[ARC5] ARC-5 Issues

[Dennis Monticelli]

I like to homebrew small CW transmitters and I always include an "R75" for
the purpose you stated and for some other practical reasons.  It is
possible to limit the cathode rise to the level where it is not dangerous
to touch the key terminals in a cathode keyed circuit.  The lower voltage
is also easier on keyer output stages.  Finally, cathode keying usually
results in key clicks unless you do some shaping.  The most common method
for shaping a cathode keyed circuit is to use a capacitor or resistor-cap
combo. The lower voltage rise limits the stored energy in the cap and
reduces the tendency to spark.

Dennis AE6C

On Wed, Feb 27, 2013 at 6:44 PM, Neil <neilb at ihug.co.nz> wrote:

> 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
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