portadyne: U56; Portadyne Table Model

Most post WWII European or UK sets appear to use a Triode/Hexode mixer/Oscillator for RF to IF. This set is quite like a 1936 to 1939 "All American Five"  AC/DC design, but with dropper resistor on my example probably only for 210, 230, 250 Volts, adjusting only the heater filament series resistance in 70 Ohm (approximately) steps. Searching the tube (valve) line up in mine there are 27 models 1936 to 1940 and none later. 19 are 1936/1937. Portadyne seems to regularly used "older" technology, the "Princess" of 1956 is Octal tubes (Triode / Hexode converter) and they have a model in 1953 repeated in different case in 1956 that is a TRF! (Universal and AG33)

The live mains feeds the top of the dropper, a 110 Ohms section with two series dial bulbs (lamps) in parallel. This feeds the two 25Z6G anodes via a pair of 100 Ohm resistors using the "no connection" pin 6 of the Octal base as a tag (the cathodes are directly connected). The next section of the main dropper is about 400 to 450 Ohms for lowest mains voltage tap to series heater chain of 25Y5G (or 25Z6G), 25A6G, 6K7G, 6A8G and 6Q7G.

The on/off switch is single pole between the mains neutral and the chassis. Thus the chassis is "live" via the heater chain and "dropper" resistor when the radio is off!

For safety I removed the 20nF capacitor to top of volume control to "gram" socket "signal" and 100nF capacitor to chassis from "gram" socket "earth". The aerial and "earth" socket wax paper capacitors (2nF aerial and 20nF to chassis)  were replaced with 2nF 1kV ceramic  rated for X & Y use.

The wax paper capacitor across the mains in had exploded as had the first Electrolytic on 25Z6G cathodes to chassis.

A large network of 70s or 80s capacitors and resistors with modern PVC wires hung between volume control, 25A6G cathode and o/p transformer secondary.  It was badly soldered and one capacitor fell off. Two wires were dry joints. This provided two feedback networks, with volume control and 6Q7G in one of them. Examining similar era schematics and the tags on the chassis suggested all of this was an addition and was removed, with only the replacement cathode resistor retained and that decoupled with 470uF 63V that had been wired from cathode to speaker.

After replacing the obviously exploded capacitors, fixing wiring to more likely and replacing other virtually short wax paper capacitors the radio operates much as I'd expect. There is a short wire connected to the heat shield on rear cover (for dropper) which functions as an aerial for very strong stations. The set performs reasonably with 3m indoor wire and very well with a longer outdoor wire (washing line!).

 

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