Some results with Eric Barbour's TS-2 tube VCO

[Tom May]

I finally built Eric's TS-2 tube VCO.  Well, the sawtooth generator
part of it.  I hope Eric is listening here, because I've got a few
comments on some things I did differently to increase the control
range to the advertised 2.5 octaves with 0-10V CV.  Following the
schematic, I was lucky to get 1.5 octaves.  Most settings of the tune
pot only yielded a little more than an octave with various thyratrons.
After making massive amounts of measurements (with a friend's Tek
THS720A "hand held" DSO to which I'm really becoming addicted, even
though it's an incongrous piece of gear to be testing tube circuits
with), I came up with the following mods to get about 2.5 octaves out
of a 0-9V CV:

1. Omit the 4.7K cathode resistor on the thyratron.  It doesn't seem
to serve a useful purpose since the "self-bias" voltage it provides
drops low enough near the end of each cycle for the 2D21s I am using
to start drawing grid current by the time the tube fires (current
starts at a grid voltage of about -1V wrt the cathode).

Essentially we've got a voltage divider with 12K/diode/4.7K, where the
"diode" drop is the grid-cathode voltage which determines the firing
voltage which in turn determines the VCO frequency.  In the case of
the 2D21, this diode drop starts at about -1V and increases (towards
zero and beyond) as the grid current increases.  This turns out to be
just the right voltage range to control the firing point for the
oscillator circuit.  Isn't that lucky?  Now, the diode drop will stay
the same if the 4.7K cathode resistor is omitted and the divider is
changed to 16.7K/diode.  Note that the 4.7K gets an additional small
voltage drop because it is also handling the current that is charging
the cap, but that does not seem to be important.  (Eric, I am wrong
about that?  Maybe the 4.7K is really some kind of linearization
scheme.)

2. To increase the range, the "diode" voltage, which controls the
firing point, must be made more sensitive to the CV voltage.  This can
be done by decreasing the 12K resistor between the CV cathode follower
and the thyratron grid.  This drops more of the CV across the "diode"
and less across the resistor (note that omitting the cathode resistor
also increases the sensitivity because it was also dropping some of
the CV voltage).  I changed the 12K to a 4.7K.  I didn't really want
to go smaller because the thyratron grid spikes to 30V or so when it
fires, and that resistor provides current limiting.

3. The cathode follower adds about 2.5 to 3.5V to the CV (with the
12AT7s I tried).  The range can be increased if the shift isn't as
large.  I replaced the 150K cathode resistor in the CF with a
2.7K/150K divider to drop the voltage somewhat.  (This may be a bad
move since the CF output now contains a fraction of the -150 supply
ripple.)  Note that this 2.7K is in series with the 4.7K and so
reduces the sensitivity; I may drop the 4.7K a bit to get it back.
Also note that the value of 2.7K was hand picked for the particular
12AT7 I am using.  Larger values drop too much voltage and prevent
oscillation by making the thyratron's grid so negative it never fires.

Instead of mod #3, I am considering trying to shift the range of the
control grid values by tweaking the voltage on the shield.  Should be
interesting.  The shield could also become a point for injecting a
pitch modulation voltage.

4. This has nothing to do with the range.  I changed the 7.5K resistor
in the 0B2 regulator circuit to 3K to ensure the 0B2 stays within its
5 - 30mA spec for all settings of the tune pot and for worst case
supply and regulator voltages.  I'll probably increase it somewhat as
the current draw seems a bit excessive, but I think 7.5K runs the tube
out of spec.  Maybe that's not a problem though.  Eric?

Weirdly enough, even with all its non-linearities, deionization
delays, etc., Eric's circuit was surprisingly linear.  I was getting
about 8 - 12Hz/V over a 60 to 150Hz range.  I haven't checked it out
with my mods yet though.  And I also haven't listened to it yet
(sheepish grin).

Tom.