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

[René Schmitz]

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. 

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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http://www.uni-bonn.de/~uzs159