[Amps] Grid resonance

[Steve Thompson]

A had a few minutes before leaving the office yesterday, so I fired up the
network analyser. I made some rough, not precision, measurements and, as
these were done on a cold tube lying on the bench I'm uncertain as to how
useful they are.

I measured the impedance between the grid ring and one cathode pin. At low
frequencies it's capacitive. As the frequency increases the inbuilt
inductance comes into play and series resonance occurs at around 250-270MHz.
The impedance then goes inductive until parallel resonance at about 1.7GHz.

I also looked at feedthrough, feeding into the cathode from a 50 ohm source
and measuring into a 50 ohm load at the anode. From 1-2000MHz the
feedthrough
is around -60dB except for around 250MHz and 1.7GHz where it comes up to
around -45dB.

Steve