Tech Forum

Some comments and concerns regarding your question. First, I am not clear if this is a repair or a retrofit. I would also presume this is a tube-type amplifier, since they were often built with point-to-point wiring and hum pick up was highly likely with so many signal wires running around the chassis.

Based on your comment of "adding a kit", I would presume that either this is a retrofit or the original noise filter cap was removed in the past. The fact that adding the new cap from line to ground reduced the 60 cycle hum (or 120, depending on the rectifier design), it seems that this might actually be a repair. Or, the filter caps themselves are old and leaky, so you are aiding them but not actually curing the problem. If the amp is old, it often needs to have the filter caps replaced and, sometimes, some interstage decoupling caps as well.

With that background, I caution that this involves principles of safety and if you are not experienced with working on mains circuits, this effort would be best left to a professional. Just because something works does not mean it is well-executed or robust. And since guitar amps get moved, jostled, banged around and often not treated caringly, there is a chance your soldered connections of this component could break loose without your knowledge and you could have a real shock hazard on your hands.

OK, assuming all that is well-appreciated, the type of cap you use is critical. This component is connecting one side of the AC mains to the accessible chassis. You cannot assume that a ground pin on a cord will save your life since you have no idea how well the ground-to-neutral impedance is controilled in the building's branch circuit you just plugged into. Hence, the cap itself must be suitably rated to be connected from line to ground. Therefore, this must be a Y2 type of cap.

If you have no idea what a Y2 cap is, then read this link first: www.justradios.com/safetytips.html. There are many good manufacturers of Y2 caps (Vishay, KEMET, Panasonic, etc.). Here is a good selection from Mouser:  www.mouser.com/new/Kemet-Electronics/KEMET_XY_Film_Caps.

Be sure that you do not rely on solder for mechanical support of live parts. Always wrap the cap leads snugly around the terminals it is connecting to and check to determine that it would tend to stay there if no solder was used. Solder is not meant for mechanical retention of massive parts. Once you feel it is well mounted, then use solder to make the electrical connection to the terminals.

Safe is a relative matter of degree. The safety ground wire in a standard three wire AC cord is there for a reason. However, the NEC does allow for devices that do not have that safety ground connection. Many electric and electronic devices are made without it.

The safety ground connection is there to insure that the fuse does blow or the circuit breaker does trip if a conductive outer case of the device becomes electrified. The current that flows from the hot wire to the safety ground blows the fuse or trips the breaker and then there is no danger of shock.

Devices that do not have that safety ground usually employ a double insulation method to prevent such a short to the outer case or to the user. Thus, if there is internal insulation AND the outer case is an insulating material like plastic, then TWO different insulators would have to fail for the user to get shocked.

Older devices, like early electric lamps, were made with metal outer cases and often the electric wires inside them were only a small fraction of an inch from that conductive case. Older electric cords were insulated with real rubber which deteriorated over time and became "frayed". The card board insulators in lamp sockets literally fell apart after some years of service, usually with over sized bulbs in the lamp (rated at 40 W so the user put a 60 or a 100 W for more light). Etc., etc., etc.

It is not possible to tell if a particular device, like your amplifier, is safe without examining it. If it was commercially manufactured in the US or imported properly, then chances are it is OK. Things like the UL (Underwriter's Laboratories) tag are supposed to tell the consumer that it has been inspected and found to comply with the code. If it is a kit or was made in someone's kitchen or garage, then it may or may not be OK.

Hum like this, is usually a result of what is called a "ground loop". Both the signal cable between the audio devices and the power cables to them are grounded and they form two different and distinct ground paths between them. If there is any 60 Hertz current flowing in either of them, then the circuit/signal ground planes in each of those devices will be at a different AC potential and the hum is introduced into the signal.

One way of eliminating this situation is to cut the safety ground on one or both of the devices. But this eliminates the safety feature of that connection. A better way to eliminate this situation is to use a balanced audio line between the devices and to connect (ground) the shield only at the source end of that line. In this way there is only one ground connection between them, via the AC power. Many times this is used in professional audio installations where long lines must be run. But this may not be possible on amateur equipment where single ended audio lines are often run.

Another solution is to use a differential input amplifier at the receiving end of the audio line. This can be accomplished with a transformer or with actual active circuitry. With a transformer, the center lead and the shield of the single ended cable are connected to the primary of the transformer so the shield is not connected to the chassis/signal ground of the amp.

Active circuitry with a balanced input is more complicated. I am designing such an amp at the present time to overcome a noise problem between my audio/visual equipment cabinet behind my chair and the computer on the desk in front of it. Due to the arrangement of the room, I had to run a 50 foot cable and it does pick up hum.

An audio transformer of sufficient low frequency response would cost a bundle. Since the audio is single ended (not balanced) this may be the only solution. I have searched and could not find a reasonably priced, differential input audio amp for this application. Professional ones seem to start in the hundreds and go up from there.

Another factor that helps to overcome noise and hum is the impedance of the audio line. Much amateur equipment uses a high line impedance which may be OK for a few feet, but longer, high impedance (10 KOhms) lines will pickup noise and hum from the environment. Professional audio uses lower impedance lines to overcome this.

The old standard was 600 Ohms but modern equipment often has extremely low output impedance (one Ohm or less) and a higher input impedance. This works in most situations. When long lines need lower termination impedance on the receiving end, a simple resistor can be connected across the line at that point.