The average voltage spike from each tube's samples was then calculated and used for comparison.A custom 12AX7 switching box was constructed to allow for a quick switch comparison of six tubes at a time while plugged into the V1 socket of a Marshall JCM800 2203 guitar amplifier.
A shielded cable was constructed to plug into the amplifier's V1 socket and transfer its pin 1, 2, 3, 6, 7 and 8 connections to one tube socket at a time in the switching box.
Each tube sample was tested in the same (Fender black face style) preamp circuit as shown in the schematic below with bass and treble set to their maximum.
A frequency generator was used to measure voltage gain and plot the frequency response from 10 Hz to 18,000 Hz.
Also included in this comparison are direct substitutes for the 12AX7 including: ECC83, ECC803, 70.
The 12AX7 is a 9-pin miniature, high-mu, twin triode. The 9-pin miniature part tells us what kind of socket the tube plugs into.