02-14-2018, 03:46 AM
Mike;
You have forgotten two overriding considerations given the choice of what capacitor with which type of dielectric to choose, at least with manufacturers, one is space, and the other is cost.
You could probably come up with an 8 mf mica cap to use a filter condenser in the power supply of a model 90, but it would probably be larger then the power transformer, and it would be cost prohibitive, if you could even find such a thing. But nobody needs that level of precision in an old radio power supply filter cap, usually a tolerance of + or - 20% is good enough, so long as the voltage rating is high enough for the surge current. In the rest of the radio, outside of the front end or the IF transformers, you can pretty much use anything that's the same value, other then an electrolytic, though you can use those as a bypass cap sometimes, and many sets did.
As you know from your own collection the earliest AC radios used large paper capacitors for filter caps, but the materials and manufacturing techniques available at the time limited their practical size to 2 maybe 3 mf, to compensate for this they used a series of filter chokes, and neither was inexpensive to use. Of course with the advent of the wet electrolytic capacitor they could build AC radios much less expensively, and more compact, they could use a single choke on the speaker field coil, and make up for having a single choke with larger value input and output filter capacitors, and make an AC set half the size and weight. Now that relatively inexpensive large value, and high voltage, film caps are available, we've come full circle, you can actually use them as substitutes for some of the smaller wet electrolytic caps, at least up to 8 mf, 10 is probably pushing it.
Regards
Arran
You have forgotten two overriding considerations given the choice of what capacitor with which type of dielectric to choose, at least with manufacturers, one is space, and the other is cost.
You could probably come up with an 8 mf mica cap to use a filter condenser in the power supply of a model 90, but it would probably be larger then the power transformer, and it would be cost prohibitive, if you could even find such a thing. But nobody needs that level of precision in an old radio power supply filter cap, usually a tolerance of + or - 20% is good enough, so long as the voltage rating is high enough for the surge current. In the rest of the radio, outside of the front end or the IF transformers, you can pretty much use anything that's the same value, other then an electrolytic, though you can use those as a bypass cap sometimes, and many sets did.
As you know from your own collection the earliest AC radios used large paper capacitors for filter caps, but the materials and manufacturing techniques available at the time limited their practical size to 2 maybe 3 mf, to compensate for this they used a series of filter chokes, and neither was inexpensive to use. Of course with the advent of the wet electrolytic capacitor they could build AC radios much less expensively, and more compact, they could use a single choke on the speaker field coil, and make up for having a single choke with larger value input and output filter capacitors, and make an AC set half the size and weight. Now that relatively inexpensive large value, and high voltage, film caps are available, we've come full circle, you can actually use them as substitutes for some of the smaller wet electrolytic caps, at least up to 8 mf, 10 is probably pushing it.
Regards
Arran
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