[Hallicrafters] 12ax7

[Fred Olsen]

DAN COTSIRILOS wrote:
> what is the difference between a 12ax7 and a 12ax7a?

Well, not QUITE "nothing".  The 12AX7A (versus 12AX7) has controlled hum 
and noise characteristics, a max Vplate of 330 versus 300, 1.2W plate 
dissipation per plate versus 1.0 (by RCA, Sylvania says 1.0 for the 
"A"), -55 max negative grid bias versus -50, and a peak heater-cathode V 
of 200 versus 180.

The ratings differences amount to squat, unless it's used in a poorly 
designed circuit that runs it too close to ratings.  The major factor is 
that the A is specified for an average total noise and hum of 1.8 uV 
TRMS, per unit; and the non-A isn't so specified.

Fred
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