• 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.

do 6h30 glow at the top only

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Most small tubes with heaters & separate cathodes (indirectly heated) are made by just inserting the heater wire into a hollow cathode. This is *not* a precision procedure for most tubes (and probably doesn't need to be). The result is that you may or may not be able to see the heater at the top or bottom of electrically identical tubes. You *should* be able to see the heater glow, but even tubes from the same run often have different amounts of glow, and sometimes visible top only.
 
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