Vacuum tube VCO with expo control.

media at media at
Mon Nov 29 01:50:20 CET 1999

At 11:20 AM +0100 11/25/99, Rene Schmitz wrote:
>I have no clue how to make an expo convertor with tubes, especcialy for one
>with a current output.

I don't either.  Paul Perry said that it has been done in the past.  There
are some tube "op-amps" at  that
show where their various resistor values would plug into their solid-state
counterparts.  However, I don't know if replacing the resistor in the
negative feedback path of a tube amplifier with a diode would work, or what
sort of compensation would be necessary if it did.

>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'm not saying we shouldn't!!  Until I heard about your circuit I thought
it was my idea :) I hadn't thought of converting it to a current.  However,
avoiding the use of op-amps would simplify the power supply and possibly
avoid etching PCB's.  It would also be more in the spirit of the "Tube

>>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.

While a triode grid is non-linear over its entire range, in a triode
amplifier it is biased to operate within a linear region.  Although I don't
think that would apply to a thyratron oscillator which needs to turn
completely off.

>(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.)

That makes sense.  The neon oscillator, the neon tube and capacitor in
parallel,  is a current controlled two-terminal oscillator.

I just took a look at Eric Barbour's thyratron VCO.  It's not your typical
oscillator circuit.  I thought that thyratrons were gas-filled triodes, but
this tube, a "2D21 or 5727," seems to be a tetrode.  I'm not exactly sure
how it works.

A sync input is AC coupled by a .001uF cap and connected to the other grid
by a 220K resistor.  (It appears as though both grids are tied together.)
I think I understand this part:  A pulse at the grid ionizes the gas
allowing the current to conduct from anode to cathode, discharging the
parallel .22uF cap, the plate current drops as the cap re-charges and
conduction through the tube stops, until it receives another pulse.  Does
that sound right??

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 output is taken from the plate of the thyratron through a .22uF cap, a
pot set up as a voltage divider ("clipping level"), to the grid of the
other half 12AT7 which appears to be set up as a common cathode amp.

So if I understand your idea correctly, you want to bias the grid and then
control the oscillation by controlling the plate current??

>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??

>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.

>Probably I'd need that transistor/triode cascode too, to take the high
>voltages away from the trannies.

Maybe that could be avoided with a tube exponential converter, although
from everything I've heard, that arrangement works fine with the neon


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