About the 9A and 9AJ Tube Basings
9A, 9LP, and 9AJ are EIA/RETMA designations for 9-pin miniature dual-triode tube types which share the same plate, grid, and cathode pin arrangement but have different filament wiring. 9AJ-based tubes have a single 6.3 volt filament connected to pins 4 and 5, whereas 9A-based tubes are have a 12-volt filament connected to pins 4 and 5 with a center tap connected to pin 9.
When the WV5 or WV11 line stage filament switch is set to "9AJ" the 6.3V heater supply power is applied to pins 4 and 5 for correct operation of 9AJ-based dual triodes with 6-volt filaments. When set to "9A", pins 4 and 5 are connected together and to one side of the 6.3V supply and pin 9 to the other side. This permits use of the 12-volt 9A-based tubes in the line stage.