Single Digit E1T Clock

  
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To me the E1T counting tube is unmistakably one of the most intriguing tubes ever made. It was invented and developed into a product by Philips in the period 1946-1954. On one of my other pages I have posted an in-depth study of it`s origins. It has long been my whish to build a clock with this wonderful device. One of the problems of making a clock from this tube is it`s limited lifetime.
Single Digit E1T Clock - schematic

The E1T was designed for high-speed counting in measurement equipment. This was achieved by using an electron beam as opposed to a glow discharge as used in the competing decatron. This implied the use of a filament heated cathode, and unfortunately even the best filaments have a limited life expectance of ca. 10. 000 hours (1. 2 year). One of the ways to improve the life expectancy is to reduce the filament temperature. Obviously this will reduce the emission, and hence the beam current. It appears that for this application, where only low speed counting is required, a lower beam current is no problem at all. The other obvious measure to improve the life time of the tube is to simply switch it off when it is not used. In general, the frequent on/off switching of filament heated tubes is not a good idea. A cold filament has a significantly lower resistance than a heated one. This will cause a high start-up current resulting in high mechanical stresses during the heat-up phase. This is the reason that many tubes (and light bulbs) die the moment they are switched on. In this clock the filament is not completely switched off, but kept heated at a very low level, resulting in strongly reduced thermally induced stresses. Two mechanisms determine when the tube is actually switched on and off. In the first place the user has to select one out of eight profiles  upon initialization. Each of these profiles determines during which...



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