• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tubes and the sun?

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Certain electrometer tubes like the Raytheon 5886 are very light sensitive and must be operated and tested in darkness. I assume because of their extremely high input impedance. Every instance of use I've seen by such manufactures as Keithley, General Radio and Victoreen shields them from any external light. Photomultipliers are also shielded from light other then the operating window.
 
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