[TCML] valve coil problems

[<lightningfor@xxxxxxxxxxxxxxxxxxxxxx>]

Hi People,
I am in the final stages of my giant valve coil (50kw).
I have changed the circuit that I am using since the first time I fired it
up, in an effort to get longer arcs.
To cut a long story short, I have a peculiar tuning problem.
I measure the secondary resonant frequency, then Tune the tank to match,
then run the oscillator with no secondary and make sure it is oscillating
correctly, then slowly introduce the secondary inch by inch.
Even with a very loosely coupled coil, as soon as the tank sees the
secondary, the valve starts to oscillate at about the 3rd harmonic of the
secondary coil...

To put this problem into perspective, I have built many valve coils, and
never had this problem.
The coil was built over a year ago, but has sat around for almost a year
since its first tests and admittedly suffered some water damage to the
valve cabinet.
Over the last week, I have tried 3 different secondarys each with a
different frequency, different coupling factors, circuits with the tank in
the plate, and circuits with tank in the grid...
But for some weird reason, every one of them gets swamped by the secondary
coil, and runs at the 3rd harmonic.

Any Ideas as to what could be going on?

Thanks,

Carlos
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