FAQ: Geiger, Ion, Proportional counters - restoration and restoring - Dekatrons

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Richard Hull
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FAQ: Geiger, Ion, Proportional counters - restoration and restoring - Dekatrons

Post by Richard Hull » Mon Apr 06, 2009 6:15 pm

Part of restoring older Scaler based systems sees one come into contact with the Dekatron at some point. Unlike the nixie tube or the "ladder lamp" decade counters and their complex drive circuitry which involved up to 5 vacuum tubes, the dekatron only required one tube to drive it.

The dekatron works totally by slight of hand trickery that involves timing constructed around the count pulse input. This trickery gets the neon glow to advance in unseen stages from one counting position to the next.

EMI was the big manufacturer of these standard octal based tubes and the most commonly encountered bore the numbers GC10A, 10B, 10C, or 10D. The US in a bid to miniaturize made the them in 7 pin miniature format, but they never took hold.

The tubes contained on average 31 elements. One central circular, disk shaped anode and 30 wire type cathodes arranged in 2 bundles of 10 interconnected wires, one bundle of 9 interconnected wires and a single output cathode wire. I attach an image of the stock dekatron. Although having 8 pins on the octal socket, the standard dekatron only needed 5 pins.

In the image, you see that the anode is central and was usually connected via a dropping resistor to about +400 volts. The 0 indicating cathode was the output carry to the next stage. The two bundles of 10 interconnected wires were referred to as "guide cathodes". As one marched clockwise from the zero indicating cathode or from all indicating cathodes, for that matter, there is a first guide electrode followed by a second guide electrode, followed by the next indicating cathode. Indicating cathodes are the only ones that glow and show the count. They are, in effect every third electrode in the tube as you proceed clockwise. This allows for some spacing between indicators for a printed legend of the 0-9 indicator on a front panel around the tubes face.

Let us say the number 5 electrode is lit a new count is introduced to external vacuum tube "time delay" circuitry associated with this dekatron. This tubes first timed pulse is applied to the first guide electrode group. As only 5 is lit the first guide after #5 is pulled down below the bias on the #5 indicator wire and the glow discharge is transfered to the 1st guide wire in microseconds. Meanwhile, a delayed low bias pulse formed by the external timing tube is applied to the 2nd guide string and the glow is transferred to this second guide pin, finally the number indicator line is dragged lower again and the #6 cathode illuminates as the glow discharge is transferred from the second guide pin. The external delay timing tube has now done what it was supposed to do and the number rests on #6 until another pulse enters the external timing tube and the sequence continues again.

As you can see, the external tube has some complex biasing and pulse timing chores. It is a tribute to the enginuity of the electronics whizes of the 40's and 50's that so many complex tasks can be handled by only one extra tube. Most often this was a dual triode like a 6SN7 or a 12AX7.

The maximum counting speed was on the order of 600-1000 cps per tube. The first tube bore the brunt of high count rates and EMI introduced the GC10D This tube could count up to 5000 CPS but required two tubes for timing as it had a third set of guide electrodes. It is not uncommon in 5 to 6 dekatron scalers to find the first "units" tube being a GC10D.

Knowing all the above should help in troubleshooting the interesting twirling and whirling dekatron scalers of yesteryear.

much can be found on Google and other internet sources

http://en.wikipedia.org/wiki/Dekatron

http://www.electricstuff.co.uk/count.html

BEST
http://www.educypedia.be/electronics/tubesspecial.htm


Richard Hull
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Jim Stead
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Re: FAQ: Geiger, Ion, Proportional counters - restoration an

Post by Jim Stead » Wed Mar 23, 2016 10:35 pm

The "BEST" link recommended above is no longer valid. This link appears to go to the same information: http://educypedia.karadimov.info/electr ... pecial.htm
Last edited by Jim Stead on Thu Mar 24, 2016 12:33 am, edited 1 time in total.

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Rich Feldman
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Re: FAQ: Geiger, Ion, Proportional counters - restoration an

Post by Rich Feldman » Thu Mar 24, 2016 12:12 am

That reminds me that I have a multi-decade Decatron-based counter somewhere nearby.
It ran fine in Test mode, last time I checked. Will be fun to use it for counting neutrons someday.

Where nixie tubes need about 200 volts, decatrons need about 400.
The glow transfer mechanism (aka "neon stepper motor") limits the count rate.
Richard mentioned those details in OP above.
Richard Feldman

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