ABC's of "Electrolytic" Capacitors for Tube Radios, TVs and Amps

Although this guide is written from the perspective of tube radio enthusiast, this guide should also be useful to those who repair vintage TVs, hi-fi's, audio and guitar amplifiers and test equipment. We hope you find this guide useful and good luck with your vintage tube electronics repairs and restorations.

ELECTROLYTIC CAPACITORS - The Basics:

• 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.
• Electrolytic capacitors are often referred to a "filter capacitors". Electrolytic capacitors help to convert (filter) AC (alternating current) power into the DC (direct current) voltage that your tubes need to operate.
• Size wise, the electrolytics are the largest capacitors and most tube radios use 2 or 3 of them. The original electrolytic capacitors are typically the size of a roll of quarters or larger. On the old AC sets (radios with power transformers) they are usually encased in aluminum and mounted on top of the chassis. With the lightweight AC/DC sets of the 1950's they are quite often under the chassis and may have a cardboard case.
• Electrolytic capacitors work hard and are probably the most unreliable part of an antique radio. As they wear out (or simply get old) you get that famous "tube radio hum". Yes, in most cases it is bad filter capacitors that are the cause of that hum.
• WARNING! If you tube radio hums "turn it off and don't use it". Bad electrolytics are not only hard on your ears; they are hard on the tubes, transformers and other parts in your radio. Capacitors are cheap….tubes and other parts can be expensive and hard to find.
• All electrolytic capacitors have both 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.
• Electrolytic capacitors have a rated "working voltage" (WVDC) which is the voltage it can handle. Ideally, an electrolytic capacitor should be operated at a voltage between 1/3 and 3/4 of its maximum working voltage. This is enough voltage to keep the electrolyte from drying out while it allows some margin of safety for unexpected voltage surges. Never replace an electrolytic with one that has a lower voltage rating.
• In service literature, parts lists & schematics, paper and electrolytic capacitors are usually expressed in terms "microfarads". Short forms for microfarad include mfd, MFD, MF, UF and uF. As a general rule, if a capacitor is more than 1 uF it's probably an electrolytic capacitor.
• In most cases AC radios can use 450 volt electrolytics while lightweight ac/dc tube radios can use 160 volt filter capacitors. However, there are exceptions so always best to refer to a schematic. 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.
• How about using NOS (New "Old Stock") capacitors? NOS capacitors are not recommended…use at own risk! As electrolytic capacitors age their capacitance values drift, they dry out and they become leaky. Would you drive a 1940's automobile with NOS 60-year-old tires?
• Electrolytic capacitors have a shelf life of a couple of years, so make sure you are buying "fresh" stock electrolytics (not new "old stock"). 
• To maximize shelf-life, electrolytic capacitors should be stored at temperatures of 5 to 35 degrees C (40 to 95 degrees F) and in non-humid conditions (less than 75 relative humidity).
• Don't waste your money on audiophile, computer grade or tantalum capacitors. Sure they are good capacitors, but 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. 
  

CAPACITOR Installation Tips:

• 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 its 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.
• Capacitors used to be much larger and much more expensive than they are today. To save on space and cost "multiple section" electrolytics were used. These are simply two, three or four capacitors in the same case. You will notice just one ground connection (black wire) as all the caps share that ground. These "multi-section" caps can be replaced with single electrolytics. Modern electrolytics are compact and are easy fit under the chassis. You can leave the old can capacitor on the chassis for original appearance. Just be sure to disconnect it.
• When replacing an electrolytic capacitor, an exact uF replacement is not necessary.  If you can't find a close replacement …better to go with a slightly higher uF value than a lower uF.  The old "rule of thumb" when replacing electrolytic capacitors is to not use more than 80% higher (or 20% lower) than "the original" uF size. If you replace an E-cap with one that has too high a MFD, the DC voltages will be higher than called for and your tubes and other parts will wear out faster. If you use too low a uF size, your radio will hum.
• 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.
• Warning ! Electrolytics have a negative end and a positive end…..if you install an electrolytic with the polarity mixed up not only will your radio not work…the electrolytic capacitor could explode. All the electrolytic capacitors that we sell have an arrow marked on them (with negative signs within the arrow). This arrow points at the negative side of the capacitor.
• On schematic diagrams the flat side of the capacitor symbol is the positive (+) side and the curved side is the negative (-) side. The positive end must be kept at the higher electrical potential (more positive voltage). 
• 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 short circuits that are very dangerous. Short circuits will cause your radio to malfunction. 
• 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 200 uF at 160 volts you could connect two 100 uF / 160 volt capacitors in parallel and you would end up with 200 uF at 160 volts. You have kept the voltage the same while doubling the uF. In "theory" connecting capacitors in series (end-to-end) should result in a higher working voltage. For example "in theory" two 100 uF at 450 volts in series should give you 50 uF at 900 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).
• Please remember to always work safely. The high voltages stored in large capacitors can kill!  Before working with these capacitors they should be completely discharged. This can be come by (bridging) connecting the two ends the capacitor in question with a high wattage 1000 ohm resistor via insulated clips and leads.
• Don't put your tube radio into storage after you have restored the electrics. Once a month let the radio sing for a half-hour or so. This will prevent the electrolytic capacitors from drying out.
• 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? You have found the right place…..JustRadios carries a complete line of electrolytic, film, mica, disc and safety capacitors for tube electronics. 

 Please visit JustRadios Ebay Store to shop for  Electrolytic Capacitors.