Telefunken 12AX7 tubes, Ribbed plate, Diamond. Lot of (5), used, tested. -copy

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

This group tested at 51/54, 52/52, 54/59, 49/54, 52/58.  

All 5 have ribbed plates with 3 ribs & 4 holes.  All are marked "Made in West Germany", with white Diamond Telefunken Stamp.  Some of the ink is legible, Fair to good condition.  

All have the Bottom Diamond glass stamp between the pins, with the following numbers:  2, 5, 11, 23, 24.  

(Genuine Telefunken Germany (Berlin and Ulm) made tubes have a diamond mark molded into the glass between the pins. If the diamond mark is missing, the tube was likely made by Siemens or another of the Dutch Philips companies. The codes printed on the glass may be Telefunken codes, but without the diamond mark in the bottom glass the tube was not actually made by Telefunken.)

Tube Data:

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.