Summary
The 12AX7 is a miniature dual triode vacuum tube with high voltage gain
and basically two 6AV6 triodes in one package. It has a center tapped
filament enabling it to be used in either 6.3V 300mA or 12.6V 150mA
heater circuits. Originally intended as a miniature form-factor follow-on
to the 6SL7 family of dual-triode amplifier tubes for audio applications,
its ongoing use in the preamplifier sections of hi-fi and guitar amplifiers
make it one of the very few small-signal vacuum tubes to continue in
production since its inception. variations include 12AT7, 12AU7, 12AD7
& 12AV7. The European equivalent is the ECC83
History
The 12AX7 was originally developed in 1946 by
RCA engineers in Harrison, New Jersey and released to the public in
September 1947.
Application
The 12AX7 is a high mu double triode, having separate cathodes and primarily
intended for use as a resistance-coupled amplifier or phase inverter.
In their classic designs of the 1950s and 1960s, Mullard used the tube's
two halves as second stage amplification and cathode coupled phase splitter,
the phase splitter being required so that the push-pull output stage
was driven correctly.
Typically a 12AX7 triode is configured with a high-value plate resistor, 220k ohms or more in high-fidelity equipment and 100k ohms in most guitar amplifiers. Grid bias and some negative feedback are often provided by a cathode resistor. In this mode each half of a 12AX7 can provide a voltage gain of about 60. With its high Miller capacitance, the 12AX7 it is not really suitable for radio-frequency use.
This tube is still found in today's designs of hi-fi audio amplifiers and almost every guitar amplifier made since the 1950s..
Variants
12AX7 Original version. The earliest types manufactured
in the USA by RCA, Sylvania, GE and Tung Sol etc.
ECC83 European equivalent to 12AX7 (Mullard, Telefunken,
Amperex etc)
12AX7A A later version which can be used in series
or parallel filament circuits.
7025 A special low noise "hi fi" version
of the 12AX7 - all other specifications identical to the 12AX7
ECC803, E83CC Premium European 12AX7 versions
ECC803S Premium "Select" - usually only found
on Telefunken branded examples and today fetching prices of around $1000
a matched pair.
12AD7 (1955 - 225mA heater - low hum)
It is interesting to note that the 'E' in the European designation explicitly classified this as having a 6.3 volt heater, whereas the American designation of 12AX7 explicitly classified it as having a 12.6 volt heater. It can, of course, be wired for either operation.
The 12AX7 has, in previous years been manufactured in the USA, Canada, virtually every European country, Australia, Japan, India, Argentina, Chile, Brazil, China and USSR. production still continues in Russia, China, Slovakia and Serbia.
Current value
Original Telefunken matched ECC83's can fetch up to $100 each
if in mint condition and testing 100%. NOS Amperex, GE and Mullards
all sell fro between $40-$80. New production Electro-Harmonix and Svetlana
types typically sell for between $15-$25 while low-cost Chinese 12AX7's
can be obtained for as little as $7.00. ECC803S Premium "Select"
- usually only found on Telefunken branded examples - can today fetch
prices of around $1000 for a matched pair.
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