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Simple vacuum tube audion

     This is a simple short wave receiver of audion type. It consists of three tubes: full-wave rectifier in 6Z31 eliminator, RF pentode EF80 as a high-frequency stage and low frequency power pentode EL84 as output audio amplifier. (instead of EL84 you can also use other tubes, eg EL82, pentode of ECL8x, after modifying the heater voltage also PL82, PL84 or pentode of PCL8x).
     The receiver is tuned by the tuning capacitors C1 and C2. Capacitor C3 and potentiometer P1 sets the feedback. P2 is the volume control. Power transformer for my Audion I made rewinding the secondary of the safety transformer of transistor black and white TV with power of about 25 to 30W. The output transformer can be either from a tube TV or radio, or you can use small transformer 230V / 9V and modify the core to have an air gap (reassemble the core so that on one side there are all "I" shaped plates and on the other side all "E" plates). I've done it this way and th transformer works without problems. The conversion ratio 230:9 corresponds to 5200ohm : 8ohm impedance transformation. If audion is well tuned, it can receive even SSB (single side band). the antenna is a long wire. The audion can receive frequencies about 3 - 28 MHz. The band is selected by changing the inductance of L1. Audion consumption is about 20 to 25W + losses in the transformer.

     Warning: The operating voltage in this Audion is dangerous! Before aby work it is always necessary to disconnect from the network and make sure that the capacitors are discharged. The receiver creates interference at his own receiving frequency and may disturb surrounding the receiver. You do everything on your own risk.

Schematic of the simple vacuum tube audion

Vacuum tube audion