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Old 31st January 2003, 11:34 PM   #1
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Default Cunningham/Radiotron RCA 58

Have a couple of old tubes and don't know their application:

- Cunningham RCA 58

- Westinghouse WL 868 (seems a diode).

Could someone shed some light on their use?
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Old 31st January 2003, 11:58 PM   #2
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I have a GE GL-868.. that's a photodiode. The large curved plate is actually the cathode, which is coated with cesium (or similar photoemissive chemical) which emits electrons in exchange for light striking it. The rod in front of it is the anode.

Data from WPS http://www.wps.com/archives/tube-dat...RPD-105A/3.JPG shows it is for audio transmission (possibly recovering signal from film soundtrack?) and starts dropping off at 10kHz, probably -3dB at 15 or 20kHz.
Be careful biasing it, in that you'll have difficulty measuring the voltage at its high impedances.

If you want to try a cute little sound transmission experiment, connect a neon light in place of a plate resistor on a 12AU7 stage. (It won't be very linear due to the low load resistance of the lamp, and possibly nonlinearity of the neon itself.)
Use a pentode or so to amplify the phototube, it'll need a lot of gain.

Tim
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Old 1st February 2003, 01:03 AM   #3
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Thanks Tim,

Its a quite stange looking diode for me, specially that gold colored cathode (which I thought it was the plate of a power diode).

I have no plans to make photo transmitting test though but apreciate your description. Didn't know for what this tube was intended for.
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