[Boatanchors] Question about a Central Electronics 20-A exciter

[WF2U]

Recently I acquired a fairly clean Central Electronics 20-A exciter. I have
the original manual, which doesn't show the schematic of the QT-1 module in
it. It has a 7-pin miniature tube socket on the side. I would like to know
what tube goes in there (if any) before I fire up the rig. BTW it'll be
paired up with a Drake 2-B receiver for a low-power vintage SSB station.

Thanks,

73, Meir WF2U / NNN0AAF
Landrum, SC