SYLVANIA 12AX7 vacuum tubes (5), used, tested. SOLD.

The test results are all above the minimum of 32 using calibrated TV7 testers.  

This group tested at 48/52, 46/48, 54/59, 39/40, 64/61.  Three of these tubes have the green sylvania stamp, one yellow, and one white ink.  

Plate conctruction varies, with one tube having a longer black plate.

The 12AX7 is a high-gain (typical amplification factor 100), low-plate-current triode best suited for low-level audio voltage amplification. In this role it is widely used for the preamplifier (input and mid-level) stages of audio amplifiers.

Typically configured with a high-value plate resistor, 100 kohms in most guitar amps and 220 kΩ or more in high-fidelity equipment. Grid bias is most often provided by a cathode resistor. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12AX7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched.