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You turn on your amp only to hear a hum coming from it that you weren’t expecting. What can you do?

Hum in an amplifier comes in two types:

60 cycle hum

This type of hum more often than not originates outside of your amplifier. However, it can also come from the filament supply, a burnt bias pot or resistor or from the filament and grid wires being too close to one another.

In a fixed bias amp, 60 cycle hum usually comes from a bad bias supply filter cap.

120 cycle hum

Unlike 60 cycle hum, this type of hum does originate within the amplifier. It most likely comes from bad filter caps or a coupling cap becoming conductive.

Troubleshooting

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To determine if your amp hum has an outside cause, unplug your guitar cable from the amplifier. If the hum stops, you know the amp isn’t the problem. Instead,  your cable or input jacks and plugs could be the culprit.

If this is not the cause of the hum in your amp, you can do a visual inspection of the components to look for burnt resistors or bias pots.  Also, check the filament and grid wires proximity to one another.

If the hum is 120 cycle, you can find out if a coupling cap has become conductive by checking the plate voltage of the tubes. One of them will be noticeably low compared to the others.

Disclaimer: Unless you are an experienced amp technician or trained in electronics, please do not attempt to make repairs on your amplifier yourself due to risk of serious injury, including electric shock and burns.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas

You want to use your tube amp at this weekend’s gig, but when you sit down to practice or jam with friends, you get a horrible popping noise. The noise goes away and comes back, but no amount of adjusting the knobs or cables gets rid of it. What you have, is a static problem.

Causes of Amplifier Static

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Some of the causes of static in your amp may include:

• Loose connections
• Bad solder joints
• Arcing capacitors
• Bad resistors
• Faulty ground
• Arcing between components

Of course, other causes may exist.

How do you find the cause of static?

The two best ways to find the source are the same as those used to find the source of hum.

The process of elimination

First, you can use the process of elimination. You do this by methodically removing and replacing your pre-amp tubes, beginning with your phase inverter tube.

The chopstick method

Begin with a visual inspection of the components. Sometimes that is enough for you to see if a resistor is burnt or a capacitor is damaged. If you don’t see anything upon visual inspection, you can use the chopstick method. This method allows you to find the source of the problem without the risk of shock or burn. It is also simple and inexpensive to do.

To conduct this test, remove the chassis from the cabinet. Hook up a speaker and turn the amp “on” and set it to “play” mode. Turn the volume controls up all the way. Using a wooden chopstick to probe, gently poke the components at their joint, wire and component connections. If a component is loose, you will hear noise when you poke or tap the resistor.

Make sure you check every component, because the noise may come from the resistor you are tapping, but one close to it. Go through the components in the same systematic way you did with the pre-amp tubes.

Next steps

Once you are absolutely certain you have found the source of your static problem, you can fix it.

If you cannot find the source of the static in your amp, or if you are a novice, take it to a reputable technician for repair.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas

Hissing Noise

This noise usually occurs whether or not there is signal to the amplifier. Additionally, it rises and falls as the volume is raised and lowered. This type of noise can have one of two causes:

Pre-amp tube problem

If you suspect that the problem originates with a pre-amp tube, the best way to rule it out is to exchange the pre-amp tube with one you know does not hum. Be careful that you aren’t simply using a low gain tube, however. Good pre-amp tubes have good gain, good tone and do not hum.

How can you know if it might be the pre-amp tubes?

Start by having the amplifier on and in play mode, then remove the phase inverter tube. This is the pre-amp tube closest to the power tubes. If you no longer hear the hum, it likely originates with your phase inverter tube or other pre-amp tubes. Now, you want to use a process of elimination. Replace the phase inverter tube and remove the pre-amp tube next to it. If the noise continues, it is your phase inverter tube. If not, keep removing and replacing the pre-amp tubes in order to discover which one is creating the noise.

Plate load resistor problem

Begin with a visual inspection. An arcing plate load resistor may look charred or burnt.

The Chopstick Test

If you cannot visually detect the bad resistor, you can use the “chopstick” test. To conduct this test, you first remove the chassis from the cabinet. Then you hook up a speaker and turn the amp “on” and set it to “play” mode. Turn the volume controls up all the way and using a wooden chopstick to probe, gently poke the components at their joint, wire and component connections. If a component, such as a resistor, is loose, you will hear noise when you poke or tap the resistor.

Check every component, because the noise may not originate with the exact resistor you are tapping, but the one closest to it that is reacting to the shock. Go through the resistors in the chassis systematically as you would the pre-amp tubes.

These two methods should eliminate the hum in your amp.

Source: Weber, G. (1997). Tube Amp Talk for the Guitarist and Tech, p. 17-25. Kendrick Books: Texas