959 Radio
Double-Pulse Dekatrons have 30 cathodes arranged around a single disc anode in a neon or sometimes argon filled envelope . Every third cathode is designated as a true cathode, the remaining ones are designated as guide 1 and guide 2. So if we pick a particular cathode and call it K0, the next cathode is G1, then G2 then K1, G1, G2, K2 and so on. All the G1 cathodes are connected together and taken to a G1 pin, similarly with the G2 cathodes. The true cathodes may have separate pins for all ten or some may be combined depending on the tube. The anode is connected through a high value resistor to a 450v - 500v d.c. supply. The cathodes are returned via a load resistor to ground if an output from those cathodes is required,typically 33K. The cathode glow is usually visible through the top of the tube. On switch on, the anode current flows through the resistor typically 470K, and the anode potential drops to the maintaining voltage of the glow, which is less than the voltage required to strike a further discharge, so that only one cathode glows. The guides are normally biassed at about 50v and are fed with a large (150v) negative pulse clamped to the 50v supply. This is fed to G1 which immediately goes considerably negative with respect to the nearby cathode which is glowing. As a result the glow now jumps to guide 1. Guide 2 is also fed with this large negative pulse via a RC network which delays it somewhat. Guide 2 goes very negative as guide 1 is returning postive so the glow now jumps to guide 2. When guide 2 returns positive the glow will transfer to the next cathode which is at a lower potential than guide 1. Hence the name double pulse dekatron. A two phase drive could be used to initiate counting but the circuit below works well for all speeds up to the dekatron's maximum of about 10KHz. This circuit explains the operation.