[Chas]

So this weekend I'll make the adapter to change a good tube to a dud tube

Use the tube with an open filament or take a tube that has too low an emission and cut off one of the filament pins.

Creating an adapter will add a few pf to the circuit, that will affect the neutralization such that it will not be null when the good tube is reinstalled...

Watching a meter will only distract when neutralizing. The ear is ample enough instrument and no leads be connected.

Theoretically, forcing a strong signal to neutralize may not be a good idea as Miller effect may come into play and cause an error. Be sure all shields are in place and any bottom plate too.

If the set had a tendency to oscillate in one area of the band, note that area and neutralize at that frequency. If the area of oscillation has moved and set still oscillates split the difference in the neutralizing frequencies and try a third time.

If still no joy then component placement, RF tube(s), voltages, grounds or wiring may be an issue, hopefully not as they are even more "cut and try".

GL

Chas