I got Eric Barbour's tube synth schems.

Tom May ftom at netcom.com
Tue Jun 10 06:11:46 CEST 1997

Not too long ago somebody here posted a pointer to the Keyboard
magazine web page about Eric Barbour's tube synths.  I shelled out my
$3 and Eric mailed me the schems.  It looks totally buildable.
Modules are:

o Thyratron-controlled VCO.  About 2.5 octaves of control.  Soft(?)
sync.  Octave switch.  No temperature compensation necessary.  The CV
is neither V/oct nor V/Hz, but Eric suggests it is possible to run a
V/Hz CV through a Burr-Brown 4302 multifunction converter to drive it.
His scheme uses a custom keyboard controller with pots to tune each
note.  He suggests retuning every 1000 hours.

o VCF is "a highly simplified state-variable circuit, using a single
CdS-cell optocoupler [available from Mouser] for center-frequency CV
control.  Its Q may be varied over a small range by applying a 0-30v
CV to the screen grid of the first pentode."  HP and LP outputs.

o VCA.  Just a pentode and a few parts.

o AR generator.  Produces impulse waveform (from gate) with adjustable
decay.  Includes a stability control that allows it to self-retrigger
for use as an LFO.  (Eric says "standard ADSR circuits are possible
but difficult to implement with tubes.")

o LFO.  Sine and square out.  Speed knob.  Looks like a guitar amp
tremolo oscillator.  Optional eye tube indicator (cool).

o Beam modulator.  Another module with just one tube and a few parts.
One signal in, two modulation inputs, one knob (could be a CV) to
control the screen voltage, one output.  Looks like a possibility for
stereo effects if you tap off both plates (Eric only uses one).  This
is just begging to be built!

o Power supply provides +/- 150V and heater supply.

I might take a shot at building this when I come back from vacation in
August.  I'm not sure I'll build his oscillator, I've got a design of
my own in mind which is more like a solid-state oscillator (including
sync) although Eric says he tried something like it and it didn't give
as much range as his design.


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