Vacuum tube VCO with expo control (2)

Rene Schmitz uzs159 at
Tue Nov 30 15:25:48 CET 1999

At 18:20 29.11.99 -0500, media at 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 :)

Thats a big if, in my opinion. But I don't say its impossible. There is a 
lot of traffic on these problems. Take a look at my website. You can find 
three alternative (and cheap) schemes that can be used for temperature 
compensation. Solid state solution in VCO4 (and 2), a circuit using a cheap
NTC thermistor (VCO6,VCO_566 second image) and the recently discussed
KTY-81 (VCO3). 
So along with the tried and tested Q81, and heaters we have 5 usable options, 
with variing tradeoffs. My work on that subject was motivated by the need
for an alternative to heaters and wirewound resistors. Unless the Q81
re-appears for 
$0.50 I'll use the $0.50 KTY-81 together with the 2SC1583 and be happy with

>>(I have to admit that the output of that VCO is just a high
>>voltage sawtooth. Nothing really exiting.)
>That's what I thought -- I've heard the Phattytron ra's.  However, a tube
>VCO circuit might be more easily matched with a tube waveshaper or
>something more interesting.  Have you tried overdriving VT2 in your VTVCO??

Well, the voltage swing at the cathode is only about 0.5Vpp not enough to
seriously overdrive the second triode. With the high voltage output
overdrive is possible. One still might have to divide the output down. If
there is another VT-processing stage 
you can do the overdrive there.

>Is Eric on this list??  Shouldn't he know?!?

AFAIK Eric is momentarily not subscribed.

>>>You lost me there!!  What's a Korg thyristor VCO??
>>I mean that three transistor discharge circuit like in some korg synths, or
>>in the TB303, this acts as a thyristor or programmable unijunction
>>transistor. Which is similar to the thyratron.
>Do you know of any websites where I could read more about that??

There has been a lot of talk here, it should be in the archives. I remember 
a good explanation came from Martin Czech. Look at Tony Allgoods TB3030 or
Juergen Haibles Korg-style VCO. Look for SCR (=thyristor) I'm sure you'll
find it. Subject 
was: "RE: 303 clone and Q27 burn out (remedy?)" Sent in Nov 98. The
component designators in the mail refer to the tb303 schematic.


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