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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Photocell connection
Date: Sun, 10 Mar 2024 20:59:20 +0000
Organization: Poppy Records
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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.


-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk