ABC's of FILM Capacitors for Tube Radios, TVs & Amplifiers

Although this guide is written from the point of view of vintage tube radio enthusiast, the capacitor information and tips should also be useful to those who restore vintage tube televisions, vintage hi-fi systems, tube run audio and guitar amplifiers, tube based test equipment, etc. We hope you find this info useful. Have fun with your vintage electronics restorations !

Film Capacitors - the Basics:

• Your vintage tube radio needs both direct current (DC) and alternating currents (AC) to operate. Capacitors act to pass AC while blocking DC. Capacitors are used to block, pass, filter and tune the various currents in your radio.
• Your old tube radio uses 4 types of capacitors: variable (tuning) capacitors, mica capacitors, paper capacitors and electrolytic (filter) capacitors. When you restore an antique radio you will replace the paper and electrolytic capacitors, but not the variable and mica capacitors. 
• Don't let terminology confuse you….."condenser" is just an old fashioned name for "capacitor".
• Capacitors have a capacitance value and a voltage rating. The capacitance value is a measure of how much electric charge a capacitor can store. The voltage rating is the maximum voltage the capacitor can handle without breaking down.
• In radio service literature (schematics/parts lists), 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).
• As a 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 likely a paper capacitor and if it's more than 1 uF it's probably an electrolytic capacitor.
• The original paper capacitors in your radio will likely be in a brown 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.
• Mica capacitors come in different sizes and shapes, but the most common shape is square or rectangular….brown in color with colored dots (sort of look like "dominos").
• Capacitors have either "radial" leads or "axial" leads. With "radial" type, both leads exit from the same end of the capacitors. With "axial" type, there is a lead at each end of the capacitors. Both types are equally good. Just be sure the capacitors you order have long leads.
• How about using NOS (New "Old Stock") paper capacitors? NOS paper capacitors are not recommended…use at own risk! As paper capacitors age their capacitance values drift, they dry out and they become leaky. 
• Don't waste your money on audiophile capacitors. Your old tube radio does not have the electronic circuitry to take advantage of those expensive capacitors. The only difference you will notice is a lighter wallet.
• 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. What is a Mylar? Mylar is simply the trade name of the synthetic film registered by duPont.
• At higher frequencies polypropylene and polystyrene are more stable that polyester, so for film capacitors under 0.01 mfd, you may want to use polypropylene or polystyrene capacitors rather that polyester capacitors.
• When replacing old paper/wax capacitors, you can't go wrong using film capacitors that have a higher voltage rating than the paper ones you are replacing. For example, if you are replacing a paper capacitor rated at 400 volts, you can use a 630-volt film capacitor (but not a 200-volt capacitor). A film capacitor with a higher voltage rating will give your tube radio better reliability and longer life.
• Why were tube radios manufactured with 200, 400 and 600 volt paper capacitors if 600 volt could have been used for all the capacitors? Two reasons… cost and size. Capacitors used to be expensive so if a manufacture could use lower voltage capacitors in a circuit, it could cut production costs. Also, the higher the voltage the larger the paper capacitor, so it was easier to install lower voltage paper capacitors. 
• Modern film capacitors are non-polar, so you don't have to worry about polarity when replacing old paper caps with new film capacitors. Although non-polar, old paper capacitors had black bands at one end. The black band indicated which end of the paper capacitor had some metal foil (which acted as a shield). The end with the metal foil was connected to the ground (or lowest voltage). The purpose of the foil shield was to make the paper capacitor last longer.
• When replacing paper capacitors with film capacitors the uF value does not need to be exactly the same. 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).
• When restoring an antique radio it standard practice to replace certain of the radios capacitors. This is known as "recapping" a radio. An old radio may work with it's original caps….but for how long ?? ….and how safely ?? If the radio is going to be sold with a guarantee or is being given to someone as a gift, you should "recap" the radio.
• You will want to replace all the paper and electrolytic capacitors. However, "do not replace the mica capacitors". Mica capacitors very rarely go bad and if you replace them it will throw off the radios tuning. Replacing the mica capacitors will do more harm than good. Only replace a mica if you are sure it is bad (which is rare).
• Old paper/wax capacitors are one of the most unreliable parts in an old radio. Don't let "molded" paper capacitors fool you. They are just paper capacitors in plastic cases and are just as unreliable as the ones coated in wax.
• Like mica capacitors, ceramic capacitors also very rarely go bad. Do not replace ceramic disc capacitors unless you are sure one has gone bad.
• Some radios use "line-filter" capacitors. These capacitors connect across your radios power line and/or go from your power line to ground. When replacing these capacitors, you should use special AC Rated Safety Capacitors. These special capacitors with improve the safety, performance and reliability of your radio.
• Get a schematic (and parts list) before you start your recap job. It is often impossible to read the values that are on the original capacitors. Also, if the radio was repaired at some time in the past, there is a good chance someone threw in the wrong size capacitors, just to get the radio working. Without a schematic you'll be guessing.
• Before replacing the capacitors, check the radios' resistors. Since you will be replacing the capacitors, you should snip one lead of each paper and electrolytic capacitor. This will help prevent false resistance readings.
• Put "heat shrink" (spaghetti) tubing on the leads of the capacitors and resistors before you solder them into the circuit. This will help prevent dangerous shorts. 
• If you need a higher uF than is available from your retailer, you can connect a couple of capacitors in parallel (side-by-side). For example if you need 0.1uF at 1600 volts you could connect two 0.05 uF / 1600 volt capacitors in parallel and you would end up with 0.1 uF at 1600 volts. You have maintained the voltage and doubled the uF.
• In "theory" connecting capacitors in series (end-to-end) should result in a higher working voltage. For example "in theory" two 0.1 uF at 630 volts in series should give you .05uF uF at 1260 volts (double the voltage and half the uF).....however, connecting capacitors in series is not recommended because with a series connection, one capacitor will usually end up getting more voltage than the other. This is because the leakage resistances of the two capacitors are rarely the same and the capacitor with the higher resistance will get a greater share of the voltage (which could result in the capacitor breaking down).
• What should it cost you to replace the capacitors in your radio? To "recap" a typical 5-tube radio you will need a couple of electrolytic capacitors and about a dozen film capacitors…..Total cost for these parts should be $10 or less.

• Last but not least…where can one buy the right sizes and right types of capacitors needed to restore vintage electronics? You have found the right place....JustRadios carries a complete line of high voltage film, dipped silver mica, ceramic disc, safety and electrolytic capacitors. 

Please visit JustRadios Ebay Store to shop for Film Capacitors.