The "Shire Three"

A group of us at the Shirehampton Amateur Radio Club are reconstructing a 1920s radio receiver. Gary Morton, M1GRY, produced a group of 1920s valves and radio components and Andy Cowley, M1EBV, found a suitable cabinet/chassis at the West of England Vintage Radio Fair at Willand, Devon. The cabinet, which was FREE, contains a marked up baseboard for a "Mullard Master Three Star" radio.

The Mullard Master Three Star

Above is the baseboard layout for a kit form of the Mullard Master Three Star. This was a conventional three triode regenerative receiver with transformer coupling. The intended valve line up was :-

V1 - Detector PM2DX

When used as a detector this valve should have a grid leak of from 1 to 3 megohms and a grid capacitor of 0.0001 to 0.0003 mfd.

V2 - First LF amplifier PM1LF

At an anode voltage of 130V this valve should have a grid bias of -6 volts approximately and will draw about 3.0 mA of anode current..

V3 - Super Power Output Triode PM252.

This is a 3 watt anode dissipation audio output triode. At 130 volts anode supply this valve should have a grid bias of from -8 to -11 volts and will draw about 14 mA of anode current.

The "Shire Three"

The "Shire Three" will be somewhat different from the original design. We haven't fully exhausted our search for suitable valves and their testing but at present it looks as if we may have a better supply of 4-volt filament valves. The output triode is probably the most difficult to source. We don't have a PM252 or an equivalent yet.

The original design would have been powered by a single lead-acid cell for the 'A' or filament supply and Leclanche batteries for the 'B'/HT and 'C'/grid bias supplies. The Leclanche batteries are no longer available. We are still investigating the possibility of obtaining a suitable lead acid accumulator.

NEWS - We have obtained two suitable second hand lead acid accumulator cells from Phil at Bristol Batteries so we have an 'A' supply for either 2 or 4 volt valves.

We plan to construct a mains 'B' supply using British 4-pin valves as might have been used in 'battery eliminators' produced in the 1930s. We have a Mullard IW4/500 rectifier (We may substitute a Phillips E451 if we use 2-volt valves) and a Cossor S130 130-volt cold cathode voltage regulator for this. The 'C' grid bias supply will be produced using modern solid state rectifiers and regulator.

We have a good collection of interstage transformers of various ratios. We are not yet certain which ratios will be required for the two stages. Measurement and testing of our stock is underway.

The aerial, tuning and regeneration coil is another problem. We do have a component, supplied by Gary, but it is a band switched device from a later radio, probably with a tetrode or pentode detector. We also lack a slow motion drive for the regeneration capacitor. Other sources are being investigated. We may build our own front end coils as we plan to extend the tuning range to cover top-band and 80m if at all possible. If we succeed we shall, of course, build a matching transmitter.