Vacuum tube VCO with expo control.

Harry Bissell harrybissell at
Thu Nov 25 21:16:39 CET 1999

Chicago Miniature Lamp "Ages" neons as an option for 16 hours. This should be a
clue for us...
:^) Harry

Rene Schmitz wrote:

> 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.
> Bye
>  René
> opinionsexpressedher   | uzs159 at
> einarethatofmyemploy   |
> erandnotofmyself....   |

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