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Amp Repair

Amp Tubes





Frequently I am asked, “Just what IS an amp rebuild?”  There is no short and quick answer, but maybe I can make things a little clearer by explaining how an amp works.


The output of the cartridge in an average juke is a very small voltage. It can barely be measured by a standard meter. To make enough sound to fill a room it must be boosted. This, basically, is what an amp does. It takes the very small voltage from the cartridge and increases it by a large amount, then turns it into an electrical signal that a loudspeaker can use to make sound waves.


The first stage is usually called the pre-amplifier. This is a very important stage; it must boost the signal while keeping the signal pure. It can't generate any noise of its own, or the noise will be amplified also. Whether tube or transistor, low noise circuitry is the key here.


The signal then goes to another stage, being boosted again, and is fed into the tone shaping circuits. These are the bass and treble controls. The circuits are designed to increase or decrease a particular frequency range by adjusting rotary controls or switches. This gives a person the ability to shape the sound to suit themselves. Some people are more interested in the upper frequencies, some like a lot of bass. For the most part, if you set the controls in the center of their range, this is what is coming off the record. Most people adjust them to their own personal preferences. This may or may not be what was originally recorded on the record.


The signal then goes to the driver stage. Here the signal is prepared to feed the output stage. In a tube-type amp, this is sometimes called the phase inverter, some transistor amps are driven the same way. There will be at least 2 output tubes or transistors per channel, they must be fed 180 degrees out or they will cancel each other out and no sound will be developed.


Most tube amps have a transformer coupling the output tubes to the speaker. These must be wound with fairly heavy wire and have a large amount of iron in the core to properly conduct the signal without distorting it. Transistor amps usually are capacitor coupled because of the lower voltages required to generate the Output.


So there we have it, a basic outline of what an amp does. Now on to the next part: what happens when an amp doesn't do what it's supposed to do? Everyone by now has heard of capacitors and resistors. To the average person I suspect this means nothing, but here's how they work:


Depending on the type of capacitor, it can do several different things. A tube amp runs on very high voltages, somewhere in the 400 volt range. In the power supply, there is a transformer that takes the 120 volts from the line and boosts it to around 425 volts. This is called a step-up transformer. A step-down transformer would take the 120 volts and lower it to whatever voltage was needed. Transformers are very versatile, they can do both at the same time. So we can get the high voltage necessary to run the tubes, and get the lower voltages necessary to heat up the tubes, usually 6 volts. Also, there is a 5 volt line to heat the rectifier tube, such as a 5U4.


Okay, the high voltage is fed into the rectifier tube, along with 5 volts for the filament and we get DC voltage out the other side. But it's not pure DC. We have to filter it and this is where electrolytic capacitors come into play. These are the large can capacitors mounted somewhere on the amp. They smooth out the pulsating DC voltage and make it usable for the tubes. When one or more fail, you get hum. This is the low background hum an amp makes and is not affected by volume. As the capacitors get worse, the hum gets louder.


Remember that the tubes operate on high voltage. This voltage must be controlled and kept in certain places. Now, how can we pass a very small audio signal past 400 volts? The larger voltage would erase the smaller voltage. Here's a coupling capacitor's job. It blocks the large DC voltage, and permits the audio signal to pass! Quite a trick if you think about it. It's holding back 400 volts, and letting millivolts pass. A millivolt is one one/thousandth of a volt. The larger voltage is gargantuan in comparison.


Capacitors fail with age, either short internally, open altogether, or start leaking. They can be affected by temperature or humidity. They can start letting some of the high voltage pass, which wipes out the smaller signal and shuts down the next stage. The number one complaint I hear is that an amp sounds fine for the first few records, and then starts fading out. Now you know why. The capacitors are not doing their job.


Let's not leave out resistors. Their function is to reduce voltage. If you have 300 volts coming in from the power supply, and you only need 100, you use a resistor to drop the voltage. But here's another little problem: sometimes they work too well and drop all the voltage. Or they fail partially and cause the tube to lose efficiency.


There is another problem with resistors, they can get noisy. This shows up as a crackling, popping sound. Sometimes it sounds like water running. There are other things that can cause the same indication, such as a tube or transistor failing, but generally resistors are the culprit. They can be temperature sensitive, too, working when cold and failing when warm.


An amp can be affected by a number of things, most of which are listed above, but the transformers can fail also. When they short, they can shut down the amp, sometimes blow fuses, or the sound quality may be drastically affected. When an output transformer fails, there will be low volume, distortion, loss of frequency response, or any of several other problems.


Output transformer problems can be difficult to diagnose. Don't assume that because you have low volume or distortion that the transformer is bad. Conversely, just because you have plenty of volume, don't assume that it's good. There might be a loss of high or low frequencies, but no other symptom.


When rebuilding an amp, the first thing to be checked is the transformers. If one or more is bad, then don't go any further until you find a good one. No amount of circuitry repair is going to correct the problem. If they are good, then proceed with changing ALL the capacitors, including and most importantly, the filters in the power supply. Then check all resistors with a good ohmmeter to make sure they are within tolerance, generally +/- 10%.


Check all solder joints. Recheck all solder joints. And here is the most important thing: KEEP THE CAPACITOR LEADS AS SHORT AS POSSIBLE. I've seen amps that looked like a forest, all the caps were standing up on their leads. This is a definite no-no. They can sag and short very easily.


Do not hang a "nest" of filter caps underneath the chassis. Mount terminal strips and attach the filters to them, then run the leads as needed. This is very critical because there is a lot of voltage on them. It's very dangerous to just let them hang in thin air with no support. You can lose an expensive power transformer this way!


Next, clean all the controls with a non-residue cleaner. Stay away from TV tuner/control spray. This can cause more problems than it cures. Check all your tubes and then "fire it up." If you've done what the amp needed, then it should work and sound great!