Flash back to a few years ago.

I have been experimenting and building circuits with dektrons for years now. Absolute highlight was a project I did for Elektor, the printed magazine in 5 languages. The original title of this was: I finally found a useful application for dekatrons. It was published in 2022 under the title Audio Spectrum Analyzer With Dekatrons. Which is exactly what it is: it uses 7 dekatrons to display the levels of audio in the different frequency. Stereo, also. You can still read this story at https://www.elektormagazine.nl/labs/i-finally-found-a-useful-application-for-dekatrons

In 2023, to get inspiration I visited the WITCH computer in the National Museum of Computing, near Bletchley Park, which is comprised of hundreds of dekatrons. The people working there have managed to get the machine running, which I think is an incredible feat. They do a daily demonstration.

In July 2024, I got the opportunity to buy a couple of 12 step dekatrons (GS12C), about which I didn't hesitate a second. I had no clue what I was going to use these for, but I knew I needed them urgently. They had been laying around for a couple of months until the idea to use them in a digital clock gradually matured. 

In my previous projects, I had been using dekatrons only as display devices. I did not use them for their original purpose: as counting (up/down) device. That was because all the "counting" was done in a microcontroller and the microcontroller directed the dekatron. This bothered me for a while. I wanted to do something where the dekatron was used for its original purpose, to count things and not be subordinate to a microcontroller.

I did some experimenting on how to couple the 'carry' on the output of one dekatron to the input of the next dekatron. Two document showed how to do this: the Sylvania brochure Decade Counter Tubes second edition and the Ericsson book Electronic Counting Circuits by J.B. Dance (chapter 4). but of course, the circuits were intended for different types of dekatrons, for different types of triodes and for different supply voltages then I intended to use. So I had to do a lot of experiments before I got it right. Luckily, the documents described very well the intended time-shifted waveforms on the two guide electrodes G1 and G2. So I could measure the waveforms on the oscilloscope and choose component values that gave the intended waveforms.

I took the idea from Sylvania to connect G1 through resistor-and-capacitor-parallel and G2 through an RC-filter-network, as illustrated below.

In the Ericsson document, they use a different RC topology, e.g.:

I found out that dc coupling the cathode of a dekatron to the grid of the triode means that the triode is conducting for a longer duration, which means that the power consumption is relatively large. This also leads to the necessity of a high-watt anode resistor. This can all be avoided by introducing a small capacitor between dekatron-cathode and triode-grid, which has the effect that only a brief pulse is given to the triode.But this technique cannot be applied for driving the nixies. So in the end I used both AC-coupling and DC coupling.

In chapter 4.4.9 of the Ericsson document, it is described how to drive nixie tubes (Ericsson calls them Digitrons) from a dekatron. Unfortunatly, the use a GCA10G type of dekatron for this puropose, which has 10 auxiliary anodes, that can be directly tied to the figures of the nixie tube. So I had to design a way to solve this for a normal dekatron. That proved to be quite easy, it only requires a lot of triodes: 10 triodes for one digit. 

I found out the existence of the E90CC double triode tube, a tube specially intended for digital purposes. Fun fact about the E90CC that it clearly states in the datasheet that this tube is 100% useless for audio applications, because of microphony and hum. I really like this, because it means that not all of them have been taken by the audiophiles.  I found someone in The Netherlands that would supply me with the desired quantities of tubes and sockets at a low price. In fact, the tubes cost no more than the sockets.

I decided to publish the on-going project on Hackaday.io and submit it to their 1Hz challenge. This meant to speed up the process. I went on to create an aluminium base plate, which was a large amount of work, given the tools that I have at hand. Some woodwork, find transformers for the power supply. Enfin, you can read this all in the project log.