Osram Music Magnet 3, 1928

 I've had this early TRF receiver for many years but somehow forgot to add it to my museum collection. It was made around 1928, a few years before rationalisation of the broadcast bands enabled set designers to use dials with station names. This set has both a log scale 0-100 plus long and medium wavelengths engraved on its dial. The wavechange switch is the small knob below the tuning dial. Tuning range was calibrated across its two bands from 2000 meters down to 250 meters. Few broadcasters worked below 250 meters in the earliest days and although the set could tune down to around 200 meters dial marking stopped at 250.

This set was supplied as a kit of parts for less skilled enthusiasts and for those unable to afford much more expensive production sets. Some would-be listeners used detailed plans issued with wireless magazines but needed to source all the parts and often use their carpentry skills to make the cabinet. I have lots of these sets and you can view them by exploring the virtual museum.

 Because Osram made valves, this was an obvious label to add. Such was the high price of valves, many owners would have taken this advice in case they bought the wrong replacement types.
 A view of the top of the chassis reveals the latest 1928 Osram S215 RF amplifier valve. Until this era sets had used triode valves, but suddenly tetrode or pentode valves became available and were capable of loads more amplification. This was not without the drawback of unwanted feedback which would make a set unstable. The cure before metallisation became the norm was a metal screen through which the new valve was poked. This enabled the aerial coil and the RF amplifier coil to be tuned to perfection without unwanted oscillation. To ensure correct alignment over each waveband it was important that the two coils both were identical. A point worthy of note is that these valves had their anode connection on the top (not the grid as was the practice later for receiving valves). This enabled the mechanical arrangement, as is used in this set, to avoid instability. You can see on the left the friction slow motion tuning arrangement and in the centre the calibrated dial. Over on the right of the chassis is another tuning condenser which is used for determining a degree of reaction to serve as a volume control. The valve in the centre is a triode having a horizontal element but the marking has worn off and that on the right a PM2 carrying an advisory label to the doubtful owner that the filament was very dull and may not be visibly lit. Early valves had filaments that were bright enough to read a newspaper. The two triode valves are mounted in what were termed "anti-microphonic" holders. This was because a set would be operated at such a high sensitivity that touching the case would result in an echoey boinging sound. The special valve holders had springs which provided enough damping to much lessen this effect.

 Below.. a close up of the aerial coil. The cylindrical part in the centre of the coil is knurled and can be rotated to adust the coil inductance so that the two ganged circuits could be roughly synchronised over the reception bands. Later, this technique would become standardised and much more accurate by having two adjusters... one for trimming the inductance and the second to trim capacitance.

 And below an identical coil complete with its fine tuner for the amplifier. Adjacent to this is an interstage audio coupling transformer. Note the aluminium chassis and the painted steel front panel. Up to this date most receivers had a wooden baseboard and an ebonite front panel. This was pretty much universal practice and sets usually suffered from hand capacity effects making them tricky to adjust.

 The main tuning condenser pair.

 And the reaction condenser. Some cheaper sets used a solid dielectric type but this wasn't as accurate as the better quality, and much more expensive air-spaced condenser.

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