NOS 12AX7 / ECC83 / CV4004 / 7025 / B759 / M8137 / ECC803S
12AX7 Vacuum Tube Reference Information
ECC83: This European nomenclature for 12AX7.
ECC803/ECC803S/E83CC: This "premium" version of European made 12AX7. The "S" on the ECC803S indicates "select" and are usually gold pin. These are usually found on Telefunken and Siemens branded tubes and a very rare Radiotron ecc803S version from the Philips factory in Australia.
12ax7: Original version of the 12ax7. Early production was manufactured in the USA by Tung Sol, RCA, Sylvania, GE etc. The earliest 12ax7 have larger 17mm plates. The 12ax7 version without an "A" suffix is to be used only in parallel filament circuits, so do not use these in a “series” circuit. This is an unusual problem to run into as they are normally found in TV sets. The best way to tell if your circuit is a series circuit is by examining all of the tubes and looking at the voltages (voltages other than 6 and 12 volts). If all tubes in your circuit start with 6 or 12 (except for 5 Volt rectifiers) , then it is a parallel.
12ax7a: Can be used in series or parallel circuits. Many manufacturers marked 12AX7's as 12ax7 without the “A” suffix well into the 60's and 70's, but these are most often 12AX7A's marked as 12AX7.
12AD7: Non-microphonic/low hum version of the 12AX7 made by Sylvania and NEC in Japan. Great for audio applications that uses a 12AX7.
12DT7, 12DM7, 12DF7: Special quality low noise/low microphonic versions of the 12AX7. Usually branded as Westinghouse or Raytheon.
7025: Hifi low noise version of the 12AX7 with all other specifications being the same as 12AX7. Tubes marked with both 12AX7 and 7025 are usually low noise as well.
5751: Military version/drop in replacement for the 12AX7, but is actually different. The 5751 has identical plate resistance to the 12AX7, but the gain is 70 compared to 100 for the 12AX7. Early production 5751 have extra thick mica spacers (often three know as triple mica) and extra support rods to minimize microphonic response. Many 5751 also have matched triode units.
7729: CBS made industrial version of the 12AX7 with gold pins for special applications requiring reliability and low noise/low microphonics.
6681: The mobile version of a 12AX7. Usually these are the same as a regular 12ax7 which have been tested and screened to perform well in circuits with large variations in filament voltage.
7058: Same as a 12AX7 but with a 13.5 volt non-tapped filament. Can be used in circuits which do not utilize the 6.3V filament tap. Performance is same as regular 12ax7.