04-11-2009, 09:22 AM
Thanks
It is in Riders vol 10, got the schematic from Nostalgia Air Archives. All the filaments are in parallel on this set, withi from pre-August 1939. It is, of course a battery set and there is a selector for 1.4 or 2 volt feed to the "A" supply. I guess on the farm they would use a lead-acid battery for the "A" supply, and a carbon-zinc battery for the 90 volt "B" supply. When toggled to the 2 volt position, there is a dropping resistor in the set to supply 1.5 to the filaments. Again this is the first set I've worked on that has parallel output tubes, although I've seen plenty of these designs in literature, and have even built a couple of PPP power amps (killers!) It occured to me that being the "hottest" device in the set, perhaps the two tubes were used as a fail safe mechanism to keep the set running for a long time, or maybe the original instructions specified removal of one when operating on a "A-B" battery pack, which there is a plug for on this set. At least the mice who lived inside didn't smoke, so the chassis cleaned up easily.
I'm going to build the power supply from the .gif attached, which I found floating around on the web. It uses the old trick of a 120/220 control transformer wired so that only one leg of the primary is used as the input, the other one is isolated from the line and used as the B source. Only 1 of the secondary windings is used for the filaments, and regulated output voltage is obtained for the 1.4 volts needed for the filaments. Since the amount of current needed for the set is very modest, a small cheap transformer can be used. I've used the same trick on a couple of phono stage projects, and it works out very nicely. DC for the filaments too!
Just waiting for the LM317s to arrive. (I know I have some, but I can't find them.)
If I ever get the cabinet together, I'll post a picture.
It is in Riders vol 10, got the schematic from Nostalgia Air Archives. All the filaments are in parallel on this set, withi from pre-August 1939. It is, of course a battery set and there is a selector for 1.4 or 2 volt feed to the "A" supply. I guess on the farm they would use a lead-acid battery for the "A" supply, and a carbon-zinc battery for the 90 volt "B" supply. When toggled to the 2 volt position, there is a dropping resistor in the set to supply 1.5 to the filaments. Again this is the first set I've worked on that has parallel output tubes, although I've seen plenty of these designs in literature, and have even built a couple of PPP power amps (killers!) It occured to me that being the "hottest" device in the set, perhaps the two tubes were used as a fail safe mechanism to keep the set running for a long time, or maybe the original instructions specified removal of one when operating on a "A-B" battery pack, which there is a plug for on this set. At least the mice who lived inside didn't smoke, so the chassis cleaned up easily.
I'm going to build the power supply from the .gif attached, which I found floating around on the web. It uses the old trick of a 120/220 control transformer wired so that only one leg of the primary is used as the input, the other one is isolated from the line and used as the B source. Only 1 of the secondary windings is used for the filaments, and regulated output voltage is obtained for the 1.4 volts needed for the filaments. Since the amount of current needed for the set is very modest, a small cheap transformer can be used. I've used the same trick on a couple of phono stage projects, and it works out very nicely. DC for the filaments too!
Just waiting for the LM317s to arrive. (I know I have some, but I can't find them.)
If I ever get the cabinet together, I'll post a picture.
