hi Mike and all
The goal of the filament choke is to prevent
a feedthrough - leakage - from the cathode to
the filament transformer, and the break down
between cathode and filaments, if filaments have
a good RF bypass path to the ground.
Some designs have a trifiler wound choke on filaments
and cathode - and a 0.01uF coupling capacitor
between them.
Considering RF potential the maximum driving power
of the tube is around 50W - 50 Vrms and 70Vp-p on 50
ohms; so some RF potential can be induced on the
filament and have a leakage to the transformer.
But I presume little or no heat caused on filaments
by the driving power.
The DC potential on the cathode at cut-off state can be
a high-voltage; it is recommended that both filaments
should not be connected to a DC ground.
So I recommend chokes on all the cathode and filaments
and, using no coupling capacitor between cathode and
filaments, with both of the filaments are not grounded.
In this case both filaments are 'floating' in terms of DC
and RF potential.
GL, de Han JE1BMJ
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