07-10-2014, 11:27 AM
Mike;
I was not sure what you were planning when you mentioned using a diode and a capacitor, which is why I didn't say anything. In terms of line droppers I have heard of three main ways, to do it, with a big wire wound resistor, with a large value capacitor, or a diode, you can use a combination of a resistor and a capacitor, or a resistor and a diode, but not a capacitor and a diode.
In truth there is also a forth way, you could use the winding of a transformer or choke in series with the tube filaments, but I've never seen anyone do this. I have heard of people rebuilding resistive line cords, but it seems like a fruitless effort unless the radio is really, really compact.
Bob Andersen has carried out a lot of experiments with various alternatives to resistive line droppers for his series string TVs. One thing that he found was that the capacitive line dropper gave the softest start, With the diode dropper a current limiter was a good idea to add in addition to any extra resistors needed to get the voltage right. Speaking of high value film capacitors, there are some Panasonic or Nichicon ones on Mouser's site, 2.5 or 3 uf at 450 volts, under $2 a piece, I think that they are polyethylene or polypropylene, they are red with radial leads.
I thought that this Century radio may have been another example of the imfamous "Plant A" radios, but those typically use a proper rectifier tube like a 25Z5/25Z6, and a 43/25A5/25L6 power output. The only sets I have heard of using a diode connected triode, like a 76, as a rectifier were home made sets based on plans in Popular Science, Radio Craft, or similar magazines in the 1930s, not something built commercially.
Regards
Arran
I was not sure what you were planning when you mentioned using a diode and a capacitor, which is why I didn't say anything. In terms of line droppers I have heard of three main ways, to do it, with a big wire wound resistor, with a large value capacitor, or a diode, you can use a combination of a resistor and a capacitor, or a resistor and a diode, but not a capacitor and a diode.
In truth there is also a forth way, you could use the winding of a transformer or choke in series with the tube filaments, but I've never seen anyone do this. I have heard of people rebuilding resistive line cords, but it seems like a fruitless effort unless the radio is really, really compact.
Bob Andersen has carried out a lot of experiments with various alternatives to resistive line droppers for his series string TVs. One thing that he found was that the capacitive line dropper gave the softest start, With the diode dropper a current limiter was a good idea to add in addition to any extra resistors needed to get the voltage right. Speaking of high value film capacitors, there are some Panasonic or Nichicon ones on Mouser's site, 2.5 or 3 uf at 450 volts, under $2 a piece, I think that they are polyethylene or polypropylene, they are red with radial leads.
I thought that this Century radio may have been another example of the imfamous "Plant A" radios, but those typically use a proper rectifier tube like a 25Z5/25Z6, and a 43/25A5/25L6 power output. The only sets I have heard of using a diode connected triode, like a 76, as a rectifier were home made sets based on plans in Popular Science, Radio Craft, or similar magazines in the 1930s, not something built commercially.
Regards
Arran
