[ARC5] ARC-5 Issues

[Neil]

> Since you are using K53 as a keying relay, you are in fact making and breaking
> the final cathode circuit.  With the contacts open, R75 limits the final plate
> current on key up .....


No, that's not the function of R75. Whenever the cathode circuit is opened,
as for example in the non-selected transmitters, the voltage at the cathode
tends to rise towards that of the plate (think of the cathode as being at the
center of a voltage divider formed by the plate, cathode, ground).

Under these circumstances, the cathode may float at several hundred volts
with respect to ground, and the nearest item it can break down to is the
filament. R75 is fitted to limit the voltage to which the cathode can rise,
hopefully preventing cathode-filament breakdown.

73 de Neil ZL1ANM