Hello All, I recently acquired a Drake TR-4C SN 39893. On removing the top cover, for a tentative inspection, I discovered that V7, the RF Amp, had been changed from a 12BA6 (Remote Cutoff Pentode) to a Telefunken 12AU6 (Sharp Cutoff Pentode). According to the RCA Receiving Tube Manual both tubes have similar operating voltages. Also, NJ7P Tube Database lists the 12BA6 as a preferred substitute for the 12AU6; I wonder if the reverse substitution holds true? The TR-4C does receive but, as the circuitry was designed using a Remote Cutoff Pentode, is the use of a Sharp Cutoff Pentode degrading the performance of the Rig ? Should I replace the 12AU6 with a 12BA6 or leave well enough alone? E.N. Lurch in 'Fundamentals of Electronics' Third Printing, Aug. 1961; p.83/84 illustrates the differences between Sharp and Remote Cutoff for the 6AU6 and 6BA6 tubes. The graphs on p. 84 show quite clearly what happens but, as an amateur, the significance eludes me. I assume the Drake designers chose a 12BA6 over a 12AU6 for valid reasons. Any comments please. Bob ZL2AAQ
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