Even though it was originally manufactured for use in vacuum
tube era mainframe computers, is the 5687 twin triode the computer vacuum tube
that got repurposed for high fidelity use?
By: Ringo Bones
Described by vacuum tube hi-fi specialist outlet Upscale
Audio as a “musical Mighty Mouse of a triode”, the 5687 vacuum tube is still
quite widely available to this very day – and yet Electro-Harmonix, Sovtek,
Svetlana and other Russian and Mainland Chinese vacuum tube manufacturers never
managed to introduce their “modern manufacture” version of this venerable
small-signal twin triode. Even though it is still relatively in plentiful
supply, I wish that vacuum tube manufacturers introduce a dedicated audiophile
version of it since the 5687 twin triode is primarily a “computer” vacuum tube
that dates back from the 1950s.
My first encounter with the 5687 twin triode was back in
1994 with the World Audio Design 300B amplifier. Since then this particular vacuum
tube is primarily used in modding or hot-rodding classic hi-fi vacuum tube gear
to make them sound better – as in using the 5687 twin triode to replace the
12AU7 twin triode. The 5687 is a medium mu (mu = 17) dual triode with a Noval
B9A base, 12.6 volts parallel AC/DC heater with a heater current of 0.45
amperes and 8.4 watt total power dissipation of both sections. Originally
intended for vacuum tube era mainframe computer applications, continuous wave
or pulsed oscillator and cathode follower circuits, it is somewhat of a miracle
that the 5687 twin triode managed to sound very good in audiophile
applications.
Even though virtually all of the 5687 twin triodes available
today are new old stock (NOS) types, the most common brands are Tung-Sol
followed by Sylvania, RCA, GE, Raytheon and CBS. Once mentioned in a Guitar Player
article back in 1988 as the most widely available preamplifier vacuum tube
that’s virtually useless in electric guitar amplifier applications when Fender
found out that the US government already stockpiled a 200-year supply of it
during the Reagan era vacuum tube shortage, things began to change in the early
1990s when audio enthusiasts found out that the 5687 is a perfect and better
sounding replacement of the 12AU7 vacuum tube in driver and phase-splitting
applications. And the 5687 is also dead linear and much easier to drive than
the 12AU7 due to its relatively high mu.
