ORIGINAL MESSAGE:
On Fri, 03 Aug 2012 17:15:09 -0700, you wrote:
>
>My problem with these is simply that with the filament floating I can't
>see what keeps the filament-cathode potential from exceeding a
>safe value, whatever that might be.
REPLY:
You are right to be concerned. Strictly speaking, the heater transformer does
not need a ground,nor does it need a heater choke.
However.... doing it that way leaves the possibility of an arc between cathode
and heater. This could easily damage the heater, the cathode or both. It also
adds a small amount of parasitic capacitance between cathode and heater which
will cause a small amount of RF current to flow between the two. Whether this is
a problem or not all depends. Can't give a definite answer on that.
The safest approach is to use a bifilar heater choke and connect one side of the
heater to the cathode.
73, Bill W6WRT
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