The 5687 Twin Triode: The Computer Vacuum Tube That Turned Hi-Fi?

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. 

Should Radio Frequency Power Triodes Be Used As Audio Amplifiers?

Even though some of them have gained fame as excellent audio output tubes, should radio frequency power triodes be used as audio amplifier output tubes?

By: Ringo Bones

They are quite big and glow brightly in a darkened listening room in comparison to “ordinary” audio vacuum tubes, radio frequency power triodes had been used to great effect during the hi-fi revival of the 1990s in making modern manufacture vacuum tube audio power amplifiers that are still coveted until this day. Modern telecommunications engineers may scoff at the practice but are radio frequency power triodes “less-than-ideal” for use in high-fidelity single-ended triode audio power amplifiers?

From an electronic engineering standpoint, most radio frequency power triodes are usually too high in output impedance to make them into successful audio output stages. The 211, the 845 and the Russian GM70 transmitter tubes R.F. power triodes are rare exceptions. Nevertheless without a proper interstage transformer to drive them, most vacuum tube amplifiers using radio frequency power triodes manufactured back in the 1990s never managed to reach their optimum power output. One Mainland Chinese manufactured 211-based power amp only managed to produce 3-watts because the designer didn’t use a proper interstage transformer to properly drive the 211 transmitter tube with the double triode preamp driver tube.

One of the few things I’ve learned doing DIY hi-fi during the 1990s is that audio output transformer primaries with lower number of windings / turns and therefore lower impedance usually result in better sound due to the lower second harmonic distortion produced. For good open-loop linearity, the load impedance seen by the voltage amplifier / intermediate stage amplifier / phase splitter must be high and the following stage should disturb this situation as little as possible. Or maybe it is just that radio frequency power triodes are not primarily designed for audio amplifier use.