[sdiy] Help with thyratron metronome - NOT A JOKE !!
Oren B. Leavitt
oleavitt at ix.netcom.com
Sat Feb 23 06:34:59 CET 2002
Hello,
You probably might like to investigate the use of an opto-isolator or a
Vactrol type photocell/LED pair inserted at the appropriate place in the
circuit. Your low voltage gate can drive the LED side.
This reminds me of a circuit I made many years ago using 3 NE-2s, an SCR
or Triac, a few caps & resistors to produce a flickering 'fire' effect
when wired in series with a regular incandescent light bulb. I am
thinking of revisiting this principle for a pseudorandom or chaotic
control voltage source...hmmmm...
TooManySynths wrote:
> --- René Schmitz <uzs159 at uni-bonn.de> wrote:
> > Hi Daryl and all!
> >
> > I think it works like this:
> >
> > Inititally the cap C1 is empty, so when there is a
> > positive halfwave
> > the thyratron becomes conductive. It can only fire
> > when the grid is
> > by a couple volts more positive than the cathode.
> > In turn the cap becomes charged, the coil is
> > activated by the high
> > inrush of current, and the cathode potential is now
> > as high as the
> > grid. The thyratron goes out of conduction. And it
> > cannot fire
> > again on the next cycles, because the cathode
> > potential is still
> > high. R5 slowly discharges the cap until the
> > thyratron can fire
> > again. This fireing threshold is dependant on the
> > setting of R2,
> > which determines how far the cap gets charged, which
> > in turn
> > determines how long it takes for R5 to discharge the
> > cap.
> > I think the neon is activated by the inductive kick
> > from the coil, or
> > maybe even directly via the thyratron, I would guess
> > its got a mere
> > indicator purpose.
>
> Thanks, of course after reading your explanation, it
> all makes sense...isn't that the way it goes.
>
> Since the initial state is "ready to fire", I am
> hoping I can hold off the triggering of the thyratron
> until I am ready. I want to leave the timing circuit
> intact but I want to be able to fire the thyratron
> from a gate signal. Since this thing is powered
> directly from an AC line I obviously need to
> completely isolate the input.
>
> I can think of brute force methods like putting a
> relay in series with the coil or perhaps even shorting
> the timer cap to force a new trigger, but.... I was
> hoping for something a bit more elegant. Would it be
> possible to somehow hold off the triggering of the
> thyratron until a gate signal is present but not allow
> retriggering until the cap has discharged normally?
> Could I use the screen grid for this ?
>
> In case it's not clear, the metronome works just fine
> currently. I'm not trying to fix it, I'm trying to
> turn it into a simple, slightly unpredictable,
> electromechanical drum voice.
>
> I've got to run off for dinner, but when I get back
> I'll post the resistor values.
>
> Thanks
> Daryl
>
> > The values of the resistors would surely help
> > quantifiying some of the
> > voltages, that could give further hints.
> >
> > Cheers,
> > René
> >
> >
> > At 10:18 22.02.02 -0800, TooManySynths wrote:
> > >Hi All,
> > >
> > >Several have offered to help, thanks. Here's my
> > best
> > >attempt at an ascii schematic.
> > >
> > > NE1
> > > 2D21 ||
> > > ,-----------------------||--
> > >110 . . | | | |
> > >---------------|. . [ | | \
> > > | . . | | / R6
> > > / | C|| / \
> > > \ R1 | C|| \R5 |----
> > > / | L1 C|| /<- | |
> > > | | C|| \ | \ |
> > > | | | | / -----
> > > / R2 | | \ \
> > -----C2
> > > \ | | / /R7 |
> > > /<- / \ / \-- ----- R4\ | |
> > > \ R3 ----- C1 / | |
> > > | | | | |
> > >110 | | | | |
> > >-------------------------------------------------
> > >
> > >
> > >
> > >Ok, first some basics.
> > >
> > >R1 Controls the tempo
> > >R5 seems to be a calibration control
> > >NE1 is a neon bulb, I can't remember the correct
> > >schematic diagram for a neon bulb
> > >The screen grid is connected to the cathode
> > >L1 is an electromagnet that draws a balsa wood
> > >diaphram near it to make the metronome sound.
> > >NE1 coincides with the click
> > >the voltage accross L1 is higher when the NE1 is
> > light
> > >and you hear the click.
> > >Both caps are .47 mf
> > >the filaments are connected through a 6.3 v
> > stepdown
> > >transformer, I didn't think that was necessary to
> > >draw.
> > >The 110 is AC, other than the transformer and ac
> > >switch, the schematic is complete.
> > >
> > >
> > >I know this circuit is simple but, well, yeah I
> > really
> > >don't get tubes at anything be the most basic of
> > level
> > >of understanding. I've read eric barbor's page and
> > >realize that the thyratron works like an SCR. I am
> > >guessing that, like an SCR, feeding AC into the
> > >thyratron is a key component to making this circuit
> > >work, iow, the polarity reversal is what causes the
> > >thyratron to open back up after it is turned on by
> > the
> > >timing circuit. But, that's about as far as I get.
> > >Tubes are hard enough, but throw in that pesky AC
> > and
> > >get lost quickly. I seem to remember something
> > about
> > >the behaviour of neon lamps that I think it is an
> > >essential part of the circuit, but again, it's just
> > >all vague memories of too many hours spent perusing
> > >the ARRL handbook as a teenager.
> > >
> > >To start with, what is the sequence of events that
> > >causes the thyratron to turn on and off?
> > >
> > >Respond in private if you wish. Let me know if the
> > >resistor values are essential to understanding
> > what's
> > >going on.
> > >
> > >Thanks
> > >Daryl
> > >
> > >
> > >
> > >
> > >__________________________________________________
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> > >Yahoo! Sports - Coverage of the 2002 Olympic Games
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> > >
> > --
> > uzs159 at uni-bonn.de
> > http://www.uni-bonn.de/~uzs159
> >
> >
> >
>
> __________________________________________________
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--
Oren Leavitt
oleavitt at ix.netcom.com
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