Vacuum tube VCO with expo control (2)

media at media at
Tue Nov 30 00:20:54 CET 1999

At 11:35 AM +0100 11/29/99, Rene Schmitz wrote:
>We certainly would have to use a device with an exponential current vs.
>voltage behaviour. Like a diode or BJT. But there are automatically
>problems with temperature then. The tube opamps at that site are either AC
>coupled or they use semiconductors, and difficult to get tubes like the
>ECC86 "low-voltage" triode.

Obviously, AC coupling wouldn't work with DC voltages.  I'm still searching
for tube log amp schematics.

>I'm sticking with the semiconductors here. I don't see the advantage that
>avoiding opamps and transistors here has.

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 :)

>I'd rather want to see something unseen: Beam power modulators,
>heptode modulators, magic eye tubes, unusual filters and the like.


>(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??

>>That makes sense.  The neon oscillator, the neon tube and capacitor in
>>parallel,  is a current controlled two-terminal oscillator.
>A fun thing that I'm going to try is to use two such CCOs in series.
>Should make a complex interaction. Coupled pendulums?!

Maybe you can mix neons and thyratrons :)

>I'm not sure, here is my explanation:
>The thyratron is sort of a neon lamp with a heated cathode. Plus one or
>more grids which can be used to prevent fireing, by applying a negative
>voltage. This prevents that the electrons can reach the anode. Or rather
>that they can be accelerated towards the anode. When fired they give back
>their energy to the neon gas in interactions. Thus ionizing the gas. The
>negative field arround the grid prevents that. The negative grid forms a
>potential barrier which most electrons can't pass. The less negative the
>grid becomes, the more likely to find electrons with high enough energy to
>get past the grid, and accelerate in the plate field. So with the grid
>voltage we can control how much plate voltage is needed to "soak" enough
>electrons into the plate field. Once enough neon atoms are ionized, there
>is sufficient energy for most electrons to overcome the grid barrier, the
>thyratron is fired.
>I'm not sure if that is what happens. Anyone else?!

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

>>The CV in goes to the grid of a triode (1/2 a 12AT7) whose cathode is
>>connected to one of the thyratron grids by a 10K resistor.  The cathode is
>>also connected to 150VDC by a 120K resistor.  Any idea what's going on
>The cathode of the input 12AT7 is connected to -150V, this is a level
>shifting amplifier, which translates the positive input to the negative
>voltage used for the thyratron.

OK :)

>>So if I understand your idea correctly, you want to bias the grid and then
>>control the oscillation by controlling the plate current??
>Rather the cathode current, but that would be the same.
>>>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.
>>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??

>>>Let the current charge the parallel cap, until the voltage across the
>>>thyratron is high enough to fire.
>>That seems quite similar to your neon oscillator.
>Indeed, I'd use the thyratron in a similar fashion as the neon lamp. The
>only difference would be the heater, and the grid biasing. The heater would
>probably help with the temperature drift of the neon circuit.

>From what I've read, the temperature drift of the neon circuit isn't so bad
if you pre-age the lamp.  I think the major advantage of a thyratron is
being able to sync the oscillator via the grid.

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