From EIMAC:
Positive ions are formed through ionization of residual gasses. High anode
voltages speed formation of positive ions. The space charge (electron cloud)
surrounding the cathode repels positive ions, and keeps the ions from poisoning
the cathode. When positive ions contaminate the cathode, electron emission is
reduced.
Tungsten has the highest melting temperature of any metal, above 3600K. Tungsten
has the ability to operate at high temperatures for many hours without
evaporation of materials becoming a major problem. Tungsten is one of the few
materials able to withstand total depletion of the electron cloud in a vacuum
tube. Tungsten emitters are not damaged by stripping away of electron clouds,
and the resulting cathode bombardment by positive ions. This immunity to damage
means emission life is not shortened by excessive current, operation before full
temperature is reached, or low filament voltage.
If a cathode material sensitive to positive ions is operated with excessive HV,
or has excessive cathode current for the cathode operating temperature, it
will suffer emission failure. This is why indirectly-heated tubes or
oxide-cathode tubes must have long controlled warm-up times before any cathode
current is drawn. We must NEVER operate the 3CX1500A7/ 8877, 3CX800A7, or other
oxide-cathode tubes at reduced heater voltage! This would allow the cathode's
protective electron cloud to be depleted, and nothing would prevent positive
ions from striking and poisoning the cathode.
73, Roger
On 10/25/19 8:09 AM, Alan Ibbetson wrote:
The datasheet for the Russian GI7B triode says the maximum anode voltage for
"continuous wave generation" is 2.5KV, so about 5KV peak. However the rating
is 12KV for pulse operation. Flashover would not appear to be an issue even
with a 6KV DC supply and the Ia/Va graph still looks sane at 8KV despite
needing well beyond -60 volts on the grid for cutoff.
Does this mean the valve can be safely operated with an anode supply beyond
2.5KV as long as the thermal ratings are not exceeded? Or do bad things
happen? If there are problems please can someone explain the physics?
The reason I ask is I've been given a 3.3KV/400mA PSU, I have some GI7Bs on
the shelf, and the cost of an 8877 or even a pair of 3-500Zs these days makes
me shudder.
Has anyone tried 3KV or more with a GI7B?
73, Alan G3XAQ
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