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Simple receivers

Shortwave receiver with only one tube

As a first example, I would like to show you a simple receiver designed by myself. Despite its simplicity, it has some finesse. It works with only one tube, namely a duo triode ECC81 (12AT7) type. With such duo triode can be built in this way a very useful regenerative receiver for shortwave reception. In this circuit, the right system of the tube ECC81 simultaneously acts in a so-called reflex circuit as audio amplifier and as aperiodic RF preamplifier. In this way, the resonant circuit is well decoupled from the antenna.

regenerative receiver with ECC81

The triode audion detector built with the left system works stably and without significant oscillation holes. It is also characterized by a gentle onset of feedback. The regulation of the feedback by changing the anode voltage leads depending on the setting only to small frequency alterations. A so-called autotransformer between the stages ensures optimum adaptation and thus good gain and loudspeaker reception sufficient gain reserve. On the then still compared to today considerably occupied SW bands could be received with this circuit in the mid-1970s, various radio stations, f.e. amateur radio stations, maritime radio, broadcast, military stations, intelligence agencies and numerous other radio services. Today, such a device is still good for the reception of analog radio stations and for amateur radio reception.

For shortwave AM radio reception, the feedback should be adjusted so that the detector stage just does not start to oscillate. If the potentiometer is turned beyond this point, transmissions in morse telegraphy and single sideband Modulation (SSB) can be heard clearly, so that amateur radio stations can be received with this setting. The most important for this is a sufficient exactly adjustment. In this reason the variable capacitor must be provided with a fine drive. Optionally, instead, a smaller capacity (e.g., 12 pF) variable capacitor may be connected in parallel with the tuning capacitor for fine tuning.

ECC81 regenerative receiver

I would like to point out that the assembly of this receiver is not uncritical! The reflex circuit must build up very carefully. Short cable routing is essential! This is especially true for all blocking capacitors! For the construction of this device I wanted to get the maximum of receiving power from a single tube. As an entry-level object, the circuit is by no means suitable.

A suitable audio transformer is hardly obtainable today. I won it by wrapping a small output transformer (about 5000 turns, tapping at 1000 turns). A usable substitute could certainly be created from an interconnection of two different transformers.

The less experienced with tube receivers should do without the reflex stage RF preamplifier and - to renounce the transformer - be content initially with headphone reception. The more promising stage of expansion would be the addition of a second NF stage for loudspeaker reception. Such a circuit can be found in the following list (Teva 0V2). For the beginner, I recommend initially to build the 0V1 receiver on top of the list.

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