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I'm now convinced its a design problem. I believe that a possible solution is
to reduce the values of the series resistor (22k) and capacitor (150 pF) coupling
the horizontal output transformer to the grid of the burst keyer tube,
perhaps to 4.7k and 50 or 75 pF. A simulator says this should reduce the
problem, and the 21CT55 does this, though only to 10k and 100pF.
I'm not going to change my set as of yet.
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