Valves/Tubes was: ez80

On 20 Jul 2010 at 20:03, Tony Duell wrote: That's true, but only in those cases where the shell (or any other element) is brought out to a base pin. The 1934 standard says that an element has to be "useful" and have a connection to be counted. So, a 6L6 has heater, cathode, control grid, screen grid, plate and shell all connected to base pins. The beam-forming electrodes aren't counted, so, for instance, the 6F5 triode has exactly one fewer "useful" electrode, even though it has two fewer elements. The other requirement is that the tube must have been introduced initially in the metal shell form (I can't think of any that were glass, then metal, but there may be some.) An "S" as the first part of a middle two-letter pair signifies a single-ended tube, which was not universally followed. Rectifiers *usually* have a high-middle letter (e.g. 5U4, 5Z3), but sadly, there are many exceptions. The most reliable part of the number is the suffix. e.g. G = glass, GT = short glass, GA = improved glass version, GY = micanol base, etc. In 1942, the RMA introduced a scheme for special-purpose and transmitting tubes, that was called the "1A21" system. The first number represents the power rating, the second, the tube type (e.g. diode, triode, etc.), the second and third numbers are assigned in the order of introduction, starting with 21. So the gas thyratron 2D21 tells us nothing more than it's a tetrode rated for 10 watts or less. That standard lasted only until 1944. --Chuck