4 Valve TRF Receiver

 I acquired this rather large and very original receiver ages ago but this is the first time it's been seen in the Virtual Radio Museum.

It has an arborite front panel and an oak board for its chassis. It pre-dates the screen grid valves which appeared around 1928 and uses uncommon square section connecting wire, none of which is insulated relying on its stiffness and rigidity to keep the layers apart.

 

 The set was probably constructed no later than 1927 and has a number of early components that I haven't seen before. The set of valves is currently as follows, but may well have been replaced over the years:- an Osram HL210 RF amplifier, a Tungsram H210 amplifier and detector stage, a Marconi P2 LF amplifier and an audio output valve whose lettering has disappeared but has the code "HL249" on its glass pinch. This might mean that it's an HL2 made in September 1929?

On the left is a large torpedo-shaped wirewound resistor of 100,000 ohms. To the right of the HL210 is a Dumethohm holder carrying a glass 2 Megohm resistor. On the left are the aerial tuning coils, the larger a Burndept 168249/191530 and a smaller Hawk 75 carrying the date 1921.

In the centre is a small HF choke and below this a fat brown torpedo-shaped resistor whose markings are erased.

 

 The aluminium can holds a Finston RF transformer which you can see if you scroll down the page. There's an unusual multi-ratio interstage coupling transformer made by RI which has selectable ratios by using different terminals. Between these two components is an unmarked HF choke and between the valves another Dumethohm holder carrying a glass 1 Megohm resistor and adjacent a green TCC Mansbridge condenser marked 0.01uF. Between the can and transformer is asecond HF choke.

 

 There are three variable condensers. The one used for aerial tuning is an Ormond 0.0005uF made from brass then in the centre, the main RF tuner is another Ormond of 0.0005uF, but made from aluminium and later than the brass version. The reaction control is a third Ormond aluminium variable condenser but having a value of 0.0003uF. All three have shaped vanes for linear tuning and slow motion mechanisms under their endcaps.

 

 Above is a good view showing the square-section copper wire.

 

 Above are the two aerial coils. The nearer connecting to the aerial terminal and the rear the main tuning coil. This type of mechanical layout is sometimes improved by using a holder carrying two sockets where one socket can be angled away from the first to adjust mutual coupling. The rheostat on the right is likely to be the set's volume control. This was simply a variable resistance in the filament supply to the first RF stage allowing for a reduction in its emission. Some sets dispensed with this and used their reaction control to reduce the output.
 
 Here's the main tuning coil with its can removed. It was made by Finston and as you can see it's designed only for medium waves, in fact only that part of the medium waveband used in the earliest days of radio. The term "split primary" is interesting. Nowadays the term applied to a mains transformer would infer a 115 volt+115 volt component suitable for UK or US mains. Here, it may refer to an auto-transformer configuration, where a tap is provided for coupling RF to the detector. However, I'm of the opinion it means it has two windings, primary and secondary but with a tap in the primary for connection of HT such that this has a reduced damping effect of tuning.

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