Re: Photocell connection

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've noticed that when gas-filled photocells were used in valve
equipment, they were nearly always supplied with a low-impedance source
of +ve voltage to the anode and the signal was taken off a resistor in
the negative return.  There is a blocking capacitor between the
photocell cathode and the grid of the valve, so the standing current and
DC conditions don't appear to be relevant.
 
This means the photocell has to be connected by a 2-core screened cable,
which was an expensive luxury in those days.  It also has to be
thoroughly screened to prevent hum, whereas the cathode half-cylinder
would partly screen the anode and reduce the amount of extra screening
needed;  so what was the advantage of taking the signal from the cathode
instead of the anode"?
 

 
At a guess, it’s because it saved headroom to have a positive-going signal.
There’s a well-defined dark level.
 
Of course, in a general AC-coupled system, that doesn’t help as much unless
the photocurrent waveform is known to be very asymmetrical.

The particular application I was thinking of was film projector sound.
The signal is in the region of a few millivots and the photocell supply
is about 80v, so headroom isn't a problem.  The front end pentode is
often triode-strapped to reduce partition noise.


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~ Liz Tuddenham ~
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