Vacuum tube VCO with expo control (2)

[Ingo Debus]

media at mail1.nai.net wrote:
> Otoh, a tube exponential converter might be something that we could use
> with solid-state circuits.  Think of how much traffic on this list is about
> matched transistors and tempcos.  Imagine if it could all be solved by
> using tubes :)

Here's a quote from one of Eric Barbour's mails on Synth-DIY from July
1997 (took me a while to find it):

>>You have built an expo converter without linearity problems
>>with tubes ?!?

>No, I didn't. As someone else pointed out, it is not possible
>with tubes. (Actually it is possible but very difficult--it 
>requires a special tube made by Raytheon in the 1950s, a 
>square-law tube called a QK329. Very rare today.)

The rest of that mail is interesting too:

>At no time did I apply all the tired old paradigms of solid-state
>synth design to my circuit. The VCOs use thyratron tubes, which
>vary considerably from unit to unit in their zero-point switching
>thresholds. The only practical way to control them is with a keyboard
>having adjustable pots in its resistor ladder, allowing an
>arbitrary voltage for each key (in ascending order).

>A nice side benefit is that you can tune it for ANY scale.....

>Although it is very difficult to make various modules (VCO, VCF
>etc.) track each other, at very least it is possible to get VCOs
>to track by using matched pairs/etc. of thyratrons. I don't even try
>to make hi-Q filters anyhow, so why bother making them track with
>the VCOs?

Ingo