Post by radioholic on Apr 19, 2020 14:41:54 GMT 12
Some people will think this is a crazy project, and others may be horrified at the wholesale modification of a vintage circuit, but I thought I would share it anyway. I have always liked the sound of TRF radios. I'm not sure why, but they seem to have a better tone and less background noise than many older superhets. This is despite TRF sets usually using a detector of the anode-bend or grid-leak types, which are known to add distortion. I had picked up a Philips 2510 in pieces at one of the NZVRS basement clear-out auctions. It had no valves, and was missing the volume control pot and the interstage transformer. As these missing bits , as well as the 4-volt valves are hard to come by, I decided to convert it to 6.3 volt valves, and to use a diode detector and a negative feedback tone control, hoping for a really good sound.
These sets are built like a diesel engine and weigh about as much. There are 3 tuned circuits, with a huge brass tuning gang. The 3 coils are in sealed cans, and all 3 had a resistance of about 1 ohm, which seemed good (high Q). The tuned circuits are capacitative coupled. I ripped out the pitch-filled capacitor boxes, the power transformer, output transformer, all resistors and the valve sockets. Leaving only the tuning gang, TRF coils and the large filter choke. I decided to use metal octal valves to avoid having to install valve shields, and installed a power transformer from a Radio Ltd model RW and a 5000/8-ohm P-T31 output TF from Antique Electronic Supply. To get the B+ voltage to 250V needed a 330 ohm dropper in series with the filter choke. The modified and original circuits are shown below.
The result: it sounds great! Good tone and very quiet background. The last tuned circuit does not peak properly when adjusting its trimmer. I suppose this is because the diode detector loads the tuned circuit too heavily. Not sure how this can be remedied (any ideas out there?) However the station separation is more than adequate as is.
These sets are built like a diesel engine and weigh about as much. There are 3 tuned circuits, with a huge brass tuning gang. The 3 coils are in sealed cans, and all 3 had a resistance of about 1 ohm, which seemed good (high Q). The tuned circuits are capacitative coupled. I ripped out the pitch-filled capacitor boxes, the power transformer, output transformer, all resistors and the valve sockets. Leaving only the tuning gang, TRF coils and the large filter choke. I decided to use metal octal valves to avoid having to install valve shields, and installed a power transformer from a Radio Ltd model RW and a 5000/8-ohm P-T31 output TF from Antique Electronic Supply. To get the B+ voltage to 250V needed a 330 ohm dropper in series with the filter choke. The modified and original circuits are shown below.
The result: it sounds great! Good tone and very quiet background. The last tuned circuit does not peak properly when adjusting its trimmer. I suppose this is because the diode detector loads the tuned circuit too heavily. Not sure how this can be remedied (any ideas out there?) However the station separation is more than adequate as is.
have you got a horn or vintage speaker to go with it? 
