Path: Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Sun, 10 Mar 2024 18:05:37 +0000 From: John Larkin Newsgroups: sci.electronics.design Subject: Re: Photocell connection Date: Sun, 10 Mar 2024 11:04:12 -0700 Organization: Highland Tech Reply-To: xx@yy.com Message-ID: References: <1qq7eb6.zb9gu01dv4dlgN%liz@poppyrecords.invalid.invalid> X-Newsreader: Forte Agent 3.1/32.783 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 29 X-Trace: sv3-coD9WaAfFg3Vs7h5cIzTFOvBXARLNIWG0pscQ9Z1blJxSg3L5x4W0ixjsbyhSBzA/1SUKt4C/DLSCFy!Ykgd+iaT+X6EWNmuyqJX7eQqFvJbto+GkWxZbYC8QCkblJi6N27hi631ASd5/z79yc0JiA1DQeSH!fo2B8A== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 2328 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.