I'm restoring a 1931 RCA and it does not have polarity shown on any of the caps. It's pretty clear that the big ones are electrolytic, but there are other smaller caps across cathode bias resistors and from grids to ground. Should any of thse be electrolytics? ( I know that coupling caps should be metal film for higher frequencies.

A more general question regarding electrolytics. Are they used in power supplies since they are polarized, or are they used because they are a cheap way to get higher capacitance. I.e. if you could use a 10 uF non-polarized, would i do the same thing as a 10 uF Electrolytic?

Thanks

Doug

:I'm restoring a 1931 RCA and it does not have polarity shown on any of the caps. It's pretty clear that the big ones are electrolytic, but there are other smaller caps across cathode bias resistors and from grids to ground. Should any of thse be electrolytics? ( I know that coupling caps should be metal film for higher frequencies.
:
:A more general question regarding electrolytics. Are they used in power supplies since they are polarized, or are they used because they are a cheap way to get higher capacitance. I.e. if you could use a 10 uF non-polarized, would i do the same thing as a 10 uF Electrolytic?
:
:Thanks
:
:Doug

Doug,
Everything I have ever read said that you replace electrolytic caps only with electrolytic caps. Again, from my understanding they are used in the power supply to smooth out the pulsating DC. On the schematic look for the rectifier tube and any of the caps that come off this tube and supply power to the rest of the radio should be electrolytic. I would guess that the parts list for the schematice will list them as such. Ron

Electrolytics are used only for economy. Non-electrolytics are electrically superior in every way--less leakage (if any), better power factor, passage of high frequencies, etc. If you saw the size of the filter caps (max 2 MFD) in my 1929 Majestic 181, you'd understand why the industry quickly switched to electrolytics once they were practical to use.

If you wish to use an electrolytic where one wasn't originally used, first be sure that the circuit doesn't require the passage of radio frequencies (because electrolytics don't always pass RF well). Then observe the basic DC voltages when no signal is present. In the case of your output tubes' cathodes, the cathode will be more positive than B-. The side of the cathode bias resistor facing the cathode will be more positive than the side facing B-. Connect an electrolytic with the positive facing the cathode.

With early power supplies (such as the 8-P-6 found in the Majestic 181), the capacitors were often not electrolytic, and were of specific values so-as to form a tuned circuit with the chokes. This overcame the problem of not having enough economy sized capacity to filter via brute-force. You can replace the non-electrolytics either with electrolytics of the same values or of values around 20 or 30 MFD. If down around the original values, be very accurate. Also, electrolytics, by nature, aren't as accurate in capacity as non-electrolytics, and so if you are going for a tuned circuit, it would be better to use non-electrolytics.

T>

:Electrolytics are used only for economy. Non-electrolytics are electrically superior in every way--less leakage (if any), better power factor, passage of high frequencies, etc. If you saw the size of the filter caps (max 2 MFD) in my 1929 Majestic 181, you'd understand why the industry quickly switched to electrolytics once they were practical to use.
:
:If you wish to use an electrolytic where one wasn't originally used, first be sure that the circuit doesn't require the passage of radio frequencies (because electrolytics don't always pass RF well). Then observe the basic DC voltages when no signal is present. In the case of your output tubes' cathodes, the cathode will be more positive than B-. The side of the cathode bias resistor facing the cathode will be more positive than the side facing B-. Connect an electrolytic with the positive facing the cathode.
:
:With early power supplies (such as the 8-P-6 found in the Majestic 181), the capacitors were often not electrolytic, and were of specific values so-as to form a tuned circuit with the chokes. This overcame the problem of not having enough economy sized capacity to filter via brute-force. You can replace the non-electrolytics either with electrolytics of the same values or of values around 20 or 30 MFD. If down around the original values, be very accurate. Also, electrolytics, by nature, aren't as accurate in capacity as non-electrolytics, and so if you are going for a tuned circuit, it would be better to use non-electrolytics.
:
:T>