ECH21 Philips Miniwatt is a heptode - triode tube with indirectly heated oxide cathode. Both systems are working separately, cathode is common for both of the systems. Heptode may be used as a mixer, hf or lf amplifier. Triodes like oscillator or lf amplifier. ECH21 was a part of many old radio receivers (for example Tesla Riava, Signal, 506A Arie, 619A Dalibor, 620A, 623A Máj, 621A Opera and many others). New in box tubes and in perfect condition. Tested on BM215A tube tester.