Monodial PSU

The original

This was also offered on Ebay at the same time as the Wireless World 1936 Monodial receiver, but I didn't win it - it made £36. (There was no contest for the receiver, so I got it for £24.99). Here's what the PSU and output amplifier looks like.
The chassis is from CAC (City Accumulator Co. London) and the smoothing choke by Sound Sales, Farnham.  The mains transformer is most likely from Sound Sales too.  The valves are MU14 rectifier and N41 output pentode.   As was common in the mid 1930s there is no output transformer as this would have been included with the loud speaker.

Here's what I built to replace it.

The new power supply

 
 
 
 Top view of power supply The smoothing chokes and rectifier The mains transformer. A genuine 1930s article.

Most of the components used were salvaged from old electronic "junk".  The mains transform was from a radiogram, where it had apparently replaced a failed smaller component.  The windings were 4V @ 5A, 4V @ 1 A, 5V and 275V-0-257V.  So with a 5V rectifier and the two 4V windings in parallel this was just the job.  The smoothing chokes are 20H each.  There are no capacitors mounted on top but there are three 7.2uF plastic film motor run capacitors fitted underneath.

The new amplifier

The original amplifier design was typical for a 1930s broadcast receiver, a single ended pentode with tone correction. Though perfectly usable, even at the time this wasn't considered ideal, better radiograms used a single ended triode, or even a push-pull triode output.  As I now had a 1930s energised loudspeaker with output transformer suitable for a 1930s output pentode I decided to build a single ended N41 amplifier, but rather than use tone correction I chose to add an extra amplifier stage and use negative feedback. The valve chosen was a high slope pentode type AC/SP3; as used by the BBC in pre-amplifiers from late 1930s through to the 1950s.
 
 
 
 Here's the circuit I used Power supply, speaker and two stage amplifier. Viewed from above.  The valves from left to right - MU14, N41, AC/SP3.
 
  
 Underside of amplifier section  


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