[sdiy] Help with thyratron metronome - NOT A JOKE !!

[TooManySynths]

--- 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
> >
> >
> >
> >
> >__________________________________________________
> >Do You Yahoo!?
> >Yahoo! Sports - Coverage of the 2002 Olympic Games
> >http://sports.yahoo.com
> >
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
>  
> 


__________________________________________________
Do You Yahoo!?
Yahoo! Sports - Coverage of the 2002 Olympic Games
http://sports.yahoo.com