I got Eric Barbour's tube synth schems.
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.
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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