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Vacuum Tube Evaluation - Mixonline

Vacuum Tube Evaluation

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12A?7 Dual Triodes

MANUFACTURER/

TUBE TYPE

Bias/
range

Triode1/min
(see Note 1)

Triode2/min
(see Note 2)

Lo Gain-
Triode 1
(see Note 3)

Hi Gain-
Triode 1
(see Note 4)

Dist Lo
@+16.7dBu
(see Note 5)

Dist Hi
@+23dBu
(see Note 6)

GT 12AX7M #1

12/B

29/32

26/32

28.37 dB

34.45

dB 0.278%

1.343%

GT 12AX7M #2

12/B

23/32

27/32

27.7 dB

33.63 dB

0.920% (Note 6)

0.695% (Note 7)

GT12AX7M #3

12/B

32/32

30/32

27.86 dB

33.87 dB

0.610%

0.953%

GT 12AX7M #4

12/B

30/32

22/32

28.08 dB

33.94 dB

0.257%

0.943%

GT 12AX7M-MPI #1

12/B

25/32

24/32

28.02 dB

33.77 dB

0.234%

0.997%

GT 12AX7M-MPI #2

12/B

26/32

27/32

28.20 dB

34.18 dB

0.328%

1.338%

GT 12AX7M-MPI #3

12/B

26/32

26/32

28.23 dB

34.17 dB

0.262%

1.017%

GT ECC83 (V-1)

12/B

34/32

34/32

28.67 dB

35.05 dB

0.547%

2.317%

GT ECC83 (V-2)

12/B

34/32

36/32

28.28 dB

34.31 dB

0.609%

2.375%

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

EHX 12AY7 #1

15/B

32/32

23/32

25.59 dB

29.61 dB

0.204%,/td>

0.480%

EHX 12AY7 #2

15/B

33/32

30/32

25.54 dB

29.53 dB

0.203%,/td>

0.469%

EHX 12AX7 #1

12/B

31/32

33/32

28.57 dB

34.83 dB

0.204%

0.884%

EHX 12AX7 #2

12/B

32/32

34/32

28.69 dB

35.07 dB

0.246%

1.096%

EHX 12AU7 #1

24/B

51/56

55/56

20.9 dB

23.09 dB

0.362%

0.549%

EHX 12AU7 #2

24/B

54/56

62/56

20.5 dB

22.58 dB

0.362%

0.532%

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

GE 12AY7

15/B

26/32

27/32

25.49 dB

29.49 dB

0.236%

0.554%

GE 12AX7

12/B

20/32

21/32

28.50 dB

37.41 dB

0.417%

1.739%

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

RCA 12AU7

24B

52/56

64/56

21.22 dB

23.51 dB

0.401%

0.633%

RCA 12AX7

12/B

33/32

32/32

28.47 dB

34.64 dB

0.420%

1.690%

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

Mullard 12AX7

12/B>/td>

25/32

34/32

28.66 dB

35.00 dB

0.320%

1.33%

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

XXXX

Telefunken 12AX7

12/B

28/32

28/32

28.28 dB

34.33 dB

0.185%

0.913%

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.

NOTES:

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.