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 ??
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