Emission tube tester mod with polarized plug
 

http://videokarma.org/attachment.php...1&d=1460669218
This is your typical emission tube tester. It's a "hot chassis" design. And you could "find" the powerline via the top cap connection, depending on the selector switch setting. Oh, another setting could give you a path to the other side of the line via a 1.8K resistor and the sensitivity control. Not exactly that safe, but if we put a polarized plug on the tester we can avoid the "harder" connection first described. If we connect the cord's hot wire to the power switch.

Don't use a 3 pin plug to ground the metal chassis of the tester. Same reason they don't ground the metal cases of toasters, as someone trying to remove a stuck piece of bread would be holding that case and touching the heating element with a knife.

All newer toasters have a double-pole switch to isolate the elements, per U/L.
Those Accurate Instrument test instruments, were the ones advertised on the back of magazines like Popular Electronics and others. :scratch2: