-and-true methods: a 25 ohm resistor in the HV lead, in front of the
filter capacitor, and a small wire wound resistor (e.g., 0.82 ohms, which is
what the L4B used) or a 10 ohm 10W ceramic WW resistor. The later is
available from Radio Shack. Also, in a grounded grid amp, you should have a
safety diode between chassis ground and B-.
73,
Jim Garland W8ZR
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