We are often asked how to determine when a tube is bad without access to a tube tester. You’ve probably been there. You’re doing sound check before a gig, something sounds wacky, and now you’ve got to figure out what’s happening. Well although not all tube problems can be found without a tester, many problems can be. We’ll go over some of the most common scenarios that a guitar player might find with his or her guitar amp. But keep in mind the same issues will arise in hifi stereo tube amplifiers and preamplifiers so audiophiles will find this article useful too.
We’ll start with visible signs. There are a couple very obvious things to look for. First, every tube will have a heater filament that, when working, will cast a nice warm orange glow. In some tubes it’s more intense than others, but this is not a problem. What matters is that it is glowing to some degree. There are some tubes that have their filaments well hidden making it nearly impossible to see. In these cases you can carefully check if the tube is hot or cold. Be careful not to burn your skin. The tube must be heated in order to function. If the filament has failed the tube is useless.
The second thing to look for is the condition of the getter. This is the greyish coating usually found at the top of a tube, but can be on the sides, or both top and sides, depending upon the tube type. Any color from grey/silver, to black is healthy. When a vacuum tube develops an air leak (a small crack or bad seal by a pin for example) this getter color will change to pure white. If you see this you know with 100% certainty that the tube is bad.
Third, look for a purple glow that is very focused around specific elements inside the tube. Do not confuse this with blue glow that is often cloudy and near the glass. The purple glow around wires or other elements indicates leakage and a tube with this should be discarded.
Perhaps the most obvious thing to look for is any loose parts that have broken off inside the bottle. You can gently shake the tube as well and listen for rattling. All tubes will have some degree of noise when you do this as the grid and screen wiring vibrate so don’t mistake that for broken connections.
We should also mention that when it comes to power/output tubes another issue can be red-plating. This is sometimes due to wrong biasing of the amp, which really isn’t a tube problem. But there are times when a properly biased amp has a tube that begins to red-plate. This is a sign that that specific tube is failing and in a “run-away” state were the current can’t be controlled by the bias voltage. A tube like this should be replaced. If not, the amp will eventually blow a fuse or worse damage other parts.
Amplifier Will not Power On
In the event your amplifier will not power on, it is almost certainly due to a blown fuse. Check and replace and blown fuse and try again. If the replacement fails immediately it’s often a sign of a failed power/output tube or amplifier fault. If the visual inspections mentioned earlier do not help locate a bad tube we recommend hiring a technician to inspect your amp.
Microphonics and Noise
Problems with preamp tubes are often due to microphonics and noise issues. A microphonic tube will ring and amplify any outside noises such as bumping the amp, tapping on the bottle, or even footsteps as you walk across the floor. All tubes will amplify tapping to some degree but an unusable tube will be very loud and often feedback or squeal. In a guitar amp with many preamp tubes it can be difficult to determine which one is microphonic. This is because tapping any tube near the bad tube appears to be bad as well. Rest assured it is very unlikely to find multiple bad tubes all at once. We suggest you tap each tube gently with a pencil or chopstick (something wooden or plastic, not conductive) and often the culprit will be louder or noisier than the others. Replace that tube and likely all will be quiet. In a home stereo amp or preamp these same steps can be followed. However one more trick that is useful is to swap a suspect tube to the same position in the other channel. If the noise follows to the other channel you know you’ve found the bad tube. If it doesn’t, you know the noise is caused by another tube and you can repeat this procedure one tube at a time until you find it. In a stereo amp you would use this same procedure to find tubes with any noise issues.
Tube noise such as sputtering, hissing, popping can be tougher to find in a guitar amplifier. If you have a spare tube of the same type it’s good to substitute it in place of one in your amp and then listen for the noise. If it’s gone you know you removed the bad tube. If not, re-install the original tube and move your spare to the next position. Repeat until the noise is gone and at that point you’ll know you’ve removed the noisy tube. Some guitar amps with more features can give you clues to where the issue is. For example an amp where the reverb is malfunctioning indicates the tube in that part of the circuit needs to be replaced.
Output tubes can also be microphonic. If you hear a rattle or ghost notes on certain lower notes you likely have a microphonic power tube. One way to confirm this is wear a glove to protect your skin from the heat and then gently hold the tube while you play the note that causing the rattle. Usually gentle pressure on the bottle is enough to stop the vibration and rattling, and will give you a clear answer as to which power tube is to blame. This problem will arise in guitar combo amps but is much less common in heads or hifi stereo amplifiers as vibrations are greatly reduced in these applications.
Loss of Power and other Odd Noises
Sometimes you will hear strange sounds, loss of power, or heavily distort sound. These are signs that a tube is failing. Often power loss that seems like the amp is performing at half power or less will be one or more bad power tubes, or even a dying phase inverter tube.
In a high gain guitar amp the clean channel may be distorted and overdrive channels will be extremely distorted and unusable. Use the isolation techniques mentioned earlier to find the bad tube.
Another symptom is when tone controls seem to have little effect and the sound frequency range has become very narrow. Bass and treble will be greatly reduced. This alone won’t indicate which tube in your amp is to blame but hopefully using the isolation steps we discussed help narrow it down.