[Amps] Tuned Input - IMD and efficiency

[Steve Thompson]

>> As an aside, adding some capacitance with low inductance leads from
>> cathode to grid won't only help linearity/efficiency, it might improve
>> vhf stability too.
> 
> 
> I can't recall seeing any systems where the cathode is involved in VHF 
> parasitics in an HF cathode driven amplifier, although that doesn't mean 
> it can't ever happen.
> 
> Every case I have seen has been a grid-anode problem.
I agree that's the fundamental problem, but the cathode ends up as part 
of the oscillator circuit. Mine is a very simplistic view, but 
capacitance from the cathode to grid acts as a potential divider with 
anode to grid capacitance. Adding grid cathode capacitance can be seen 
as increasing the 'pot down' for the feedback, or it can be viewed as 
reducing the vhf gain which makes the 'loop gain >1' part of the 
oscillation condition slightly less likely.

The effect of an external capacitor will be limited in tubes with grid 
structures that give the worst problems, but in most amplifiers the 
input switching wiring means that there's little vhf effect from the 
capacitors in the input network.

Steve