[Amps] input matching question on the YC156?

[Steve Thompson]

On Thursday 16 September 2004 16:16, Neil Carr G0JHC wrote:
> I'm currently playing around with a YC156 and have and have discovered an
> interesting phenomena which I'm having difficultly explaining.
>
> On previous amps  I have never really bothered tuning the  i/p matching
> until I've hit it with RF (I build single band amps, so its quite easy to
> guess).
>
>
>  Now I have a MFJ I'm able to "play a little" and do a cold test setup
> first.
>
>
> I set my input matching up for 1:0 SWR with the MFJ, at this time the
> heaters are not on. Soon as I switch them on SWR shoots up to 2:1, then
> over a 6 minute period gradually to 4:1.
As the cathode heats up, a cloud of electrons build up around it - I'm 
guessing that this changes the capacitance to ground. The usual form of 
biassing in gg amps won't prevent the cloud forming and spreading. It might 
prove interesting to apply a significant cutoff bias to the cathode and see 
if the effect goes away.

Steve