byPhil O'Keefe06-27-201304:50 AM - edited 06-26-201310:38 PM
Amp sounding weak? Is it making funny noises? Maybe it's time for new tubes
By Phil O'Keefe
It happens gradually over time as the amp is used, but the sound of even the best tube eventually deteriorates. The next time you're struggling with your tone and wondering why it's just not quite "right" anymore, or why it doesn't seem as cool as you remember from days gone by, ask yourself this - when was the last time I changed my tubes?
THREE TYPES OF TUBES
There are three main types of tubes. Not all "tube" amps use all three types; often, solid state components are substituted for one or two of them, depending on the amplifier design. True tube amps generally have both tube preamp and tube power amp sections, and may have a tube power rectifier, or a solid state rectifier. Hybrid amplifiers utilize a tube or tubes in either the preamp or power amp section, with the other section of the amp being solid state. The vast majority of hybrid amps use a solid state rectifier instead of a tube. Let's take a closer look at each tube type, and some of the symptoms to watch out for.
These are found in the vast majority of tube amps. Even the majority of "hybrid" amps use a preamp tube, although occasionally you'll find a hybrid amp that flips that paradigm and uses a solid state preamp and tubes in the output or power amp section of the amplifier. Examples include some of the old Music Man amplifiers from the 1970s and early 1980s, as well as the Fender Super Champ X2.
Preamp tubes are often used for other functions within the amp, such as for the tremolo (mislabeled as "vibrato" on many Fender amps), and sometimes for the reverb driver and recovery circuits too. If the reverb or tremolo on your Fender amp is dead, the repair might be as simple as a quick tube replacement. Occasionally a preamp tube will be used as a phase splitter (often called a phase inverter), which splits the signal into two, with one positive and one negative polarity to feed the two halves of a push/pull (Class AB) tube power amp.
Some common preamp tubes include the 12AX7 / ECC83, 7025, EF86 / 6267, 12AY7 / 6072A, 12AU7, 12AT7 and 5751. A few examples are shown below. (Fig. 1) Preamp tubes are occasionally covered with metal shields. They help protect the tubes and provide electronic shielding that helps reduce noise, so if your amp is so equipped, make sure you replace the shields after you test or replace the tubes.
Figure 1: A few examples of preamp tubes, including the 12AX7, 6072 / 12AY7, and EF86
These tubes are generally larger than preamp tubes; taller and (with the exception of the EL84) fatter in diameter than their preamp tube cousins. They can be found in the power amp section of the amplifier, and along with the output transformer, they provide the final amplification "oomph" that drives your speakers.
Power amp tubes wear gradually. Unlike preamp tubes, if you've been using the same set of power amp tubes for quite a while, you could very easily notice a dramatic improvement in tone by replacing them - especially if you've been driving the amp hard on a regular basis. Unlike preamp tubes, which can usually be replaced without having to consult a tech, some amps need to be biased after replacing the power amp tubes for best results. When in doubt, check your amp's manual, or ask on the Harmony Central forums.
Common power tubes include the 6L6, 6V6, 6550, EL34, and EL84. (Fig. 2) Unlike preamp tubes, they are almost never covered with metal shields, but they may have retainers to help hold them in place that you may need to remove before taking the tube out of its socket.
Figure 2: Common power amp (or output) tubes include the EL84, 6V6, 6550, and EL34
This tube converts the incoming AC mains power from the wall outlet into the DC current that the amp needs to run. Without a rectifier, the amp won't even turn on. No sound, no pilot light - nada. If your tube amp fails to power up, check the fuse and the rectifier tube, if it has one. In most cases, one or the other is blown and needs replacement. The rectifier is a large tube, similar in size to many power amp tubes. When looking at the back of the amp, if it has a rectifier tube, it is generally on the far left hand side of the amp, right next to the power amp tubes.
A tube rectifier is more commonly found on lower-wattage amps (under 50W), while high-power amps tend to use a solid state rectifier instead of a tube. A tube rectifier doesn't make as big a difference to the sound of the amp as much as it does to the feel of it. With a tube rectifier, note attacks can "sag" a bit, and they don't punch out as immediately or as forcefully as you'll normally get with a solid state rectifier - the attack transients of notes are a bit more compressed, especially when you really dig in and hit a note forcefully, and when the amp is being run loud and hard.
As long as the amp powers up, the rectifier tube is doing its job, and normally there is no real benefit to be gained from replacing it prior to it failing. Some common rectifier tubes include the 5U4 / GZ32, 5Y3 / GZ30, 6CA4 / EZ-81, and 5AR4 / GZ34. (Fig. 3) Since rectifier tubes generally run until they fail, it's normally not something you need to worry about, but if you gig or tour frequently, it wouldn't hurt to keep a spare on hand - just in case.
Figure 3: Common rectifier tubes include the 5AR4, 5Y3, and 6CA4
DIAGNOSING TUBE PROBLEMS
Rectifiers are easy. When they fail, it's usually pretty obvious, but what about preamp tubes and power amp tubes? Both can show signs of wear or have problems that can affect your tone even before they fail completely.
On preamp tubes, you can generally use them until they start to cause noise (such as crackling, hissing, or hum) or other audible issues, or barring that, until they fail completely. Swapping out a suspect tube for a known good tube of the same type is a time-honored way of troubleshooting preamp tube issues. Just make sure you power off the amp and unplug it before changing any tubes, and remember that tubes get HOT! Always wait until the amp has cooled, or use a oven mitt to grasp the tubes with. Another way to test preamp tubes is to power up the amp, turn it up to a moderate level, and then gently tap on the preamp tubes, one at a time, with the eraser end of a pencil. If the tube is microphonic - if it makes a hollow sound that you can hear through the amp's speaker, or if it pings, or makes any kind of objectionable noise, it's probably time to replace it. Individual preamp tubes can be replaced one by one - there's usually no need to replace them all at once.
Like tires on your car or strings on your guitar, power amp tubes start wearing the moment you put them in your amp and fire it up. The more you use the amp, and the harder you push it, the less time you can expect them to last. How long will they last? It's impossible to say. I've had old vintage amps that still sounded fine, even though they had the factory-installed original tubes in them. I've had other amps that needed to have the power amp tubes replaced after a year of heavy use. If your amp seems listless, lacks punch or feels flabby or weak in the bass, then it's probably time for a fresh set of power amp tubes. Since they wear out gradually over time, you might not notice that their sound has deteriorated, but when you replace a worn set of power amp tubes with a fresh set, the dramatic difference in tone can often be easily heard.
THE BEST INSURANCE
If you play a lot, you might want to consider buying a full replacement set for your amp. Test them when you first get them to make sure they're all working properly, then store them somewhere safe. A padded tube carrier / case is a good investment, especially if you tour. If you do gig, make sure you bring those spare tubes along with you. And since it's not at all uncommon for the fuse to blow when a tube fails, make sure you also carry a few spares of the ones used in your amp - you never know when they might save the day!
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazine