This vacuum tube amplifier preamplifier uses 6K4 vacuum tubes and S.M.E.R.T power supply technology for perfect sound quality. I was fascinated by the op amp, and designed quite a number of audio circuits based on them. For the audio aficionados among us, the Baxandall tone control circuit is an op amp. In 1956 Heathkit. "/>. The 2019-2023 General License question pool asks about the inner workings of vacuum tubes used as ... Which element of a triode vacuum tube is used to regulate the flow of electrons between cathode and plate? A. Control grid B. Heater C. Screen grid D. Trigger electrode Vacuum tube amplifiers are widely used in amateur radio to boost RF signal.