Paperweights, parts of the PTR175

 I spotted these radio assemblies on the Internet being offered for very little money as "paperweights". Clearly not intended to be sold as working pieces of equipment, being a described as a paperweight, neatly absolves te seller from any responsibility. However they looked interesting, and I believe having been made by my old employer Plessey I decided to buy them.

The assemblies were part of an aircraft transmitter receiver known as the PTR175 and comprise an AM modulator plus the PA stage of the airband radio transmitter. They just pre-date the use of semiconductors for these type of equipments and employ some quite exotic valves.

The modulator uses a pair of 2E26 beam tetrodes first introduced in 1947. Although designed primarily for use at VHF they are used here as push-pull modulators. Their anode dissipation is rated at 6 or 7 watts so a pair operated at 60% efficiency could develop around 20 watts of audio. This is enough for modulation of an RF stage running about 20 watts or so RF output. As the modules are fitted in a relatively small case cooling is necessary and this is achieved by blowing air through the case, assisted by internal blowers..

 To keep the module as small as possible the audio pre-amp stage uses three miniature wire-ended valves buried in the wiring and other parts inside the box.

 Under the cover on the right are the three wire-ended valves, V105/V101 and V102. The paxolin board carries a printed circuit on its underside.

 Turning to the RF module. This uses a 4X150D power amplifier which has a very high power rating but is operated in this module in a manner that demands little cooling because of its relatively low power output of circa 20 watts. The module uses two other valves, a 6J4WA, which is a UHF triode and a very odd-looking disc seal triode type 6442, seen below.

General views with screening covers detached.

View showing the two triodes. The lower is the 6442 disc seal type. Also the tuning condenser and trimmers in the drive circuit.

Another view of the two triodes.

 A notable feature of this transmitter module is the method used for tuning. Like all airbourne equipments, dating back to the SCR522 for example, a transmitter must be accurately tuned to the selected operating frequency. This module has a drive input connecting to three tuning condensers. Two are operated in the same plane as the input drive shaft and one at right angles driven by bevelled gears. The tuning condensers kept in track across the airband by using very oddly shaped tweakable vanes and by trimmers.

The input coupler (left centre) for tuning the RF module and RF output at J602.

Cover over the PA valve

 Under the cover is the 4X150D with cooling fins. Because it uses plate modulation the valve was probably run in Class C having no real reason to be operated as a linear amplifier.

 Looking at the design of the two modules it's clear that they must have been expensive to manufacture, although, if my memory serves me correctly the hourly labour rate used by Plessey for costing a graduate engineer back in the 1960s was only a couple of pounds or so and that for manufacturing much less. By the time I left Plessey nearly 30 years later the manufacturing hourly labour rate was over £60. By this date valves were long obsolete having been superseded by semiconductors whose characteristics enabled wideband amplifiers to be designed that needed little by way of tuning, and mass production using automated assembly machines had made meticulous setting-up a thing of the past.

The PTR175 operated on a number of preset crystal-controlled channels in two bands: 117.5 to 135.95Mc/s and 225 to 399.95Mc/s.

What to do? I could work out the connections to the modules and power them up and see how they perform? Or, I could just use use the pair as paperweights. Fifty years ago it would have made the ideal basis for a 2 meter transmitter.


Return to Reception