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A tube is described which utilizes the cyclotron motion of electrons to provide a resonant system which is tuned by variation of the magnetic flux density. The amplitude of the cyclotron motion depends upon the relationship of the applied signal frequency and the electron cyclotron frequency; at resonance, the amplitude is maximum. Resonance is detected by shooting the spiraling electrons through a honey-comb type mesh grid; the current intercepted by the grid is greatest at resonance and is proportional to the RF signal power. The tube is a complete TRF receiver (less video amplifier) within one vacuum envelope. The resonant frequency is a linear function of solenoid current; the tube can be tuned over a wide frequency range (at least 10:1) with a single control. The characteristics of the device have been investigated for resonant frequencies from 65 to 650 mc; the sensitivity and 3-db RF bandwidth can be varied within limits; typical signal sensitivity is -45 dbm (500-kc video bandwidth) with a 4-mc 3-db RF bandwidth. The measured characteristics and the theoretical predictions are in reasonable agreement.