> Planning on using a single power supply for two GS35-B
> amplifiers. Only one amp. will control the HV power
> supply. This
> amp. has a three minute delay from the time the filament
> applied
> before the HV can be turned on. The other amp. will have
> a three
> minute delay from the time its filament is turned on
> before the PTT
> can be used to key the amp.
>
> Question: Do you have to wait three minutes from the time
> the
> filament voltage is turned on until the HV is applied?
Mike,
Whenever cathode current demand exceeds emission capability
for the cathode operating temperature or surface area, the
cathode can be chemically poisoned. This is why
indirectly-heated tubes or oxide-cathode tubes must have
long controlled warm-up times before any cathode current is
drawn. We should also never operate oxide-cathode tubes at
reduced heater voltages! Low temperatures can allow the
cathode's protective electron cloud to be depleted, and
nothing would be there to prevent chemically poisoning the
cathode.
Low temperature operation or operation that strips away the
space charge around the cathode can actually cause sparking
to occur. This localized sparking can trigger an anode to
cathode or anode to grid arc.
While it generally is a better idea to not apply HV until
the cathode is fully operational, you can get by if you can
ensure cathode current is not drawn until the cathode is
fully warmed up. You have to maintain the electron cloud
around the cathode before current generates any positive
ions, and you have to include arc protection in the anode
supply line. Whatever you do, you must be sure the cathode
circuit is fully open and that no cathode current is drawn
until the electron cloud around the cathode is fully formed.
It is the bombardment of the cathode by ions or
contamination by gas that ruins the cathode.
You also have to watch grid current and remove drive within
milliseconds if excessive grid current occurs. This is
because grids are plated with a soft metal that can
contaminate the cathode, and that material can evaporate or
be displaced by kinetic energy of electrons striking the
grid. This can produce accumulated damage over time. A fuse,
meter, or warning light won't do a proper job of protecting
the grid.
The four things you have to avoid are excessive HV,
poisoning by emission demand exceeding available space
charge, cathode sparking by low temperature or low emission
operation, and excessive grid current (even for very brief
periods). Operation at low filament voltage will ruin an
oxide cathode power tube, so maintain the minimum allowed
heater voltage and be sure the cathode is fully warmed up
before drawing any current.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|