• 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.

6GM8/ECC86

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If pin 9 is the shield, then it probably has nothing to do with the heater. If a shield is present, then it should either be grounded or attached to a cathode - basically you want it grounded for AC, and negative for DC. The former ensures that it does act as a shield and reduces capacitive coupling between valve sections; the latter ensures that it does not pick up stray electrons.
 
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