What about a '1-V-2' regenerative receiver ? '1-V-2', according to the long time tradition, means: 1 Tube RF preamplifier, a regenerative detector ('V', for Valve), and 2 Tubes in the audio chain.
In the prototype below, the RF amplifier is an EF805S, the detector is an ECC81/12AU7, with both sections in parallel (I hate to have unused tubes in a device ...), and the audio amplifier section is made by a triode-pentode, ECL84/6DX8.
Single band, from 3.5 to 4.0 MHz, the RF amplifier uses a tuned circuit, separately from the main tuning. The detector and the front end coils are not mutually shielded, but the RF tube is: it was necessary, otherwise any tuning variation in the RF section around the frequency tuned by the detector, makes the tuning setpoint of this whole thing unstable.
Soon it has been discovered that feeding a preamplified signal to a regen detector might be an overkill often, especially if long antennas are used. The receiver as it is allows to listen easily on 80..90% of the stations on the 80 mt band, by using only 5 ft piece of wire as antenna.
The detector is fed by a VR90 gas voltage stabilizer and the regeneration is set by tapping this voltage with a potentiometer. With 470 kOhm as bias resistor on the 12AU7 grid, from 1 to 2 Volts are sufficient (!) to enter the oscillation and therefore to copy CW and SSB. The greater this resitor, the greater the anode supply voltage to enter the regeneration. And less distorted tends to be the SSB reception.
Overall, the same sensitivity, stability and excellent signal to noise ratio have been possibile by this radio receiver, as compared to those achieved with 'Polyphemus'. The only substantial difference is regarding the dial resettability, which in this case is moderate, being this essentially the consequence of a regenerative detector, which is a free running analog oscillator and therefore the stability entirely depends on the mechanical rigidity of the all assembly.
In any case, it is again very pleasant and satisfying to tune in radio stations on the Shortwaves, by simple means.
Posted by Radioman | Permanent Link