WHAT IS AMP REBUILDING
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
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
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
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
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
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!