How to Troubleshoot Your Amp

As promised earlier in the article: “What Tubes are Best for Your Amp?” here is a guide to help you in the event you encounter problems with your amplifier.  Enjoy!

September 1, 2020 – Being that Covid-19 has all but brought live music to a halt, it seems like things are at least starting to get back to normal. Bands like Steel Panther, Dirty Honey and The Allman Betts Band are playing either live Drive-In concerts or via livestream. On the flip side, one benefit of everyone being a virtual shut-in is that a lot of artists have hunkered down in the studio. I look for a lot of new music releases in 2021. If you’re a gigging musician I’m sure you can’t want to get back to playing live. To that end, you might want to check out your gear so that when the live venues are ready, you will be too. We’ve all heard horror stories about your amp being dead when you turn it on or dying in the middle of a gig. Perhaps when you hit a chord your amp responds with buzzing, clicking or clanging. You might turn it on one day and everything looks fine but no sound comes out. You might hear a loud hum that doesn’t seem to go away whether the volume is turned up or down or you hear a hissing or popping sound when you take it out of Standby. These are all symptoms of an amplifier that may need service. So what do you do? Is it something you can fix yourself or do you need to have it serviced by a technician? Here are some tips on how to troubleshoot your amp. Even if you can’t fix it yourself, if you follow these steps you can still tell the tech the things you’ve tried so he/she has a better idea of what might be wrong. Some symptoms of an amp needing service are:

• No power. You turn the amp on but it doesn’t actually turn on. If you get no indication of power from the amp try the following: On amps with lighted power switches or power indicators, sometimes the lamp burns out. If your amp actually works but the lamp is burned out, you can use it as-is or have the lamp switch replaced. If the amp doesn’t work, make sure it is plugged into the wall outlet. If it is, try plugging something else into the outlet to make sure there is power to the outlet. If your amp plugs into a surge strip (recommended), make sure that is working as well. Some surge strips have a built-in circuit breaker in case something draws too much current. Often there is a reset button on it. Press it and see if you have power from it. If you plug another device into the surge strip and it won’t turn on, replace the strip. On some strips the overload protection is the strip itself. When it receives too much current it dies and all you can do is get a new one. Don’t rely on the pilot light on most surge strips – they burn out and make it look as if the strip is dead; often they are not. It may sound unusual but surge strips do go bad. I’ve replaced more than a few, especially the lower priced ones.
• Repeatedly blowing fuses. Most tube amps and some solid state amps have built in fuse protection in case something goes wrong with the amp. You can try replacing the fuse. Make sure you replace it with the same value as the one that blew. A lower value fuse can be used temporarily if you don’t have the exact rating fuse as the one that blew. NEVER use a higher-rated (current) fuse in place of the one that blew. It may work but you could end up damaging the amp, which in the end will cost you more money to fix. This could be more dangerous as well. If you replace the fuse and the amp turns on, play through it for five or ten minutes to see if it blows again. Often an amp will seem fine at low volumes but when you turn it up, it blows a fuse. This means you have a more serious problem. So what causes a blown fuse? Remember, fuses are there to protect a circuit against too much current. Current is the flow of electrons through a circuit. Think of the wires and circuits of an amp as a pipeline. They are designed for a certain amount of current flow. If too much current flows it would be like forcing too much water through the pipe. The fuse is designed to blow instead of the circuit so it doesn’t damage any components. If you put a larger rating of fuse in the circuit, you are allowing more current to flow through the pipe than it was designed for, and we’ve all seen what happens to water mains in the street when they blow – you get leaking current everywhere (I’m joking.) If the amp is still blowing fuses it’s time to start looking at other causes, possibly the power tubes (see below.)
• Preamp tube failure. The preamp tubes are the smaller ones inside your amp. There could be as few as one but often three or more. Preamp tubes need to be replaced when:
  • You hear hissing coming from your amp that seems unusually prevalent. Amps with less than five preamp tubes should be relatively quiet. Five or more tubes and some hiss may be normal. If your amp’s hiss wasn’t noticeable when you first bought it, it is an indication that one or more preamp tubes need to be replaced.
  • You hear ringing through the amp when hitting certain notes or chords or “ghost” notes. You could have a tube that is microphonic, meaning it is amplifying external signals from outside your amp. A good test is to lightly tap on a preamp tube with your fingernail or something made of plastic or wood like a chop stick or pencil (never metal) to see if you hear it through your amp. If you hear the tapping on one tube but not the others, replace the tube.
  • Your amp doesn’t seem as gain-y (distorted) as it did before. Modern tube amps get a lot of their distortion in the preamp section. If your amp sounds cleaner than it did before even with the gain turned up, you could have a preamp tube wearing out. Sometimes when preamp tubes wear out, they sound just the opposite – fizzy and weak. Try swapping them one at a time and see if the sound improves (let the amp cool down for a minute or two first.) Keep in mind that most amp preamp tubes feed the amplified guitar signal from one into the next – these are called gain stages. Usually the one farthest to the right is the phase inverter. It doesn’t amplify the signal to the degree the others do and may not wear out as quickly. I would test this one last and make sure that if you remove it, to return it to its proper socket because it is balanced internally to directly feed the signal to the power section. If you replace it, make sure that tube is balanced as well.
  • The amp turns on but no sound comes out when you switch it out of Standby (tube amps.) Several possibilities exist that can cause this: It could be due to preamp tube failure. Each preamp tube is a link in a chain. If one of those links fails, the signal will stop right there. Other issues such as power tube failure could cause this problem as well. Another thing to look for if your amp turns on but there is no sound or the sound suddenly goes away while you are playing is red plating. In this situation there is a problem with your power tubes. They will glow red, orange or white but very brightly like a light bulb and you will get weak or no sound. Some authors say you can let the amp cool down and try it again to see if it red plates again but my experience is this problem does not go away. If you see this condition turn your amp off immediately and have it serviced. No sound could also be a sign that the output transformer (OT) has failed. Running your amp always requires that a load (usually speakers) be connected to the amp and the impedance setting of the amp matches the setting of your speaker(s.). If you ran your amplifier but forgot to plug it into a speaker cabinet before taking it out of Standby, you could have mistakenly blown the transformer. It isn’t necessarily your fault however, sometimes transformers just fail. Have the amp serviced.
• Power tube failure. When tubes fail or start to fail they can cause various problems. Power tubes (the larger ones in your amp) gradually wear down over time (as do preamp tubes but less often.) If you run your amp at high volumes or fully cranked with an attenuator this can shorten tube life. Tubes can short out, meaning instead of current flowing in the designed path, it seeks a shorter path to follow through the tube. This can cause tubes to fail or otherwise damage the amp, another cause of blown fuses. Sometimes when a power tube fails it will turn milky-white in the upper portion of the tube. These failures are easy to spot – others are not so easy. Electronic tube testers can check for shorts and other problems but this will need to be done by a tech. Sometimes tube sockets develop cracks in them, especially if the amp is played hard at high volume for extended periods of time. Age can be a factor as well. When you replace the tubes and you are still blowing fuses it could be that one or more tube mounting sockets need to be replaced. Often when an amp is serviced the tech will clean the sockets and realign the pins in them to make sure they are in working condition. One way to check to see if your tube sockets are causing a problem is to play through the amp with the lights off in the room. Facing the back of the amp where you can see the power tubes, play a power chord or note with a quick, abrupt sound to it. If you see ANY sparks or arcing when hitting the notes, shut down the amp immediately and have the amp serviced. Another hint that your tubes may be reaching the end of their life is if they glow very dimly when you turn the amp on (in Standby.) New tubes will glow at their base with either a whitish or red/orange color when newer. As they age the glow becomes diminished; sometimes becoming hard to see at all when your room lights are on. This can be a good indication that the tubes are wearing out so if you’re buying a used amp you may be spending money on new tubes sooner rather than later.
• A loud humming. In most amps, a small degree of hum is OK. Transformers hum, just like the ones in your Lionel train set, if you remember those. If you turn an amp on into Standby and you hear a tiny bit of hum that is normal. If however, you take it out of Standby and the hum is louder and coming through your speakers, this is a problem. It could be caused by:
  • Leaking filter capacitors. These are the big, cylindrical, can-looking things that usually are sticking out from the top of the amp chassis (or bottom if a combo.) They are usually blue or black, sometimes green or silver. Their job is to filter the incoming AC voltage and remove any 60-cycle hum while converting it to a smooth DC voltage that the amp uses in its circuitry. Over time, these capacitors start to lose their filtering ability and so hum may not get filtered out. When this happens, you will hear humming through the amp that won’t go away. If your amp has a Preamp Out jack you can try feeding the signal out of there into the input jack of another amp and if you hear it through the other amp you know the hum is coming before the power section. The rule of thumb is generally if your amp is ten years old or more, the filter caps should be replaced, especially if the amp sits unused for long periods of time. Most caps are date coded. Usually there is a four-digit code separated by a hyphen (-) showing the last two digits of the year and the month the cap was manufactured. Example 95-06 would be June of 1995. If your filter caps are more than ten years old, have them replaced.
  • Improper grounding. When you have a loose ground wire or a bad ground from a failing component it can introduce hum or noise into the circuit. I have also seen potentiometers (pots) such as the master volume or preamp volume pot develop poor contact and introduce hum into the system. It usually happens on the most used pots on the amp and those two get adjusted more than the others, by far. If cleaning the pot with a spray such as Deoxit™ or equivalent lessens the hum then have it replaced. If you want to check for a loose ground wire be very careful. Do this with the amp turned off and don’t touch anything with your fingers. Use a pencil or a chop stick and see if any wires that connect to the amp chassis appear loose or broken off. Ones that use locknuts can come loose if they were loosened previously due to thread locking fluid being worked off during servicing. If you attempt to tighten any, always unplug the amp and only use one hand when working on the amp and remove any rings or jewelry. Keep the other hand in your pocket. Voltage may still be present in the amp even when unplugged and touching the chassis with both hands could cause current to flow through your body instead of the circuitry so NEVER USE TWO HANDS WHEN WORKING INSIDE AN AMPLIFIER!
• Improper tube bias. If you replaced the power tubes with a different brand or type and it just doesn’t sound “right”, you may want to have the amp checked out for the proper tube bias. Without getting too technical, tube bias controls how much current (and signal) flow through the power tubes. It should be checked any time you replace the power tubes. Some amp makers (EVH, Marshall and others) put test ports on the back of the amp on some models so you can check the bias voltage yourself with a voltmeter. Consult your amp maker for the proper bias setting for you amp. This is as far as I would go with checking bias voltage yourself. Other methods involve measuring bias current and require specialized equipment and is best left to a service tech.
• Scratchy volume controls. If you turn the EQ (Bass, Middle, Treble) or the volume pots and hear a scratchy sound or the volume seems to cut out when the knob is set at a certain position, the pots need to be cleaned. This is done with the chassis removed from the amp and a spray such as Deoxit™ is sprayed on the wiper of the pot. This removes oxidation on the contacts and should remove the scratchiness. The knobs will turn easier as well. I have seen some Marshalls from the eighties that had volume pots that were “all or nothing” when adjusting the volume. It was either very low volume or blasting your ears off when you turned the knob. In this case they may have been in short supply of quality components and used one that wasn’t up to snuff. Replacing it fixed the issue.
  • No sound when using the effects loop. If you encounter this, try taking the effects loop out of the circuit. On amps with a switch for the effects loop, turn it off and disconnect your effects coming in/out of the loop. Sometimes the contacts for the input and output jacks of the effects loop become dirty or corroded. You can try cleaning them with Deoxit™ or equivalent and see if that fixes the problem. Sometimes the jacks have to be replaced.
  • A buzzing sound at any volume. This may actually be a blown speaker rather than an amp problem. When speakers fail is it usually due to excessive volume or playing through speakers with a lower power rating than that of the amp. It will sound fizzy at any volume and worse as you increase the volume. If you’ve ever blown out the stereo speakers in your home, this will sound familiar. If you have a 100-watt amp and you play through a speaker rated at 25-75 watts at full volume, it is only a matter of time before the speaker fails. Amp makers solved this by connecting multiple speakers to spread the load. You still want to make sure the combined rating of all your speakers either matches or exceeds the power rating of your amp. A speaker’s power rating is given in watts but it is misleading. The speaker is most often advertised at its continuous power rating (“RMS”.) Speakers are designed to take some punishment at peak power (their absolute limit) but not for very long. If you are overpowering your speakers by constantly playing at maximum volume they could soon fail. Buzzing may also be caused by a loose ground, faulty pots as stated above or even a bad guitar cable. If you use a pedal board or chain of effects using patch cables, they are known to cause problems. I have seen brand new patch cables that add noise to the signal. Get good quality cords and cables. They may cost you more initially but save you time, money and aggravation in the long term.

Hopefully this article will guide you in some basic troubleshooting of your amplifier. If you’ve run into an unusual problem and were able to fix it, please tell us what you did to fix it, we’re all ears!

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