[Amps] HV Turn On Delay

[Tom W8JI]

> 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