AW: SCR VCO?

[Eric Barbour]

UJTs and four-layer devices (SCRs, triacs, etc.) have all
been used for building VCOs. Their major problem is one of
temperature drift--it's serious. Especially in the case of the
UJT, which is often referred to as a "junk part" by analog
design engineers. (If you can find a copy of ANALOG DESIGN 
ENGINEERING: ART, SCIENCE AND PERSONALITIES by Jim Williams,
EDN Electronic Series, 1994, you will find a section in there
by Bob Pease about the agony of making a linear V/F converter
(VCO) with a UJT.)
  The gas thyratron is vaguely similar to a four-layer device
in electrical operation; it does not conduct until the control
electrode reaches the threshold, whereupon conduction occurs.
Shutting the device off requires shutting off or reversing the 
voltage across it. In a relaxation oscillator, this is done by
the capacitor which sets the oscillator rate.
  Where the difference comes in is in the utter un-transistor-like
behavior of the thyratron. It is dependent on a hot cathode and
gas ionization, not on avalanching of a 4-layer semiconductor structure.
The 4-layer device varies greatly with temperature, while the
thyratron shows almost no variation. Both devices are not very
linear V/F converters, unless many schemes are undertaken to make
them linear. Still, there is one advantage of the tube over its
analogous semiconductor device.
  As for the sound and behavior, well, Bob Edgar got a chance to
try my modular tube synthesizer this past Sunday. Ask him if it's
real. Any comments Bob?

-- 
Eric Barbour
Svetlana Electron Devices
Portola Valley CA USA