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
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