[ARC5] ARC-5 Issues

[Christopher Bowne]

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.




________________________________
From: Neil <neilb at ihug.co.nz>
To: Christopher Bowne <aj1g at sbcglobal.net>; arc5 <arc5 at mailman.qth.net>
Sent: Sun, February 24, 2013 8:20:01 PM
Subject: Re: [ARC5] ARC-5 Issues

 
> 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