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From: John Larkin <jl@997PotHill.com>
Newsgroups: sci.electronics.design
Subject: Re: Photocell connection
Date: Sun, 10 Mar 2024 11:04:12 -0700
Organization: Highland Tech
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On Sun, 10 Mar 2024 12:30:18 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) 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"?

Tube systems generally had a positive power supply, with a bit of
ripple, so taking the small signal off the anode was a nuisance. A
phototube usually needed to be shielded anyhow, unless it was
measuring a very slowly changing light source.

I just hung a cathode follower next to mine, phototubes or PMTs. That
kept the speed up too.