Vintage Radio Capacitor

Tube radios use 4 types of capacitors: variable (tuning) capacitors, mica capacitors, paper capacitors and electrolytic (filter) capacitors.

Radio service parts lists and schematics, paper and electrolytic capacitors are usually expressed in terms “microfarads”. Short forms for microfarad include mfd, MFD, MF, UF and uF. The mica capacitors in your tube radio will have lower capacitance values than the paper and electrolytic capacitors. Micas are expressed in terms of micromicrofarads (picofarads). Short forms for micromicrofarads include mmfd, MMFD, MMF, PF and pF. A pF is one-millionth of a uF. For example, a mica capacitor valued at 500 mmfd (pF) would be 0.0005 mfd (uF). When reading schematics and buying capacitors, you sometimes have to be able to convert uF to pF or pF to uF. For your convenience we have a Capacitor Conversion Chart https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-capacitance.

General rule, if a capacitor in your vintage tube radio is less than 0.001 uF, it is probably a mica capacitor. If it is between 0.001 uF and 1.0 uF it is most likely a paper capacitor and if it’s more than 1 uF it’s probably an electrolytic capacitor.

Paper capacitors used in vintage radios will likely be in a brown or cream color paper tubular case (sometimes coated with wax). They are usually 1 to 1 1/2 inches long and 1/4 to 1/2 inches in diameter.

My thought is don’t waste your money on audiophile, computer grade or tantalum capacitors. They are good capacitors, but the old tube radio does not have the electronic circuitry to take advantage of those expensive capacitors. Only difference you will notice is less money in your bank account.

Plastic / polyester film capacitors are now used in place of paper capacitors due to their smaller size, lower cost and superior performance. There are many variations of plastic / polyester capacitors. Good types of film capacitors for tube radio restorations include metalized polyester, metalized polypropylene, metal-foil polypropylene, polystyrene and mylar.

Replacing old paper/wax capacitors, use film capacitors that have a higher voltage rating than the paper ones you are replacing. My thought is a film capacitor with a higher voltage rating will give your tube radio better reliability and longer life.

When replacing paper capacitors with film capacitors keep in mind that capacitance values are “easy to please”. The uF value does not need to be exactly the same. For example; if replacing a 0.04 uF capacitor you can use a 0.039 uF; if replacing a 0.008 uF you can use 0.0082 uF. These replacements are virtually identical. If you are +/- 10% you be well within your radios factory specifications. (Just be sure your replacement capacitor has a working voltage is equal or greater than the original paper capacitor)

An example of an oil paper capacitor value of 0.1 uF at 1KV DC
the cross or alternate that would work in film is https://www.digikey.com/short/z479r2 .

List of film we carry https://www.digikey.com/short/z479f4
List of ceramic we carry https://www.digikey.com/short/z479fv
List of mica we carry https://www.digikey.com/short/z4792m

Oh and just a reminder we now do carry some vacuum tubes as well, see link below.
https://www.digikey.com/short/z479wv

Here are a few sites I enjoy gleaming through…
https://antiqueradio.org/howfix.htm
https://www.vintage-radio.info/