Here is an excellent vintage film about vacuum tubes called “Electronics At
Work” courtesy of the Prelinger collection at the Internet Archive. While not
specific to just audio tubes, this WWII era film gives an excellent visual
representation of how tubes work and their innumerable uses. Among other things
the film illustrates how vacuum tubes can be used to amplify as well as
rectify. Of particular interest is the use of tubes for radar during WWII.
You already knew that vacuum tubes were capable of producing shimmering cleans
and overdriven crunch, but did you know they also helped win the war? So next time
you pop open your Marshall to put in a new batch of 12ax7’s you should have a better
understanding and whole new respect for our friend the vacuum tube!
Bradley Simpson of The Gear Page has done an excellent comparison review of 7
different classic NOS (New Old Stock) 12ax7 tubes. The tubes reviewed are among
some of the best ever made. One must remember that the scarcity and quality of
these NOS tubes has caused prices to skyrocket compared to new production tubes,
but many tube aficionados find the sonic rewards to be well worth the price.
Also remember that any tube review is subject to the personal opinion and most
importantly the ears of the reviewer. Regardless, this is an excellent review
and well worth reading. Check it out here.
12ax7 Tubes are the most famous and widely used guitar amplifier preamp tubes
in history. This venerable tube can be found in almost every famous (and not
so famous) tube guitar amplifier ever made. Whether it’s a simple single ended
amp like a Fender Champ or Epiphone Valve Junior, or the bigger Fenders and
Marshall amps and on to the more modern super high gain amps with cascaded
preamps, the 12ax7 tube was there. A single ended preamp almost always will
consist of only a single preamp tube. On the other end of the spectrum some
super-charged channel switching amps might have as many as eight 12ax7
The 12ax7 is what is known as a dual triode. This means that it is that
there are actually two tubes in one. This allows a clever amp designer to
create a more versatile amp by either taking out half of a triode for a cleaner
sound or adding an extra triode in a circuit for some added .gain.
Choosing the right 12ax7 tubes for your amp can be a confusing process. It
wasn’t too many years ago that a guitarist had somewhat limited options for
purchasing new tubes. Sometimes the only option was to purchase expensive and
hard to find NOS tubes (New Old Stock). New tube production had dwindled and
there even seemed to be a possibility that tubes could become obsolete
altogether. This was due to the fact that vacuum tubes are expensive and
somewhat difficult to manufacture. The good news is that in the last decade or
so the quality and number of choices of high quality and reasonably priced new
12ax7 tubes has grown by leaps and bounds.
Some name brands that are producing high quality new 12ax7 tubes are
Tungsol, Mullard, Sovtek, Electro-Harmonix, JJ, Groove Tubes, Svetlana and
others. These tubes all share similar designs with sometimes subtle
differences. Some of these tubes are regarded to be sweeter sounding than
others and some might sacrifice some sound quality in the name of reliability.
It is important research which tubes you need for your amp. Often times a a mix
of different types of 12ax7 tubes in different positions of your amp will
provide the best performance.