RCA and Sylvania
first introduced the 12AX7 to the world in the late 1940’s. Today, it is by far the most common preamp
tube used for electric guitar amplification and there is an almost overwhelming
supply of current production options to choose from. The purpose of this comparison is simply to
provide guitar players with a frame of reference to help in finding the best
current production 12AX7 for their needs.
Also included in this comparison are direct substitutes for
the 12AX7 including: ECC83, ECC803, 7025
and 5751.
12AX7 Basics
The 12AX7 is a 9-pin miniature, high-mu, twin triode. What does that mean? The 9-pin
miniature part tells us what kind of socket the tube plugs into. It has nine thin pins arranged along the
circumference of a circle with a diameter of just under half of an inch. There is a large gap between pins 1 and 9 to
ensure that the tube can only be plugged in one way. The high-mu
part is referring to the electronic specification called “amplification factor,”
abbreviated by the Greek letter μ (mu - pronounced myoo). This gives us a general category of
amplification that the tube was designed for as compared to other preamp tube
types like the “medium-mu” 12AU7. The twin triode part tells us that there
are two identical and independent triodes contained within the one tube. A triode is the most basic tube structure
that allows for signal amplification and it has three electrodes: plate, grid and cathode.
The Gain Test Setup
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. From the frequency plot it
could be seen that there are three extrema at about 50 Hz, 400 Hz and 4,000 Hz. These three frequencies were then used to quickly
measure the low, mid and high frequency voltage gain on the same number of
samples for each tube in our comparison.
The average from each tube’s samples was then calculated and used for
comparison. The smallest average low
frequency gain tube was used as the reference for a gain of “1” and the largest
average low frequency gain tube was used as the reference for a gain of
“10”. This gain reference method was
then repeated for both the mid and high frequency averages.
The Noise Test Setup
Each tube sample was again tested in the same preamp circuit
and an oscilloscope was connected to the amplifier output to monitor the
amplitude of the voltage spike produced by tapping on the tube’s glass
envelope. The average voltage spike from
each tube’s samples was then calculated and used for comparison.
The Listening Test
Setup
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 separate filament transformer was used on the switching box to
simultaneously heat all six comparison tube filaments without overheating the
amplifier’s power transformer. 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. Listening tests
were then conducted while playing guitar and switching between the tubes at
various amplifier control settings to come up with tonal descriptions for each
tube.
Kurt Prange (BSEE) is
the Sales Engineer for Amplified Parts (www.amplifiedparts.com)
in Tempe , Arizona ,
United States Michigan
Results PDF
Results PDF
Posted 03/2013
