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Really? Huh... let's see. Trying this with a 6SN7, lowering the filament voltage sort of bumps it up a tiny bit, and then it drifts back to about where it was. A 6BK5 does about the same, it shifts but lingers about in the same spot, maybe just a hair higher. I also tried a NOS 12BY7 which reads around 68, also a very small shift. But... my 12AX7 tests good at about 60 with the filament set according to the manual, but if I drop it as you suggest... it's bad at 20 on both grids. That's the 12AX7 in my Zenith that controls the A.G.C and serves as the vertical oscillator.
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