• 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.

anyone able to decipher a 539 Sylvania date code?

Month 9 of 1953?
Month 5 of 1939?

What is the tube type?
Has the tube been tested?
Is the tube any good?

Date codes can not prove the condition of the tube;
And date codes can not prove the match, or non-match, to a tube of another date code, or even of the same date code.

Is this a valuable tube that will be sold for $$$?
Test first, ask questions later. :^)
Find a tube tester, and test it again. 25 years can make the tube less capable, and can do that to me too.
I have always found the best tube testers are a working tube circuit.
Yes, you have to be careful as you bring the tube up to full voltage; a Variac and one or two DMMs can monitor the tube as you slowly bring it up. Lots of work to do it that way, but worth the try.
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