Apache TX-1 "RF Hiccup" Puzzle

[digital-conjurers at ADELPHIA.NET]

Have an Apache TX-1 running AM only with an unusual problem:

About 250 milliseconds after unkeying, there is a small "RF Burp" or small RF pulse of very short duration (100 ms or less), and small energy, that comes out of this transmitter.  This is especially puzzling, given:

1.  The finals seem perfectly neutralized
2.  Voltages, tubes, everything checks out as nominal
3.  Coax routing seems very orderly
4.  The complaint _slowly disappears_  as you tune the final tank circuit slightly off the resonance dip; i.e. if you're dipped at 200ma, and slowly swing up to, say, 240ma, the phenomenon goes away.
5. SWR is low; even does it on the dummy load.

[BTW we're using 6293 industrials instead of convetional 6146's. (The primary difference between these and the "normal" 6146 is that the plate is much "heavier" in its construction. )]

So, the sequence is:

1. Unkey, the Dowkey releases, all power meters swing to zero, 250ms later there's that >100ms RF "hiccup", then all is fine.

Wierd.

Ideas?

Thanks to the group in advance,

Lin/KJ6EF

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