ABOUT TABLE-1: Each triode section was first evaluated in a
tube tester, the resulting "readings" (in bold) are relevant only
to the "/ the minimal reading" (not bold) as per Notes-1 & 2.
Triode–1 of each tube was then evaluated for LO GAIN and HI GAIN in a preamp
circuit that emulates a conservative mic preamp and a more aggressive guitar
preamp, respectively. With the exception of GT-12AX7M-2, all measured tube
distortion is primarily 2nd order (octaves).
For tubes of same type but different manufacture, the tester readings
do not seem to correspond with the gain measurements. A future test will
determine whether there is relationship between each triode’s gain, within
the same tube, and the respective tester reading.
NOTES:
-
tube tester reading / minimum tester reading (For
Triode-1:
Plate = pin-1, Grid = pin-2, Cathode = pin-3)
-
tube tester reading / minimum tester reading (For
Triode-2:
Plate = pin-6, Grid = pin-7, Cathode = pin-8)
-
Triode-1 gain with no cathode bypass cap
-
Triode-1 gain with a cathode bypass cap
-
% Distortion of Triode-1 at approximately +16dBu (lower for non
12AX7 types)
-
% Distortion of Triode-1 at approximately +23dBu (lower for non
12AX7 types)
-
This is the only tube that exhibited a significant 3rd harmonic
component
-
Distortion Range Notes: The nominal levels chosen for the LO-GAIN /
HI-GAIN measurements are 12dB to 18dB down from +35dBu. Using the GT-ECC83
as reference, this is the level just before an obvious asymmetrical flattening
of the waveform.
-
A.) @ +35 HI-gain dist = 10 % w/ bypass cap
-
B.) @ +29 LO-gain dist = 2 % w/o bypass cap
-
C.) @ +35 LO-gain dist = 5 % w/o bypass cap but at same level as A.
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