Hi Dick, I guess we all have our pet designs and as usual, there is more than
one answer.
I have seen enough KAPOWS! over the years that I tend to design towards
minimizing them. If the cathode is connected to one side of the heater then
there
is no chance of a cathode to filament breakdown in case of a fault condition.
If the cathode is connected to the heater and the heater is connected to ground
then the possibilty of excess fault current blowing the filament open exists.
Therefore: I would connect cathode to heater, bypass the heater for rf and
float the heater transformer above ground. To make sure all of the drive power
is applied to the cathode, I would use a bifilar heater choke with appropriate
bypasses to ground at the transformer end.
A common failure mode in case of a plate to ground short (including plate to
grid flashover), is the B- can rise to near the B+ potential without proper
surge protection circuitry. If this happens, you can have a cathode to grid
surge, possibly damaging the tube. You can also subject the plate and grid
current
meters to destructive currents. In addition to the usual 25 to 50 ohm B+
resistor, a single hefty diode connected from B- to ground can shunt the fault
currents to ground. If the cathode and heater are not connected together, the
same fault condition can damage the tube cathode and heater.
There are definitely some "gotchas" here!
Hope this make sense.
73,
Gerald K5GW
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