[Amps] why does input SWR change when HV applied?

[jeff millar]

Neil,

The cathode match works into both the input capacitance and the 
resistive component of the cathode.

The resistive component comes from the fact that the cathode circuit 
carries both the plate and grid current due to the voltage swing. The 
input impedance basically computes as cathode voltage swing divided by 
the sum of plate and grid current...except that both values vary widely 
through the RF cycle, and the ratio varies even more widely as the tube 
goes between cutoff and full conduction. To some extent, the resistive 
component of input impedance also varies with drive level.

I tune up drive circuits by installing a temporary resistor from cathode 
to ground to simulate the documented input resistance, and use the tube 
and socket to provide the real capacitance. This tune-up can happen with 
the filament off.

jeff, wa1hco

Neil Carr G0JHC wrote:

>I have a question..maybe  dump, but I don't know the answer.
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>Just finished a YC156 amp .all working well.
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> I was surprised to have to re-tune the input  circuit to such an extend
>that I had to open up and re-size the coils (Single band 6m amp).
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>Input tuning was done after 6 mins heater warm up with an MFJ analyser, but
>no HV.  Soon as I applied the HV the input SWR shot from 1.0 to 3.0. This
>would vary slightly between 4kv and 5kv.
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>Why would the i/p swr change so much once HV was applied? ..and then change
>slightly dependant on how much HV? 
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>Looking forward to any replies.
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>73 Neil G0JHC 
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