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
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.