Expo convertors with tubes ?

[Debby and Gene Stopp]

It's good to see so much discussion about tube-based synthesizers - I have
recently become involved in tube circuits quite heavily (rebuilding a bunch
of old dead amps that have been sitting around for years). I think that
there is a lot of potential here (pun intended) for a unique category of
sound. It seems to me that the transfer functions involved in the audio
chain and modulating voltages would be sufficiently different from those of
traditional synthesis circuits that it would justify this kind of project.
The slopes of the envelopes and the tone of the output and the shape of the
waveforms would be determined by active elements that are using different
rules than their semiconducting cousins. Things would resonate and ring and
charge up and down differently.

It looks like it's going to be tricky to get the VCO's to do the same thing
as in traditional synthesizers. Wide-range voltage control of frequency with
the stability to reliably produce musical intervals seems to be too
important a function to abandon. Using discrete steps with switched passive
elements, although highly practical, has the disadvantage of not allowing
things like interval-tuned multiple oscillators, envelope pitch modulation
and portamento - three highly distinctive attributes of synthesizer playing.
It would be a shame, however, to have to resort to a hybrid approach whereby
the VCO's are solid-state and the rest of the signal chain processes these
function-generated raw waveforms.

Perhaps it would not be too much of a travesty to use solid-state VCO's in
the core of the waveform generators. These VCO cores could perhaps drive
tube-based waveshapers, so that the actual waveforms are created using tube
electronics. Or maybe, the tube circuit could be a phase-locked loop,
creating the waveform by itself but being "coached" to the proper frequency
by a solid-state oscillator. Much like the tone source in the ARP Pro
Soloist, for example. The PLL concept could even have the potential of
allowing a small amount of pitch wobble or wander, another way of getting a
unique sound. May it be justifiable to "pollute" the pure-tube concept just
a bit in order to gain the synthesis advantages of wide-range tracking
oscillators? The thought of a few tube-created oscillator waveforms tuned in
unison with some portamento through a warm-sounding VCF... this just might
be one hell of a killer sound.

Also, the curves of tube-generated envelopes and LFO's could be interesting.
Subtle variations between these curves from one kind of synthesizer to
another do tend to distinguish them apart sonically. The difference between
simulating an ambulance siren with a slow triangle wave into a VCO, and the
real thing, is primarily the curve of the pitch rise and fall.

Just some food for thought....

- Gene


-----Original Message-----
From: Paul Perry <pfperry at melbpc.org.au>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: Sunday, December 05, 1999 7:07 PM
Subject: Re: Expo convertors with tubes ?


>At 01:16 AM 6/12/99 +0100, Rene Schmitz  wrote:
>
>>I don't see how this tube could be used to build an expo convertor.
>>The QK329 has a square law.
>>
>>However I could imagine that an exp (or log) law tube could be made using
>>the same deflection principle. Replaceing that parabolic shape with an
>>exponential one.
>>
>During the Heroic Age, people had oscilloscopes with a cardboard screen
>stuck to the tube, cut in the shape of what transfer function they wanted
>(log, square, Macdonalds arches, you name it).
>They had a photocell looking at this, in a sealed box.
>As the cro trace moved across, a feedback arrangement moved the spot up
>and down so it rode on the edge of the screen.
>And, looking at the Y signal, gave you the required.
>
>paul perry melbourne australia
>
>