Telefunken and other manufacturers of counters with the E1T used two simple changes to the design to be able to adjust the circuit to be able to use tubes with wider tolerances, the first was to run them in a design with up to 400V anode voltage, the design was compensated for this higher voltage, but they also had a much simpler design which just incorporated a 10k potentiometer in series with the 15k cathode resistor. This potentiometer is then used to adjust for different tube characteristics. I haven't seen Philips present similar circuits but they were probably using them too. I've tested both designs but find the one with the 10k potentiometer a lot easier to use. It let me run quite a few more of the tubes in my scrap bin, but more importantly I think it is a good way of sorting tubes that behave like new from tubes that will soon end in the scrap bin, apart from letting them run in an unmodified original circuit. I haven't found any documents describing this change but it appears in some of the Telefunken counters.
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