3 Prong Conversions

Important Disclaimer:

ELECTRICITY IS DANGEROUS AND CAN KILL YOU.

THIS IS NOT A MANUAL. THIS IS PERSONAL NOTES FOR MYSELF THAT ARE VIEWABLE,

BUT SHOULD NOT BE TAKEN AS INSTRUCTIONS. ONLY QUALIFIED TECHNICIANS SHOULD PERFORM WORK ON ANYTHING THAT PLUGS INTO AN ELECTRICAL SOCKET. PROCEED AT YOUR OWN RISK.

2 prong equipment (like old amplifiers) can shock! Will removing an old power cable affect the value of your vintage amplifier? All I can say is that there is no value in getting shocked, so outside of units going to museums, please update those power cables!

Which wire is which:

Hot (smaller prong on the right of an outlet, often black or brown wire) goes to the fuse, switch, and one side of the power transformer.

Neutral (larger prong on the left of an outlet, often white or blue wire) goes to straight the other side of the power transformer.

Ground (round prong, green wire) goes straight to the metal chassis.

Parts and Tools Needed:

New 3 prong cable, same gauge or larger than existing wire (18 AWG, SVT is typical for most old amps)

New strain relief to match size of new cable (try the Heyco brand)

Strain relief tool (optional, or pliers can work)

Drill with appropriate drill bits

Screw, nut, lock washer, and Loctite (or other anaerobic adhesive)

Crimp terminal ring connector

Crimp tool

Screwdriver with appropriate bits for amplifier disassembly

Power cables: SVT vs SJT

SVT is a UL code for standard household appliance power cables. They only come in AWG 17 and 18. They are meant for regular, non-heavy duty applications. Most old amps ('50s/'60s) fall in this category. When in doubt, match the gauge of the old cable.

SJT is a UL code for heavy duty power cables and these cables will be thicker than SVT even with 18 AWG wire. They range in size from 18 to 10 AWG.

Most of the time, you will want the SVT 18 AWG cable. If you try to install a SJT 18 AWG cable in an old amp, you will generally find that you have to enlarge the original hole, and the old Heyco strain relief will not fit, so you'll need a new large strain relief as well.

"Where do I attach the ground wire?"

The best place for the ground wire is in the immediate area of the power cable's entry hole. Drill a new hole, use locking fasteners, and Loctite (or other anaerobic adhesive). Use only a crimped ring connector for attaching the ground wire. Solder is not acceptable by modern standards for the terminating the line power cord.

"Cheater" methods include:

-Soldering. This is inadvisable as it is very difficult to solder most chassis metals. You will most likely just end up with a blob of cold solder that may appear to be attached OK, but may fail in the future. Old amps were often assembled this way. If you are determined to solder to the chassis, I would suggest further research and specialized equipment to get the solder to flow correctly and bond with the chassis. Some 1970s amps use a crimp connector with solder melted on top.

-Using a transformer screw. This allows you to use a crimped on (or soldered on) ring connector, but instead of drilling a new hole and adding new fasteners, you just piggyback on the existing transformer screw. Not acceptable by modern standards, but often seen in 1970s amplifiers.

Disassemble the 2 prong unit (power cable unplugged, please) and safely discharge the filter caps as soon as you can. Heyco strain reliefs can be easily removed with a handy strain relief tool. The same gauge or larger gauge copper wire should be used. For most amps, 18 gauge is fine. If the outer jacket of the new cable is larger than the old, you may need to enlarge the hole and get a new strain relief of correct size.

The ultimate goal for the hot and neutral wires is to be across the power transformer primary (hot at one end, neutral at the other). Between the power cable and the power transformer, there needs to be a fuse, and there is often a power switch as well. Whether the switch and fuse are on the hot leg or the neutral leg is not guaranteed (the amp works either way). Make sure in the new installation (regardless of the original) that the hot connection goes into a fuse (the "tip" connection, not the "ring") and then into the power switch, and then to the power transformer primary. The neutral goes straight to the power transformer primary's other side.

The advantage in the above wiring can be appreciated when servicing the amplifier. When the fuse is blown, the technician can be assured that the hot line is terminated at the fuse holder. Usually this can be confirmed visually, without the use of any test equipment. Otherwise, if the hot immediately enters the power transformer it can be unclear where the hot terminates, and increase the risk of shock.

In the beginning there were 2 prongs per power cable for most home electric devices. After many reports of shocks, fires, and other undesirable consequences of using electric appliances, the National Electric Code was changed, and the grounded outlet (3 prong) era began. In the United States, January 1, 1974 was the day that all new 15A outlets (your average home outlet) were required to be grounded.

The "Death Cap"

Two prong amps will have a smallish cap, usually in the nano Farad range, that is meant to connect the line voltage to the chassis, thus providing some RF filtering to the primary of the power transformer. If the amp has a "ground switch," then the cap is usually connected to the common terminal and the switch selects between the line and neutral, allowing a reverse wired outlet or a flipped over plug to find this filter cap. This type of cap is called a "Y" cap, and they are still in use today, but with strict manufacturing standards to be a modern "Y" cap.

The danger in old "Y" caps is that they are just regular coupling caps, and it is not guaranteed that they will fail open, as modern "Y" caps are guaranteed to do. If an old cap fails short from line to chassis, the chassis will be "hot" and electrocution of the user becomes a serious possibility.

When upgrading to a 3 prong power cable, it is common practice to simply snip out the old "Y" cap, since the chassis will provide good RF rejection after being "grounded." Of course the user could defeat the 3 prong later on with either a "cheater" adapter, or just breaking off the 3rd prong, in which case the "Y" cap is needed again. The best method then is to purchase a new UL approved "Y" cap and replace the old "death cap" in addition to upgrading the power cable.

The "Y" term means "line-to-ground," while "X" type capacitors are "line-to-line" or "across-the-line." The X symbol sort of goes with "across," while the Y symbol sort of symbolizes the 2 lines (hot and neutral) going down to ground. Modern equipment may have both an X and 2 Y caps, one for hot and one for neutral. These may be included just to achieve UL, FCC, or other standards, which may be more concerned about blocking what the device is radiating out. For vintage amps, or hobbyist builds, you probably do not need all those caps to filter RF coming in.

Home Power Basic Info

Home power in the U.S. uses single phase ac of approximately 120Vrms (check your socket for the real number). Numerous outlets and lamp sockets are wired in parallel, and each group of parallel outlets share a fuse or circuit breaker usually set to trip at 15 amps.

Looking at a modern socket (NEMA 5-15), the long slot on the left is the "neutral" bus, the short slot of the right is the "hot" bus, and the round hole is the "ground."

The "ground" should literally connect to the ground via a dedicated stake driven into the ground or via a cold water pipe, or other method outlined in building and electrical codes.

The "neutral" is all connected to the "ground," but should only make that connection at the main fuse box of the building. It is "cheating," and incorrect, to connect neutral and ground at an outlet box in the living area.

The "hot" is carrying the 120V relative to the neutral/ground, and this is coming from the power company, entering your building either from a utility pole or through wires in the ground. It may enter as 120V, or it may enter as 220V, and your main electrical panel will have a 220V circuit, and a 120V circuit.

The circuits are arranged so that a short (low resistance) connection will blow a fuse, or trip a circuit breaker. To restore power, the offending short must be removed, and then blown fuses must be replaced, while circuit breakers usually just need a reset.

Floating Grounds and their Danger

If an electronic device has no connection to another circuit, like a hand held radio for instance, then it is said to be "floating." Inside the device, the circuit has positive and negative currents, and if one calls the negative terminal of the battery "ground," then voltages within the radio will be positive or negative with respect to the battery's negative terminal. However, the radio's ground and the literal earth's ground are not connected, and it is unknown what the voltage (potential difference) between them is.

2 prong appliances are floating, and the metal chassis inside will be at a voltage different than all the "grounded" metal in the room. If you are floating as well, and "open circuited" by wearing thick rubber shoes and not touching anything else, then touching the 2 prong appliance's metal chassis might not shock you, but you will get energized to that voltage. If you then, or are already, touching something at a different (higher or lower, doesn't matter) voltage then you get shocked. If energized to 120V from the wall power, a circuit can form through your shoes and into a concrete floor, so don't experiment with power outlets and your body.

Working in music stores, there will often be a '50s or '60s vintage tube amplifier about with the original 2 prong cable still attached. Often this will be displayed, plugged in, and next to a 3 prong amplifier. All you have to do is accidentally touch the metal panel of the vintage amp and the modern amp at the same time to get a shock. It's easier than you think. When you plug in a 1/4" cable into the panel, those metal plugs on the 1/4" cable become an extension of the panel! And when you plug it into a guitar, the strings themselves become an extension of the panel! This is where the greatest safety concerns lie, especially for singing guitarists because the same thing can happen to a plugged in microphone. Many musicians have been injured or killed due to receiving an electric shock from a voltage between microphone and guitar strings. It is very important that microphones and guitar strings be at the same potential.

Do not fear your modern 2 prong appliances though (TVs, kitchen appliances). If properly designed, you will only interface with a plastic outer casing that will have little chance of shocking a user. Musical keyboards and other sound generators or processors that have 2 prongs are assumed to eventually get "grounded" through a 3 prong amplifier; at least that's the general wisdom. Often having multiple "3 pronged" pieces of gear interconnected can cause the dreaded "ground loop hum" problem. Amplifiers should never have the 3rd prong removed. Signal generators and processors are often designed without 3 prongs to avoid this issue. If they are fitted with 3 prongs, it is advisable to leave the ground pin and try severing the signal's ground connection somehow. A special cable can be made for this purpose (just break the shield connection on one end), or if you're handy you can install a "ground lift switch" on the output(s).