[sdiy] Re: did someone say "tube"?

[Thomas Holley]

    Thanks a lot for the suggestion. It sounds doable if my original idea
tanks, or maybe even if it doesn't. Now I have a simple voltage clamp
circuit using 2 12au7 dual triodes that works great as a sampling circuit.
It essentially clamps the output (Sample) at a proportion of the input
(Keyboard Buss) and is activated by a negative trigger pulse. The output can
be scaled to a near 1 to 1 relation with input and the output is seen across
a capacitor similar to a conventional sample and hold opamp circuit. So far
so good. The problem comes in, at least in simulation, when I connect the
output of the sample and hold cap to another tube to generate the CV. The
simulation shows the cap bleeding off too rapidly and causing nasty pulses.
I must breadboard the circuit to find actual results. I thing the SPICE
simulation is false in this area.
    The Gate is done with a bistable multivibrator and diode. Trigger uses a
thyrotron. I can send you a preliminary schematic if you are interested.

Thomas
----- Original Message ----- 
From: "Ken Stone" <sasami at hotkey.net.au>
To: "Thomas Holley" <thomasholley at adelphia.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, June 08, 2003 9:09 PM
Subject: Re: [sdiy] Re: did someone say "tube"?


>
> I've been thiking about your tube based keyboard with the tube sample and
> hold problems. Perhaps you are approaching this the wrong way.
>
> Instead of trying to capture the voltage, capture which key is pressed
> instead. Use a neon lamp based latch, where only one neon can remain lit
at
> a time, then take your voltage from that via an appropriate divider or
diode
> "binary encoder" and a D/A resistor ladder. Sure, you'd need to use
diodes,
> though as this was common practice in a lot of tube equipment, I don't
think
> that it is cheating. Wurlitzer organs used a similar latching arrangement
> for their organ pedal boards, though of course they were gating existing
> pulse trains rather than deriving a voltage.
>
> Ken
> _______________________________________________________________________
> Ken Stone   sasami at hotkey.net.au
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