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.
2 comments:
Even before the days of regenerative / negative feedback, audio frequency amplifiers that operate in audio frequencies - i.e. 20-Hz to 20,000-Hz and radio frequency amplifiers with a high-Q that operate only at a very narrow range of frequencies between 15.000-Hz up to 300-MHz or more are very different animals. Even back in the days of directly-heated triode vacuum tubes.
Whereas the audio frequency amplifier must handle its entire audio frequency range at one time, the radio frequency amplifier / transmitter using radio-frequency triode vacuum tubes, is only called upon to handle only one frequency at a time - or at most, a very narrow band of frequencies. For this reason, we deal with a non-resonant or untuned circuits when we consider the audio-frequency amplifier, whereas we deal with resonant or tuned circuits when we consider the radio-frequency amplifier.
In the frequency response of tuned circuits and filters, Q describes the circuits' selectivity or the degree of attenuation of unwanted signals. In general, the higher the circuit's Q, the more selective it is. And audio amplifiers tend to have way, way less Q in comparison to radio-frequency amplifiers / transmitters.
Another difference is that the chief bugaboo of the audio-frequency amplifier is harmonic frequency distortion. Because of this, in the audio-frequency amplifier world, we can operate a single output power vacuum tube only in class-A. Output power vacuum tubes must be operated in push-pull for class-AB and class-B circuits. In the case of the radio-frequency amplifier, however, harmonic distortion has very little effect because of the selectivity of the tuned circuit in radio-frequency amplifier have a much higher Q in comparison to audio-frequency amplifiers. By their very nature, harmonics do not distort radio-frequency amplifiers. The second harmonic of a 500-KHz r-f signal, for example is 1-MHz. It would have to be a poorly tuned circuit, indeed, to pass both the fundamental (500-KHz) and the second harmonic (1-MHz). Because of this, radio frequency power triodes / transmitting tubes are not primarily designed for audio - thus the need for an improved version of the 300B audio triode like the Svetlana 811 Power Triode.
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