[AMPS] abnormallly high plate current

[Mike Willis]

> > I forgot to mention that I am getting the same readings with other known
> > good full out Eimac 3-500Zs so it is not the tube(s). Filament voltage
> > measured at the tube socket is 5.22 volts.

5.22 Volts is 0.22V too high. You should consider fixing this as tube life
is affected greatly by filament voltage. The 100mA bias is OK. A little high
perhaps but quite within spec.

You have 3500V, 400mA. My similar AL80A has just produced 500W into the load
on 10m just now at 2800V 350mA plate, and 90mA grid. I could get more, but
there is no point in the UK. The efficiency is only 50%, ignoring the drive
feedthrough. I can do better, for example 300mA plate, 150mA grid,
efficiency 60% and so on. I try and stick to a 3:1 plate/grid current ratio.

I wonder if you are doing the same, or loading very heavily ? Could it be
that you have a problem only on the lower bands (If so, I suspect your plate
block is too low,  or the Q is being wrecked by the proximity of the coil to
the case).
Talking of Q, Cue Rich and parasitics that may also cause this effect.

Assuming you have tuned up correctly, then there does seem to be a problem
but I can't help without more information to go on.

Mike


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