Low voltage tube crystal
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).
Back to main site