[Mondial]

Skip, if you make a neutralizing adapter, be sure to make the interconnecting wires as short as possible. The adapter sockets and wires have capacitance between them, so it affects the interelectrode capacitance seen by the circuit.

Perhaps the most accurate method of neutralization is to insulate the filament pin of the actual tube used for each stage, one at a time. This way you are neutralizing the actual grid-plate capacitance of each specific tube and not adding any stray capacitance.

I usually wrap a small piece of clear Scotch tape on one of the filament pins for neutralizing. The tape is thin and doesn't interfere with inserting the tube in the socket, yet insulates the pin so the filament is not lit.