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.