Musing out loud mode:
Centre taps of T1` primaries appear to be intended to be at steady DC.
L1 makes it look like a Royer but seems no need for CT's to vary.
Small prospect that are intended to develop some sort of AC component but I
think not.
May well be wrong :-).

So just maybe adding decoupling at either CT may help.
May already exist.
Most liable to be at collctor winding midpoint.
Adding a 10 u - 100U at base promary CT may help.
Rationale is that signal may ride through L1 undecoupled and just perhaps
lift both bases at once.

Tubes may last muchly better if filaments were driven but probably nobody
cares.

Small C-C cap across transformer primary may be interesting.


    Russell
--=20
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