[Amps] What limits DC plate supply voltage for GI7B?

[Roger D Johnson]

 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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