Low voltage tube crystal oscillator

by sv3ora

The biggest disadvantage when operating tubes on high voltages is not the high voltage PSU, this can be relatively easy to build. The biggest disadvantage is that most of the other components in the circuit must be also of high voltage. This increases the cost at a geometric rate, since one cannot use common modern low voltage components anymore. When the specific application you build, operates on  low signal handling, then it is perfectly fine to use low anode voltages in some stages and then use high voltage only when great amplification is needed and not throughout the whole circuit.

Here is a vacuum tube crystal oscillator I have designed and tested, which works fine with low anode voltage of 6V, just like the filaments of the tube. I have tested the oscillator on 6MHz but since it is a pierce oscillator, I do not see why it should not work at many other frequencies by just changing the crystal. I set the 6-50pF trimmer so that the oscillator has very low distortion and at the same time achieve a sure-start when the oscillator is turned on. The oscillator of course can be set for a higher level (and higher distortion) if you do not care about harmonics. Even at its lowest level, it can be loaded to 50 ohms without stopping oscillation, but of course the level reads lower. Despite being a pierce circuit, I could not detect any harmonics on my FFT when set for minimum distortion (thus minimum signal output).

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