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.