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

[Will Matney]

Neil,

Without any plate voltage, the internal resistance is different than 
when the tube is operating and the anode impedance drops. Checking it 
without voltage, it will be different (higher resistance), then when the 
anode voltage is applied, the resistance drops.

Will Matney

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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