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

[Neil Carr G0JHC]

I have a question..maybe  dump, but I don't know the answer.

 

 

Just finished a YC156 amp .all working well.

 

 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).

 

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.

 

Why would the i/p swr change so much once HV was applied? ..and then change
slightly dependant on how much HV? 

 

 

Looking forward to any replies.

 

73 Neil G0JHC