The orphan receiver described in this article is owned by Mark who restored it to full working condition with the help of fellow club member Marcus. Mark regularly browses secondhand and antique shops and he obtained this particular console a couple of years ago.
The job of tracing out the circuit fell to Marcus and some of the odd component values are those that were fitted when the set was obtained – see Fig.1. The purpose of some components has us mystified, while others have strange values so someone in the past had had a fiddle and got it very wrong. These errors have since all been corrected in the chassis, so the set now works quite well.
As shown in Fig.1, the first stage consists of a 58 valve (a 6U7G is identical electrically) which functions as a conventional RF amplifier. This then feeds an autodyne converter stage based on a 57 valve (6J7 equivalent) via a tuned circuit.
Autodyne frequency converters were
used before good pentagrid and triode hexode type frequency converters came onto the scene. They can be critical to set up but a well-designed circuit will give few problems. As an aside, autodyne converters were also commonly used in solid-state receivers.
Following the converter stage, the signal is fed to the first IF transformer which is tuned to 175kHz and amplified in the following 58 valve. The amplified output from the 58 then goes through the next IF transformer and is detected using a 55. AGC is also generated in this stage and it is applied to the 55 and to the two preceding 58 valves. To me, this appears to be a significant mistake and the set would not work at all well if it were wired this way. That’s certainly not the way the chassis is wired now.
After detection, the audio signal is applied to the triode section of the 55 and then fed to a 59 audio output stage. The 55 has only quite low amplification so the audio amplifier isn’t the most sensitive in the world. The resulting audio output is fed through a speaker transformer to an 8-inch (200mm) speaker which is mounted on the substantial baffle board that forms part of the cabinet.
Other features of the circuit include a tone control which is wired between the plate of the 59 and earth, to provide a degree of high-frequency audio attenuation. In addition, the unit can be used to amplify the signal from a record player turntable. The 55 audio appears to be wired as a cathode-driven stage, with the grid supposedly grounded for audio signals. However, the circuit as drawn won’t work, as the grid is not earthed for audio signals.
In practice, the chassis has since been modified so that it works as originally intended.