My last article on substituting valves in the AMI jukebox reminded me of a repair I did of a Sparton console from the USA way back in 1999. Back then I was presented with a very nice Sparton Equasonne model 930 nine-valve TRF set made by the Sparks-Withington Co. in 1928.
Technically and historically, this is a very interesting radio. Back in 1928, most USA manufacturers were RCA licensees, that is, they paid 7.5% of gross sales value to RCA for the use of the patents they owned, and they were required to use RCA valves. Sparton had its own valve manufacturing subsidiary - Cardon - and was not a licensee.
Sparton began to become very successful so the RCA licensees insisted that it sue Sparton, which happened in February 1928. This forced Sparton to adopt circuits which did not require an RCA license. In particular, they could not use the Alexanderson TRF patent which covered a TRF amplifier where the tuned circuits were interspersed with the amplifier valves.
The Technidyne circuit
Instead, Sparton adopted a circuit invented by Lester Jones, where all the tuned circuits were ahead of an untuned, multistage RF amplifier. This "Technidyne" circuit was introduced for their Equasonne models in 1928. See circuit diagram below. RCA later sued under other patents, forcing Sparton to finally sign up in May, 1929, so the Technidyne circuit was abandoned.
The fact that Sparton had its own valve manufacturing facility was the source of most problems for this radio restoration. As received, the set had type 227 valves in the RF and detector stages instead of the original type C-485 triodes. One of the output valve sockets had a Cardon 183 triode; the other socket was empty. The rectifier was, luckily, a common type 80.
227 valves were not suitable for the RF and detector, due mainly to their requirement of 2.5 volts at 1.75 amperes for their heater. This compares to the 485 valve’s requirements of 3.0 volts and 1.25 amperes. The 183 output triode also has different characteristics to the C-182 or 182B originally used. The circuit does show a 482B used in this position but this is a later type made as a replacement for the C182 and 182B. There are no suitable replacements commonly available for these valves.
Luckily Antique Electronic Supply came through with two 182B valves in usable condition. I was very lucky; AES told me they’d previously never had any in stock and just happened to have a couple when I inquired. They could not help with the 485s. I decided to try the type 56 in the RF and detector stages, dropping the 3 volt supply to 2.5 volts with a resistor. This is preferable to using the 227s, as the heater winding is not overloaded with the 56's.
The 485 valve was specifically designed for use in the Technidyne circuit and I was worried that the 56 wouldn’t work in its place. All seemed well though, when I finally had the radio up and running. I made a suitable heater dropping resistor (0.5V/6A = 83.3mΩ) out of copper wire wound over an old jug element former.
Unfortunately, the speaker was a write off. I had to substitute a modern speaker with a resistor replacing the field. I gave the old speaker to the owner in case he ever wanted to go to the expense of having it restored. The remainder of the restoration was fairly straight forward, requiring a handful of resistors and capacitors, and an output transformer rewind.
When fired up, the radio worked very well, with more than adequate sensitivity for local stations. It picked up most of the Brisbane stations from here on the Gold Coast with just a couple of metres of aerial wire.
The radio ended up in Canberra and I did get a complaint. The tuning range of this radio is restricted at the high frequency end of the dial and it wouldn't tune in the station that the new owner wanted to listen to. I replied that there wasn't much I could do about that.
Perhaps just as well. The thought of this radio, with its rare output valves, being used as an everyday radio, is not a pleasant one.
© Copyright Stuart Irwin and published with permission of the author.
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