It’s not often that you come across a made-in-the-1930s radio in good condition. But that’s just what we have here – this Palmavox receiver looks almost new in its polished wooden cabinet, although it’s no longer original and has had quite a lot of work done on it.
The first clue we have to the age of the set is a sticker attached to the base of a type 58 valve. It’s marked “Palmers, Park Street, Sydney. Valve is guaranteed until 14.8.34”. If we presume that the valve was tested some time after the radio was originally purchased, the set is nearly 80 years old.
While everything outside and inside the set looked original, a close inspection revealed some modifications. Instead of the expected five valves, there were only four valves and two coil cans on the chassis. I also found two hand-drawn schematic diagrams, one for a 5-valve superhet and another for a 4-valve TRF. Someone had modified and simplified the original design!
But why would anyone do this? To understand their motivation, we need to delve into the circuit.
The original design used an “autodyne” frequency converter with a type 57 valve. This arrangement was used at a time when radio valves were very expensive and in the case of domestic sets, allowed a superhet circuit to be designed with one less valve than used in costly professional models.
The chassis is bolted to a shelf inside the timber cabinet, while the electrodynamic loudspeaker is mounted on a heavy wooden baffle immediately below. The baffle isn't original though - it's made of chipboard, a material that didn't exist in the 1930s. The grille cloth has been changed as well.
There was one potential problem though – the oscillator function of the autodyne was sometimes marginal, being dependent on the quality of the coils and the amplifying ability of the valve. As a result, it’s possible that the original owner just wasn’t able to get the oscillator to work and so decided to convert the set to a simpler TRF (tuned radio frequency) arrangement. He may not have fully realised just how poor the performance of the set would be with one less valve and no IF (intermediate frequency) amplifier stage with its superior selectivity.
The chassis in good condition but some of the parts, including a 57 valve and an IF transformer, are missing due to its conversion to a simpler TRF circuit.
Both the original hand-scribbled circuits have been redrawn and are reproduced here. Bearing in mind their origins, neither of them may be completely accurate. My immediate aim was to check out the power supply, the audio amplifier and loudspeaker. Then, a little further down the track, my aim is to convert it into a much better-performing radio with a frequency converter and IF amplifier stage, much like the original circuit.
Not surprisingly, it’s no longer possible to obtain original spare parts and that includes the oscillator coil and one of the IF transformers. That means that suitable substitutes will have to be found.
The previous owner did a beautiful job of restoring the metal chassis by filling in the holes left by the valve socket and the two coil cans he removed. Metal pieces have been soldered into the holes, the gaps filled and the outside of the chassis repainted. Only by close inspection can these mechanical repairs be seen.
It would be a pity to undo this workmanship, so when restoring the receiver to the original circuit my plan is to mount the converter valve and other components underneath the chassis, where they won’t be seen. Hence the Palmavox will end up as a questionable mixture of 80-year-old and more recent technologies but to anyone looking into the rear of the cabinet, it will look original . . . almost!