B.B. King was once quoted as saying, "There ain't but two kinds of music - good and bad." I guess that something similar could be said about the sounds that come out of an amplifier. Ideally an amplifier will enhance and make louder the true tonal character of the guitar plugged into it. Unfortunately, there are times when you will get sounds that you never wanted, sounds that are not musical, not enjoyable and would typically be described as noise.

     If you've been following this last series of articles you will recall that we have been talking about ways to "clean up" an old tube amp and get rid of the unwanted noises it has developed and, at the same time, restore the sound quality back to its original state. The amp we've been working on has been a Fender Deluxe Reverb, although the same ideas can be applied to any tube amplifier. Before we get started, read this warning.

     The first thing we do before working on any amp, is to make the amp safe by draining the filter caps. The procedure to do this was discussed in our last article. It is equally important that we do it for this project as well, as we will be changing the plate loading resistors which are directly attached to the filter caps. Any leftover voltage stored in the filter caps could find its way to your body, a sure-fire way of ruining your day.

     Once the caps are drained, and the amp is safe to work on, give the amp a quick visual inspection. Look for any parts that have broken loose or are burnt. Obviously loose components will need to be re-soldered to the circuit board and burnt or damaged components will need to be replaced. Many times everything will look fine externally, but internally the component is burnt. When checking resistors for instance, I will give the leads on the ends of the resistors a tug with my pliers to see if it has been damaged internally and become brittle. Many times the resistor will just crumble or the lead will pull free from the body of the resistor. Be sure and give a thorough check of the plate loading resistors and the screen grid resistors as these have a higher rate of failure due to the higher voltage being applied to them.

     The plate loading resistors are attached to pins 1 and 6 of your preamp tubes (12AX7, 12AT7, 12AU7, 12AY7, etc.) with typical values being between 82k and 220k, the most common value being 100k. Fender makes it easy to find these resistors in most of their amps by arranging them in an inverted "V" shape with one leg of the "V" attached to pin 1 and the other to pin 6. Make sure you check these resistors over real well. I've noticed that as these resistors start to go bad, your amp will sometimes get intermittent crackling noises.

     You'll also want to pay close attention to the screen grid resistors which are soldered directly to the power tube sockets between pins 4 and 6. These resistors recieve a lot of voltage and excess heat from the power tubes. Typical value here would be 470 ohms. Make sure that when you are changing resistors in your amp that you replace it with components that are of the same value (470 ohms, 100k ohms, etc.) and with the same wattage rating (half-watt, one-watt, etc.). Most resistors in a Fender amp are half-watt. I have seen some Fenders with half-watt plate loading resistors, and others with one-watt resistors - half-watt will work just fine. Screen grid resistors should never be less than one-watt.

     The next component that has a high failure rate is the cathode bypass cap. These capacitors have one end attached to pins 3 or 8 of your preamp tubes, and the other end attached to ground, in parallel with a resistor. When these caps go bad your amp will lose power and bass response. Typical value in a Fender is 25 mfd. Keep in mind that these caps are electrolytic and have both positive and negative leads. The positive side is attached to the tube and the negative side goes to ground. Some Fender amps used double 25 mfd. caps that housed two separate caps in one container. There would be two positive leads and one shared negative lead. These caps aren't readily available anymore (and weren't that great anyway) and should be replaced with two separate 25 mfd. caps. As far as the voltage rating goes, 25 to 50 volts is fine.

     Next check the solder joints throughout the amp. A good solder joint should look somewhat shiny, a bad solder joint, also known as a cold solder joint, can have a dull, cloudy look to it. Add some fresh solder to the joint to make a good solid bond. Make sure whatever component is attached to the joint (resistor, capacitor, wire, etc.) is solidly in place.

     Lastly, there are some areas of the amp that are susceptible to corrosion due to humidity and other environmental factors, and should be cleaned occasionally. The components most prone for this are the pots (volume, treble, bass, speed, intensity, etc.) and the tube sockets. Pots that are in need of cleaning will usually develop a "scratchy" sound when you turn them. Get some tuner cleaner (not WD-40 which is a lubricant) and squirt it into the pot. Turn the pot from one extreme to the other a few times to work the cleaner into it. Do this for each pot and try not to use an excessive amount of cleaner. If the problem persists, you will need to replace the pot. Keep in mind that all pots are not the same as they have values on them just like resistors. Fender amps have pots on them that range from 10k to 3 meg. Make sure you replace it with the same value.

     You will also want to clean out the tube sockets, both power, preamp and, if applicable, the rectifier. Simply remove the tube, squirt some cleaner into the socket, and work the tube in and out of the socket a few times. Do this for each socket. You may notice that some of the sockets feel somewhat sloppy and do not hold the tubes very securely. This happens as the metal fatigues over a period of time and needs to be re-tensioned. Get a small object like a fine-tipped screwdriver or safety pin and insert it into the socket and bend the metal clip so that it will grab the pins of the tube more tightly. You should do this for each socket. If the socket does not seem to respond to this, you will need to replace the socket.