Vacuum tube VCO with expo control.

Rene Schmitz uzs159 at
Thu Nov 25 11:20:32 CET 1999

At 14:18 24.11.99 -0500, media at wrote:
>Excellent!!  Any idea how an exp converter can be built out of tubes??
>I've looked but I haven't found anything.  By generating its own heat, it
>might be less sensitive to the heat thrown off by neighboring tubes.  I'm
>glad to see that you built your circuit using 12AX7's, a popular dual
>triode that grows on trees.

Infact almost any dual triode with the 12AX7 (ECC83s) pinout can be used. 
I've put in a ECC81 (12AT7?) and it works nice too. I varied the filament 
voltage: no shift of frequency. I unplugged the anode supply, powering the 
circuit only from the charge on the electrolytics. The frequency didn't
shift until the 
oscillation suddenly dies away. Eeeeeeeeeeeeeeeup. 

I have no clue how to make an expo convertor with tubes, especcialy for one
with a current output.

Why should we refuse to use the often discussed transistor expo convertor?
Its a well known circuit, and there are many possibilities to make it
stable. Following your idea: Why not use a oven stabilized 3046 here?
Generates his own heat too.

>>I think there could be other schemes, employing heated thyratrons, but I
>>don't have a source for them at the moment.
>Do you think a thyratron can be made to function as a current controlled
>oscillator??  I was under the impression that their response was
>non-linear, and hence the need to switch between hand selected resistor
>values in the Phattytron.

The *grid* of the thyratron has a nonlinear response. Like a triode grid too. 
(I first had just VT1 in the circuit, the cathode grounded thru a resistor,
and steered it from the grid, you get nonlinear response as well.) The
magic of this circuit is in the current steering thru the cathode. (Grid
grounded amplifier situation is linear for currents: Ic=Ip.) I think
keeping the grid of the thyratron at a fixed potential and forcing a
current thru the cathode would be analogous to a Korg style thyristor VCO,
with its "overhead" triggering. Let the current charge the parallel cap,
until the voltage across the thyratron is high enough to fire.
Probably I'd need that transistor/triode cascode too, to take the high
voltages away from the trannies.

>At 10:46 AM -0500 11/22/99, harrybissell at wrote:
>>What about neon aging as the cathodes decay... and limited life ???
>>does this mess up the expo conformity of just a pitch offset ???
>I don't think this would have any effect on the exponential converter, or
>cause any noticeable detuning while playing.  In 1957, a neon lamp was used
>in the Kinsman Organ.  After they deliberately pre-aged the lamps, the
>breaking voltage decreased gradually over time requiring a minor adjustment
>every several thousand hours.

The expo issues are the same as often discussed here.
My neon lamp is a older used one, I think it came from a phase check
screwdriver. I don't see any erratic switching behaviour. (Within the
3Hz-3kHz range, above that the circuit doesn't oscillate anymore, the
triode switches the lamp permanently on.)
But I don't know for a fresh lamp.


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