[Amps] GG tuned input with autotuner

[Steve]

At high power, the current flowing through the tube is not a nice neat 
sinewave, it's spiky with lots of harmonic content. If the cathode 
circuit doesn't give the harmonics a path to go through, the tube 
struggles to work properly at high power.

Your auto tuner will probably configure itself as a 'L' match with 
series inductance and shunt C. If the shunt C ends up being switched in 
at the rig end, then the cathode is looking at an inductor - not good 
for the harmonics.

10' of normal cable adds around 300pF of shunt C, just what the 
harmonics need.

Try adding 100pF or more fixed C at the cathode with a short lead to the 
tuner and see if that works.

Steve

> So .. out of curiousity..  ( yeah I know ... meow ..  ) I put a longer length 
> of coax
> between the amp and the tuner output .. not sure how much .. maybe 10 ft or so..
> About the same as was between the tuner and amp in the other config .. 
> 
> And .. viola ! The output came right back up again .. .
> 
> So .. whats going on here ?? inputs .. ( be nice )
> The added capacitance of the coax per foot made a difference? 
> 
> Do I need some input loading caps to be added to the amp at the grids ??