Small glowing tube/lamp

[Stylo N M]

Hi,

When I was young, I remember regularly taking the cover off our conventional dial telephone (usually when mum and dad went out) which in the early 1970s was just one standard phone in the hallway. I noticed though there was a small black coated tube which was wire ended, and on parts of it the coating would be worn off, or fallen off, so I could see inside the little holes, and what I saw was a tiny orange glow like a valve, when the handset was lifted.

So what is this little lamp, what is it called and what does it do?

Thanks,

Paul.

[Station X]

It was a barreter which formed part of the regulator circuit housed on a plug in PCB.

[russell_w_b]

Originally Posted by Stylo N M

So what is this little lamp, what is it called and what does it do?

It's part of a current regulator, Paul, used to bias shunt rectifiers to limit speech current when the telephone is on a short line. For long lines the regulator can be strapped out. 'Resistor, Bulb, No: 15'. There's an excellent description on Bob's telephone page, here, about 7/8 down under the heading 'Automatic Regulator': http://www.britishtelephones.com/t706.htm They were used on the later 746 instruments too, but hard-wired, as opposed to a separate plug-in PCB.

There're apocryphal tales of worried old ladies dunking their telephones in water (or somesuch) upon seeing the glow from within and thinking their telephones were on fire, in the days before the bulbs were painted black or covered up.

[Stylo N M]

Thanks graham, thanks russell, wow that's a good old story about the old laddies dunking the telephone! brilliant

Paul.

Fascinating, you learn something every day.

[TonyDuell]

The version I heard (which seems more likely) is that the bulb was painted black because the GPO were worried subscribers would see the glow and think the telephone was on fire, not that anybody actually _did_ think their 'phone was on fire.

706 type telephones (separate plug-in PCB for the regulator) normally have one bulb with 3 leads, a centre-tapped fillament. The 746 has 2 bulbs with 2 wires each in a plastic holder, they are wired in series externally to give much the same circuit. I have seen a 706-style plug-in PCB with the 2 bulbs and holder from a 746 on it. No it wasn't a bodge, it looked original.

[Radio Wrangler]

I heard the black paint was added because the GPO feared people might use the light to read something by, without paying them for the added functionality

David

[Herald1360]

Maybe nowadays you could hijack enough post office electricity using a switching converter to drive a high efficiency LED without tripping the line