[ARC5] ARC-5 Issues

[Neil]

From: Christopher Bowne
To: Neil ; arc5
Sent: Monday, February 25, 2013 11:04 PM
Subject: Re: [ARC5] ARC-5 Issues

> Good point Neil...thanks for pulling me off the guard rails....without R75
> between the cathodes and ground the tube would be completely cut off, with it
> there there will be some current, so saying R75  limits the plate current is
> incorrect.  It actually establishes the cut off plate current, presumably at a
> low value.

Christopher, I've spent 3 days trying to figure out what you mean.
I've come to the conclusion that you misinterpreted what I wrote.
I did not say that R75 is fitted to limit the plate current. I wrote this:

--Quote--
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.
--endquote--

R75 has nothing to do with setting the key up (or relay contacts open)
grid bias or plate current. R75 prevents cathode-heater breakdown by
restricting the cathode to a much lower voltage than it might otherwise
float at when the contacts are open.

73 de Neil ZL1ANM