Hi,

That was perhaps the "Selectoject" which used a pair of
12AX7 (ECC83) valves in a phase-shifting audio peak or
notch filter. It was designed to go between the detector
and the first audio amp but could, I suppose, be used
equally well externally. If you will let me have an
address, I'll copy it for you from the 1956 Handbook or
from the appropriate QST CD.



73 de Joe, G3LLV