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Vacuum Tube Evaluation

12A?7 Dual Triodes

About Table-1: The “#1,” “#2,” etc. designations in the tube manufacturer/type headings indicates that several tubes were tested within each type. 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 second order (octaves). The GE, RCA, Mullard and Telefunken tubes were added to provide a reference, but here again tubes can vary-even within the same brand/type. 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.


1. tube tester reading / minimum tester reading (For Triode-1: Plate = pin-1, Grid = pin-2, Cathode = pin-3)

2. tube tester reading / minimum tester reading (For Triode-2: Plate = pin-6, Grid = pin-7, Cathode = pin-8)

3. Triode-1 gain with no cathode bypass cap

4. Triode-1 gain with a cathode bypass cap

5. Measured % distortion of Triode-1 at approximately +16dBu (lower for non-12AX7 types)

6. Measured % distortion of Triode-1 at approximately +23dBu (lower for non-12AX7 types)

7. This is the only tube that exhibited a significant third-harmonic component.

8. 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 occurred.

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.