[Amps] RF coupling to 8877 filaments?

[Han Higasa]

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