The Importance of Line Filter Capacitors in Tube Radios:

• Your vintage tube radio needs proper line filter capacitors to sound good and to operate reliably and safely. These important capacitors are called line filter capacitors because they are used to filter-out/remove RF (radio frequency) line disturbances and interferences that are picked up by your homes power lines.
• Line filter capacitors also help to keep you safe from nasty (possible fatal) electrical shocks, by isolating the “hot” side of your 120 volt power line from your radios chassis.
• Noise interference is picked up in power lines from all kinds of household appliances such as washers & dryers, electric shavers, refrigerator motors, fans, air conditioners, light dimmers, lawn mowers, etc. Line filter capacitors pass noise interference ground (rather than your ears).
• Back in the good old days (when you tube radio was made) there was a lot less interference as there were far fewer electrical appliances in a typical home. Also, safety standards were no where near as strict as they are today …and, of course, safety capacitors had not been invented. If tube radios were manufactured today in 2004, AC rated line filter safety capacitors would be required.
• The capacitors that filter out interference from the 110-120 volt AC (alternating current) power coming from your wall plug go by a number of names. The general term for these capacitors is AC Line Filter Capacitor or AC Interference Suppression Capacitor.
• There are TWO basic types of Line Filter Capacitors”, namely “across-the-line” capacitors and “line-to-ground” (line by-pass) capacitors.
• In AC radios (radios with power transformers) where one side of the capacitor is connected to your 110-120 VAC power line and the other end of the capacitor connects to radio chassis/ground, that capacitor is referred to as a line-to-ground capacitor.
• Other names for Line-to-Ground capacitor include line bypass capacitor and line isolation or isolating capacitor.
• In AC/DC (radios that do not use power transformers) radios where a line filter capacitor is connected between the “hot” and “ground” side of the power line, that capacitor is known as an across-the-line capacitor. Across the line capacitors serve to keep RF noise disturbances from affecting the receiver. (AC/DC radios sometimes also utilize line bypass capacitors.)
• Line filter capacitors are hard working capacitors and operate under very harsh conditions.  They can short or explode if their dielectric breaks down. They have to handle 110-120 volts on a steady and ongoing basis. Depending on where the on-off connection is located in the radio circuit, the line filter capacitor(s) may be working even with the radio is turned off. Yes, if the on-off connection is located after line filter connection your filer caps will be working 24 hours a day, 365 days a year (as long as your radio is plugged in). Keep your radio unplugged when not in use.
• Over and above the ongoing 110-120 VAC, there are lighting storms/strikes and power surges and spikes in your power lines that can generate several kilovolts!  A proper a line filter has to be able to withstand ongoing high voltages and voltage surges and spikes for years, and if it fails, it should not explode, catch fire or put people at risk of shock. AC rated safety capacitors are designed to safely handle these steady high AC voltages and surges. Should they ever fail, they are designed to fail is a “safe way”.
• In the vintage tube radio you are about to restore you will not find safety capacitors. If your radio was made before WWII you will find that ordinary paper/wax capacitors were used. If your radio was made after WWII you will find either paper/wax condensers or ceramic disc capacitors. Once in a while you will even find that mica capacitors were used. Theses non-safety certified paper, ceramic disc and mica capacitor are all subject to FAILURE.

 

What Happens when your Line Filters Fail?

• If a line filter capacitor fails (and they do) your radio will work poorly (or not at all) and you may be at risk to electric shock (possibly fatal) and/or fire.
• If a line filter capacitor opens, your radio will be subject to the noise interference that the capacitor would normally filter out.
• If your AC radio does not have a true “earth” ground connected to it, it means your line-to-ground line filter capacitor is your receiver’s only connection to ground (i.e. coupled connection to the ground side of your 110-120VAC power line). This is why sometimes reversing the plug on an AC radio will improve performance. It is also why you will sometimes get a “spark” when you connect a true earth ground to an old AC radio.
• If your radio uses a single line bypass capacitor (and no earth ground connection) and your line-to ground bypass capacitor opens, your AC receiver has lost all ground contact. This loss of ground contact can cause a bad side effect known as is known as “modulation hum”. With “modulation hum” there is no hum between channels, but as soon as a radio station is tuned in you get a bad hum. To see is an open line-to-ground capacitor is the cause of modulation hum, just (connect in parallel) bridge it with a capacitor of the same capacitance.
• An improvement in circuit design is where an AC radio has two line-to-ground capacitors (see below schematic) to filter both sides of the line. This will give you better power line filtration and you don’t have to worry about polarity when you plug in your radio. However, let’s not complexly knock the single bypass design as some early AC tube radios used NO line filter capacitors at all.



• If a line bypass capacitor shorts-out or becomes leaky (and all old paper capacitors eventually leak), you may be in DANGER of an electric shock (possible fatal).
• With some AC/DC radios often the only thing between the chassis your 110- 120 volt line cord is a lonely line bypass capacitor (sometimes in parallel with a resistor).  What you end up with is known as a “floating ground”. If that by-pass cap becomes shorted or leaks badly, you could have a “hot” chassis capable of giving you a lethal 120V shock.
• As for AC receivers, the same thing can happen. If a line-to-ground capacitor shorts out, the chassis could be 120 volts “hot”.
• In AC-DC radios, if the across-the-line capacitor (C2 in the below schematic) opens up, radio performance will suffer from interference.



• The schematic below shows an AC/DC type power line circuit with just one line bypass line filter capacitor. When an AC-DC radio has just one line-to-ground (bypass) capacitor, that cap is only working effectively when connected to the “hot” side (rather than the ground) of your 110-120 VAC power cord. This is why some radios work/sound better is the plug is reversed. If your radio is picking up a lot of interference, try reversing the plug.



• It is a good idea to use a Ground Fault Interrupter (GFI) in every electronics workshop. This little device is inexpensive and looks almost identical to a regular 120-volt outlet. It is designed to shut off the power off if abnormal current to ground is detected (as a ground leak through you body can kill you). If you use old test equipment such as a tube tester or an oscilloscope, you may find that the GFI switches your power off for no known reason. The likely cause off this is leaky line filter capacitor(s) i.e. too much electrical current is flowing through them. If you are using old test equipment, be sure to replace the old line filter capacitors with new safety approved capacitors
.

 

To the Rescue - The Advent of Safety Capacitors:

Types of Safety Capacitors:

• There are two major types of interference-suppression / AC line filter safety capacitors; namely, type X and type Y. The purpose of these capacitors is to reduce radio frequency interference and to ensure safety from shock and fire.
• In addition to helping to block out noise interference from being received, they are also used in electrical devices and appliances to help prevent noise interference from being sent out. Below is a schematic of a modern day “line filter circuit” which shows the safety capacitor(s) C1 and C2. Capacitor C1 would be a type X “across the line” capacitor and the two C2s would be type Y “line to ground” safety capacitors.



• Class X capacitors are used in “across-the-line” applications where their failure would not lead to electric shock. Class X safety caps are used between the “live” wires carrying the incoming AC current. In this position, a capacitor failure should not cause any electrical shock hazards, rather, a capacitor failure “between-the-lines” would usually cause a fuse or circuit breaker to open.
• Class Y capacitors are used in “line-to-ground” (line bypass) applications where their failure could lead to electric shock if a proper ground connection were lost. The failure of a “line-to-ground” capacitor would not open any safety fuse.  In other words, the failure of a line bypass capacitor could create a 120 volt “hot” chassis that could give you a potentially fatal shock.
• Because the terms X and Y are pretty non-descript, it can be hard to remember which type go where. To remember, just think of the X as being A CROSS on a 45 degree angle, and there you have it ….X is for “A CROSS the line” applications.
• Most safety capacitors have voltage ratings of 250 VAC and can be used in circuits up to 250 VAC.  With a 250V voltage rating the capacitors can used in the USA and Canada where typical household voltage is 110-120 VAC as well as in Europe, Australia and other countries where 240 volts is used.

 

Classes of Safety Capacitors:

• Safety capacitors are grouped into a number of different classes. For X Type capacitors there are class X1, X2 and X3. For Y Type capacitors there is class Y1, Y2, Y3 and Y4.
• The only types you will probably see for sale are X1 (impulse tested to 4000 Volts), X2 (tested to 2500 V), Y1 (tested to 8000 V) and Y2 (tested to 5000 V).
• Of the above capacitors, type X2 and Y2 are the most popular and the type that you will probably want to use. X2 and Y2 safety capacitors are used in appliances that plug into ordinary household wall outlets, while type X1 and Y1 are for heavy duty industrial use. For example, a type X1 capacitor would be used in an industrial computer or industrial lighting ballast that is connected to a 3-phase line (the main power truck lines within a building).
• You could use type X1 and Y1 in your tube electronics if you wanted to, but all you require to meet safety standards is the X2 and Y2. The type 1’s will cost you more money and may be more difficult to install due to their larger size.
• Are X2 and Y2 capacitors interchangeable? Yes and no! You can safely use an Y2 capacitor in place of an X2 capacitor for an “across-the line” application, but you should not use an X2 capacitor in place of an Y2 capacitor for a “line to ground” application. The X2 type would work and remove noise interference, but would not meet line-to-ground safety standards. This is because Y2 capacitors are more robust, take higher test voltages and are designed to open, (rather than short) should a failure occur.
• What do Y2 safety capacitors have in common with your cars windshield? They are both built “not to break” and if they do break, they are built to “break safely”.
• By now you are probably asking yourself…if Y2 capacitors are so great and can be used for X2 purposes … why not just forget about X2 capacitors and use Y2 type all the time? There are two good reasons … cost and size. Y2 safety capacitors are more expensive than X2 type and Y2 capacitors are larger (which can may make installation harder). Using Y2 type capacitors when an X2 type is all that is required would be like re-capping your tube radios with 1600 volt capacitors rather than 630V caps. All you have done is spent more money that you had to.
• Once in a while you will see a safety capacitor that is as combination of X and Y classes.  For example, an X1/Y2 ceramic disc safety capacitors. This simply means that the capacitor meets safety qualifications as both an X1 capacitor and an Y2 capacitor. If I might use a car analogy again, you might think of an X2 capacitor as a special summer tire, an Y2 as a rugged winter tire and an X1/Y2 as a versatile all-season tire.

 

Construction of Safety Capacitors:

• Safety capacitors are available in ceramic disc and metalized film or paper. The film type are made of self-healing metalized polyester, polypropylene or paper and usually come in a “box” style casing as the capacitor is encased is a flame retardant or flame-proof case.
• Ceramic disc safety capacitors have the advantage of being economically priced and compact in size (making installation easy). One limitation ceramic disc safety caps have is that the largest uF capacitance size available is 0.01 uF (which is sometimes lower than what is needed per the schematic). If you require Y2 safety capacitor that has a capacitance over 0.01uF you will need to use Y2 capacitor made of Poly Film.
• Type X2 film capacitors are also very economical, are relatively compact and are available in a wide range of sizes.
• Y2 film capacitors are more expensive and larger that both disc capacitors and X2 film capacitors. Most film capacitor manufacturers make Y2 capacitors up to 0.047uF maximum.
• All safety capacitors should have certifier’s symbols on their casing. For example UL (for USA), CSA (for Canada), VDE (for Germany), etc. To ensure you are using properly certified safety capacitors check that certification symbols (see below) are on the cases.



• There are also certain safety standards/certification tests that you want the capacitors to meet. For Radio, TV and certain other telecom use the requirement the standard that should be met is: UL1414 (USA); CSA C22.2 No.1 (Canada) and EN132400 (Europe).  If you buy safety capacitors for your radio or TV, check with your vendor to be sure the caps you are buying meet the above standards.

 

Wrap-up:

• When you restore a vintage tube radio it is standard practice to replace all old paper/wax capacitors and electrolytic capacitors. This is known as re-capping the radio. All line filter capacitors should be replaced with approved safety capacitors.
• You will need to determine which capacitors are “across-the-line” and which are “line-to-ground”. You will be able to easily identify these capacitors with the help of a schematic. Don’t guess …. use a schematic.
• “Across-the-line” capacitors should be replaced with type X2, X1/Y2 or Y2 safety capacitors.
• “Line-to-ground” line filter capacitors should be replaced with Y2 or X1/Y2 safety capacitors. (do not use X2 type).
• As a general rule one does not replace disc or mica capacitors, but if they are being used as line filter capacitors, it’s time to break the general rule. Non-safety rated ceramic disc and/or mica capacitors have a high failure rate when used as line filter capacitors. Non-safety certified disc and mica capacitors should be replaced with a proper safety capacitor.

 

JustRadios Services:

• Capacitors for Vintage Electronics: We sell a full line of high voltage film, mica, electrolytic and safety capacitors for vintage tube radios that include interference suppression / AC rated line filter safety capacitors. All capacitors come with long leads for easy under chassis installation in tube electronics. We carry X1/Y2 Ceramic Disc AC-Rated Safety Capacitors, Y2 Metalized Polyester Safety/Interference Suppression Capacitors and X2 Metalized Polypropylene Interference Suppression/Safety Capacitors. All of our safety capacitors meet UL1414, CSA 22.1No.1 and EN132400 for use in televisions and radio receivers.

• Capacitor Kits for Tube Radios: We carry a number of capacitor kits which offer savings over buying capacitors individually. These cap kits contain the sizes most often needed for tube radio repairs and restorations. Our Safety Capacitor Kit is Kit # 12.
  
  
• Resistors for Tube Electronics : JustRadios carries a complete line of Resistors for Tube Electronics. For added safety all of our metal oxide, carbon film, metal glaze and power resistors are "flame-proof".
  
  
• Resistors Kits for Tube Electronics : JustRadios carries a nice selection of Resistors Kits Tube Electronics. For added safety all of our 1 watt, 2 watt and 5 watt resistor kits are "flame-proof".

• Schematics and Service Data: We carry schematics and service data for vintage tube radios, televisions, hi-fi systems, audio and guitar amplifiers, CB radios, test equipment and more.