ABC's of SAFETY (Interference Suppression) Capacitors for Tube Radios
Line filter safety capacitors can improve the reliability, safety and performance (sound) of your vintage tube radio. This page discusses in "non-technical" language: the importance of using proper "across-the-line" and "line-to-ground" line filter capacitors; what can happen when a line filter capacitor fails; the TYPES of line filter / line bypass / interference suppression capacitors; the CLASSES of type X and type Y safety capacitors; the construction of safety capacitors; and how to choose the right safety capacitors for your tube radio restorations. Although this article is written from the perspective of tube radio enthusiast, the information should also be useful to those who repair vintage tube TVs, vintage hi-fi's, tube based audio and guitar amplifiers, tube testers, scopes & test equipment, etc. We hope you find this safety capacitor information useful and good luck with your vintage tube electronics repairs and restorations.
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 linefilter capacitors because they are used to filter-out/remove RF (radio frequency) line disturbance and interference picked up by your
home 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
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 InterferenceSuppression Capacitor.
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
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 in a
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.
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 “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 if 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
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 (possibly 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 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 of 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:
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.
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.
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.
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.
capacitors have voltage ratings of 250 VAC and can be used in circuits up to250 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
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.
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”.
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 a 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.
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.
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
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.
Q & A:
The two most common questions I receive regarding safety caps are noted below:
Question: The old paper capacitor(s) currently being used as line capacitor(s) are rated at 630 Volts. The safety caps are rated at just 250 Volts. Is it safe to use the lower voltage safety capacitors?
Answer: The old paper caps are standard capacitors and are rated at 630V DC. The new safety caps are rated at 250V AC. The 250V AC rated safety caps will take much more punishment that regular 630V DC capacitors.
Question: My old tube radio does not use line capacitors. If I want to install line capacitor(s) .....what uF size should I use?
Answer: I do not know of any formula to determine what uF size should be used... but an exact uF size is not critical. Since the most common size line capacitor(s) is 0.01uF you may want to start with that size. Since safety caps are also "interference suppression" capacitors, the installation of line capacitors "may" help to reduce interference.
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.
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
capacitors should be replaced with type X2, X1/Y2 or Y2 safety capacitors.
line filter capacitors should be replaced with Y2 or X1/Y2 safety capacitors. (do not use X2 type).
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 Kits for Tube Radios: We carry a number of
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.