Tube amps w/ Geoff Farina

DISCLAIMER #1: DO NOT TRY ANYTHING BELOW UNLESS YOU HAVE READ MY PREVIOUS COLUMNS, HAVE A THOROUGH UNDERSTANDING OF ANALOG ELECTRONICS, USING MULTIMETERS, DISCHARGING FILTER CAPACITORS, AND GUITAR AMPLIFIER REPAIR IN GENERAL. TUBE GUITAR AMPLIFIERS CONTAIN LETHAL VOLTAGES THAT CAN KILL YOU, EVEN WHEN THEY'RE UNPLUGGED! IF YOU DO ATTEMPT THESE REPAIRS, YOU DO SO AT YOUR OWN RISK. WE ARE NOT RESPONSIBLE IF YOU FRY YOUR AMP OR YOURSELF!

DISCLAIMER #2: I'M NO EXPERT. I LEARNED EVERYTHING I KNOW FROM READING ABOUT AMPS, EXPERIMENTING, AND BLOWING THINGS UP. THE FOLLOWING INFORMATION IS NOT COMPREHENSIVE, AND IS SIMPLY WHAT HAS WORKED FOR ME OVER THE YEARS.

DISCLAIMER #2 : I'M NO EXPERT. I LEARNED EVERYTHING I KNOW FROM READING ABOUT AMPS, EXPERIMENTING, AND BLOWING THINGS UP. THE FOLLOWING INFORMATION IS NOT COMPREHENSIVE, AND IS SIMPLY WHAT HAS WORKED FOR ME OVER THE YEARS.

Great issues of Tape Op have been flying by without me, so I figure it's about time I chime in once again, this time with my own Rhode Island recipe of quick tube-amp fixins. Since I provided the first installments of my amplifier quackery to the pages of the fledgling Tape Op some years back, tube amp knowledge has proliferated thanks to the Internet, the analog re- revolution, and to operations like Torres and Kendrick that know the value of sharing knowledge. Unfortunately, it seems that common sense has not seen the same proliferation, and for that reason this article starts with very simple troubleshooting recommendations that may seem insultingly simply to some readers, such as checking your guitar and pedals, tubes, and mechanical components. Later on in this article, I'll explain the effective "click tests" ubiquitous among ham radio geeks since the early days of electron tubes, and I'll explain other advanced ways to get you and your guitar amp back on the right track. This article will help you use a logical approach to troubleshooting that will solve 90% of your amp problems in just a few minutes.

If you think have a problem with your guitar amp, the obvious first step is to make sure that the problem is with the amplifier and not with your pedals, guitar, or other miscellaneous junk that you might have plugged into your amp. The quickest way to do this is to take a cable that you know is good, plug it into the amp directly, turn the volume up, and touch the end of the cable. If you don't hear a nice strong click, you know your amp is the problem. This may sound obvious, but do it or you won't be the first guitar player to rip apart your amp at soundcheck only to find that your stompbox needed a new battery.

Now that you know your amp is in fact the problem, the next thing to consider is the frequency of tube amp problems. Having unabashedly broken my own tube amplifiers for close to 20 years now, my experience is that things go wrong in the following order:

1) tubes

2) old soldering and wiring, particularly my own

3) transducers and mechanical parts (speakers, jacks, RCA plugs, pots, reverb tanks, and anything else that moves)

4) electronic components: burnt resistors, dried-up caps, fried diodes, etc.

5) power supply components

6) output and power transformers The idea here is that you're probably wasting your time doing heavy math checking to see if your output transformer is shorted out before you've checked for more common failures. With this in mind, the logical starting point is to check your tubes.

Checking the Tubes

Generally speaking, thoroughly checking the tubes will solve your problem about half the time. A great way to do this is to simply replace each tube, one at a time, and listen each time to see if the problem goes away. For troubleshooting purposes, I always keep a couple of my old, worn tubes on hand. For my trusty '68 Twin, I carry a set of worn out 5881s, a 12AX7, and a 12AT7, and they come in handy all the time. Starting with the power tubes, I replace each tube one by one, each time turning the amp back on and listening for the problem. About half the time I find that the problem is solved before I'm finished with this process, and that I simply had a bad tube.

If you hear a ringing sound along with your guitar sound or when you're not even playing, don't even bother replacing the tubes one by one. You probably have a microphonic preamp tube. This is a preamp tube that plays its own little song right along with your guitar solo, and it's usually a preamp tube with a loose component that gets amplified along with the AC signal voltage from your guitar. The easiest way to find the tube is to unplug your guitar cable from the amp, turn the volume up, and gently tap each preamp tube. The tube that rings the loudest when you tap it is the one that you need to replace.

You'll probably find microphonic tubes more often with new tubes than with tubes that you've used for a while. Here's my little hypothesis of why this is the case:

1) Tube testers used by tube companies do not have the functionality to determine if a tube is microphonic, and the only way to tell is to put the tube in an amp and listen to it, a rare practice among companies that market tubes for guitar amplifiers.

2) Tubes rarely "become" microphonic. Whatever makes them microphonic seems to rarely occur with use, and seems to be a defect that occurs during the production of the tubes.

Another thing you should check for is tubes that are not seated properly in their sockets. Wiggle the tubes around and make sure that they're making good contact with the tube sockets. The socket pin-holders often loose their tension over time, and need to be gently bent back into place. To do this, turn the amp off, unplug it, and discharge the filter caps. Take a strong pin or needle and stick it into the holes where the tube pins go, but try and get the pin in between the plastic of the tube socket and the tensioned pieces of metal that make contact with the tube. Then, gently bend the metal toward the center of the hole. Also, spray Tuner Cleaner or other like-chemical [Blue Shower followed by CAIG PreservIT] into the holes to get rid of any corrosion.

If you're the lucky owner of an amp that hums incessantly or makes continuous noises without a guitar plugged in, there's a quick way to narrow down your problem while you're checking for bad tubes. But first, you need to know a bit about the layout of the amplifier: Generally speaking, your guitar amplifier is actually a series of little amplifiers, known as "stages". Each stage consists of a series of components that amplifies your guitar signal a little more before it reaches the output transformer and the speakers. Your amp's innards may look like a maze of components and wiring, but each one of these little amplification circuits only has a few resistors and capacitors associated with it. Fortunately, the components for each stage are usually in the same area as the tube associated with that stage. If you open up your amplifier and look at it from behind, the first circuits to the far right are tone-shaping circuits that adjust the frequencies and then increase the amplitude of the guitar signal and send it to the reverb stage, and the first pre-amp tube or two, depending on your amplifier, are also part of the tone-shaping stage. The reverb circuit, the next one to the right, adds some Pacific surf and then amplifies the wave a little more and sends it to the next amplification stage to the left, etc., etc. Marshalls, Mesas, and other more compressed-sounding amps might have five or more of these stages, each one slightly distorting the signal voltage and contributing to the amp's characteristic compressed-distortion sounds.

Back to reality. You're amp is howling away without you, and I promised you I'd show you how to narrow down the problem. A quick way to do this is to pull out the preamp tubes one by one, starting at the far left preamp tube, the first tube the guitar signal sees. What this does is disables the entire stage and all the components associated with that stage. This allows you to test each stage one by one until you disable the one responsible for the cacophony. For example, on a Fender Twin, the first tube you pull out is the Channel 1 preamp tube. If the sound doesn't go away, you can assume that some stage other than the Channel 1 preamp stage is making the noises, since the components that are associated with that tube are disabled without the tube in place. Replace it and move to the next tube, which is the Channel 2 Preamp tube. Keep moving towards the power tubes and when you pull out the tube that stops the noises, you've found the amplification stage that is causing the problem. In fact you've narrowed down the problem from the entire amplifier to the few components associated with that tube's amplification stage. (Or stages, to be precise. Remember from my past installments that most preamp tubes like 12AX7s and 12AT7s actually have a set of 2 cathodes, 2 plates, and 2 grids, allowing one tube to be used in two amplification stages.) After you find the faulty stage, 9 times out of 10 you can test the resistors around the tube with your trusty multimeter to see of they match their coded value, or you can check for DC voltages on the caps around that tube, and you've found your problem quickly and efficiently. (Also remember from my past articles that caps are not supposed to pass DC voltages!)

Checking for Bad Components and Soldering

(Note: It is at this point that you should write your suicide note if you don't know what you're doing. Please re-read DISCLAIMER #1 above.)

Still got problems? Time to look under the hood. Checking back at our list of frequent failures, the next thing to check for is bad soldering. While you're at it, you can give the components a visual once-over to see if they appear to be damaged or burnt. Often you'll see black soot around components that indicates arcing or that something simply blew up and should be replaced. If you're an impatient hack with a Radio Shack solder gun like I am, you'll find that bad solder joints are a common problem. The best way to check for bad soldering is to open the chassis, turn the amp on, and poke around. Taking all of the necessary precautions, turn the amp on and turn the volume up. Take a chopstick or other non-conductive probe and poke around at the solder joints and components. If you hear a sound when you tap something, you may have found your problem and it's an easy fix. While you're at it, check the pots. Turn all the pots up and down and see if they make noise. If they do, they may be arcing and may need to be cleaned [CAIG CaiLube MCL] or replaced.

Checking Transducers

A transducer is a fancy word for something that changes physical energy into electrical energy, or vice versa. There are two common transducers on your guitar amp. The first one is your reverb tank. It takes part of your guitar's signal and changes it into physical energy that vibrates a spring. This spring continues vibrating under its own inertia at similar frequencies and harmonics of the original signal, and these continued vibrations are in turn changed back into an electrical signal and added back to the AC signal voltage, producing the surf sounds we all know and love. The second kind of transducer your amp uses is the speaker. The speaker takes your guitar signal, after it has been altered by the tone shaping circuits, extended by the reverb circuit, and amplified by the amplification circuits, and changes it into physical energy that moves your speakers that moves air at frequencies that are analogous to the frequency of vibration of your guitar strings.

As pan-disciplinary gear-geek acumen dictates, things that move are the first to die. Anything that moves creates friction and heat, causing all kinds of stress and strain and other problems. The lesson to learn here is, until the super-cooled quantum-plasma stacks hit the market, the speakers and reverb are going to cause us woes. The simple way to check your reverb is to turn it completely down, or to physically disconnect the tank from the amp. Noises gone? Problem solved. Checking the speakers is a bit more tricky because you should listen to them one at a time. This may seem unnecessary, but in reality it's often very difficult to hear if one speaker is not functioning properly in a multi-speaker setup, especially if you're not expecting that to be the problem. The difficulty in listening to speakers one by one is that you can't simply disconnect all-but-one of the speakers and expect the amp to work properly. Without going into too much detail on this subject, although most tube amps can tolerate an impedance mismatch of up to 100%, it's not always smart to simply randomly disconnect speakers. Also, such an impedance mismatch may radically change the tonal characteristics of the speaker and the amp, leading you to believe it's broken when it may be fine, or that it's fine when it may be broken. Fortunately, with some amps there are ways around this problem. If you have a Twin or Twin-like beast, a quick way to test your speakers is to pull out the two outer (or inner) power tubes, and disconnect one of the speakers. This will make your amp run at half-power, but it will run fine, and you can test each speaker individually to see if it's working properly.(This is also a classic mod if you want the sound of your cranked Twin without the excessive volume.)

Everyone blows speakers, but another less-known but common problem is that if the amp gets smacked around in the back of the van, the speaker's pole pieces can shift and render your speaker a refrigerator magnet. If you test one of your speakers and it sounds weak and distorted, this may be the problem, and unlike a blown speaker that can usually be re-coned, the problem is irreversible. Also, sometimes miscellaneous grime finds its way into the speaker's voice coil, causing all kinds of problems. You needed to replace those Eminence paperweights with real speakers anyway, so now's your chance.

Using the "Click Test"

So you've checked your tubes, transducers, and components, and your amp is still not working right. Now it's time to dig in and find out exactly what part of your amplifier is not doing its job. Most of the tests above will rid you of unwanted sounds, but what if your amp simply isn't making any sounds at all, or is sounding weak? A down-and-dirty way to do solve this problem is to use what the old ham radio operators called a "click test". This is simply a test that checks each stage, one by one, to see if it is amplifying a signal. Instead of using a signal from your guitar, you simply use the tiny signal produced when you disrupt the components of a tube with the probe from your multimeter. This is much like the test we performed above by pulling out each tube one by one, only this time we'll start at the other end of the amplifier with the phase inverter tube. This is because we're now looking for the stage that's NOT amplifying, rather than the one that is making noise. For this reason, we want to start "down stream" so that we'll know that everything between the stage we're testing and the speaker is working properly.

You should also have a schematic of your amp so that you can set up your multimeter to measure the voltages around the tube, and so that you know what voltages to expect and don't blow the meter's fuse. Set up your multimeter to measure the voltages on the plates and grids of the tube, taking all the necessary precautions. Start at the phase inverter tube, the tube closest to the power tubes. If you touch the plates (pins 1 and 6 of a 12AX7 or 12AT7) and grids (pins 2 and 7)of the driver tube and hear a nice fat click, you'll know that everything past that amplification stage is live and working, simply because it's amplifying the signal of your probe. Go to the next preamp tube touch the grid with the probe. When you get to the tube that doesn't click, you'll know that stage is the weak link that's not amplifying the signal you produce by touching the grid. You've just taken an amplifier with hundreds of components and narrowed down the problem to a tube and a few components! Now check for shorted resistors around the tube using your multimeter by seeing if any of them show insanely high or low resistance values, and check for DC voltages on the downstream side of your capacitors, and you'll probably find something that's not right and you'll know exactly what to replace.

You can also use the same procedure to track down unwanted hum and other noises, assuming the methods above failed you. Simply check for AC signal voltages on the grids of the tubes, again starting upstream at the first preamp stages of the amplifier. The first tube that shows AC signal voltages on the grids will indicate the problem area. Poke around the tube with your multi meter, and you're good to go.

Checking Voltages

Still no luck? My guess is that if you've come this far and haven't fixed your amp, you're at least having fun and learning something in the meantime. With that in mind, and seeing as your already sitting there with your meter and schematic scratching your head, a nice way to really get your hands dirty is to check your amp against the schematic. Also, there are great layout diagrams for most Fender amps that also show voltages, so see if you can get your hands on one of those as well. Take some time and check the voltages and the values of the components of your amplifier and see how closely they match up to the schematic. (Keep in mind that the old-timers only had around 110 VAC or less coming out of their walls, as opposed to our 117-120 VAC. Voltages in your amp may fluctuate by more than 10% for this reason.) Of course checking each and every voltage in your amp isn't a time-saving troubleshooting approach, but it's a great experience to see how your amp is more than the ebb and flow of electricity that Physics 101 makes it out to be. Instead, it's more of a complex situation with tenuously balanced components, each one affecting all the others, but that's another story all together...

If you notice some voltages aren't right but you just can't pinpoint the problem, you may have a more complex problem like a bad output or power transformer, or other power supply problems. You also may have something like a shorted cathode resistor that is hard to detect because cathodes can be grounded without necessarily throwing off other parts of the amp. A future installment we'll cover some more advanced troubleshooting techniques that will help us find these more difficult problems. We'll cover resistance and capacitance tests, power supply tests using a Variac, and maybe even some things I've read about but never tried myself, like checking the output transformers and the gain of a tube.

In the meantime, feel free to send me any questions or comments at geoff@secretstars.com, but don't expect me to do any math. Thanks to everyone who emailed and wrote requesting more installments.