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Amp Repair

Amp Tubes





Many times I'm asked how to locate certain problems. Intermittent conditions are the toughest to find and correct, oft-times so frustrating that they seem like a cruel joke concocted by Satan. Perhaps a nearby bottle of Holy Water couldn't hurt, but maybe some of these more practical suggestions will help you solve those reoccurring curses.


First, determine if the trouble shows up when the juke is hot or cold. It would be a waste of time to look for the problem if it hasn't started yet. Begin by making sure that all the tubes are ht. All tubes have a filament inside, in the center of the tube. Some tubes have more than one, and are called multi-section tubes. A good example of this type is a 12AX7. One filament can go out on one side and give trouble, and you might not notice it, since you can see a light in the tube.


In some amps, the 5U4 tube will not come on until the mech is running, so don't get sidetracked by that. Look carefully to see if all filaments in all sections of the tubes are ht. Just feeling a tube to see if it's hot is not good enough. Another problem might be a tube going out after the amp has been on for a while. A loose tube socket is one way this might happen, corrosion on the pins is an- other. The amp would fade in and out under this condition. Push the tube you suspect from side to side or in a circular motion in the socket. If the trouble clears, then you know what's causing it.


On some amps, the filament voltage is supplied from outside the amp. There will be a cable, or a connector on the side of the amp that mates with the adjacent power supply. This is another place to lose voltage. If this is the case, all the filaments will go out at the same time. This is one of the easiest to find and correct. The same situation applies to the 5U4 tube if it is supplied with voltage from outside the amp. If just the 5U4 goes out, look for a similar fault, a loose connection or dirty contact.


The high voltage on most amps is turned off if the speaker plug is unplugged. This is to protect the output transformer. A crackling noise in the sound, or sudden loss of sound can be caused by the speaker plug coming unplugged or simple corrosion building up on the pins. The quick fix here is to polish the pins on the speaker plug.


On early stereo Seeburg's, you can lose one or both channels by dirty mute contacts on the mech. Sound fluctuation is another sign of the same thing. To determine if this is the cause, unplug the large 5-pin mute plug from the amp. If the problem goes away, then you know that the mute switches on the mech are dirty. Clean them with denatured alcohol, and allow to dry completely. Check the spacing of the contacts to make sure they open and close properly.


Dirty controls are another source of trouble. Volume, bass, treble, balance and gain controls can cause hard to find intermittent troubles. Cleaning all the controls at the same time is always a good idea. And this brings me to a point: NEVER, NEVER, NEVER spray TV tuner cleaner or WD-40TM on ANY control. First, WD-40 TM is kerosene. It is flammable, and leaves a greasy residue. TV tuner cleaner also leaves a residue. Either can attract dirt and make the problem worse over time. Use a nonresidue cleaner, such as might be used to remove flux from a printed circuit board. Computer-type cleaners are good. Just be sure that it is a non-residue type, and is non-flammable. The same holds true for the switches on mechs. This is very important!


If you are willing to spend an hour or so, chances are that you can locate the source of your intermittent sound. Tube sockets can be cleaned with a round file, such as a welding torch tip cleaner file. These are available in sets of various sizes from an auto parts store. They work well. Controls can be cleaned, tube and speaker plug pins can be polished, and bad solder joints can be located and resoldered with a little patience.


While you are working on your amp, look at all the electrolytic caps. One end will be sealed with a rubber or phenolic seal. This applies to all, including the can type. Look at the seal and make sure nothing is leaking out.

Sometimes you will see a black substance resembling tar leaking out. This is not good, and means the cap is failing. If you see a white liquid dripping out, or where it might have been leaking but has dried, this is cause for immediate action! Replace it now before it shorts and burns up the power supply. Electronic selection receivers have the same problem, if your juke has a Tormat system, check the receiver also while you're checking the amp.


Check all the coupling caps for cracks in the outer shell if they are plastic. Make sure they are sealed up all the way around, as well as the ends. Check resistors for discoloration or burned spots.


If you suspect that a tube might be bad, tap it with a plastic screwdriver handle. If you get a noise or crack- ling in the speaker, then it will need to be replaced. Sometimes you will get a ringing noise when you tap it; this is not necessarily a bad thing, unless it makes the noise while a record is playing.


It's impossible for me to cover every problem that an amp might have, the aforementioned situations are simply guidelines. Start with trying them, and if nothing helps, then it is time to go into the circuitry. Before diving into the amp, just be sure that the cartridge and needle in the tone arm are good, as well as the speaker. This is a good place for distortion and poor sound to develop. A bad cartridge or needle may sound good on some records, and poor on others. If an amp is bad, generally it will sound the same from record to record.


Several people have called in or emailed me lately asking which tubes are in short supply, and if they should stock up on certain types. Obviously, I can't tell which ones will be in short supply in 5 or so years, but generally, tubes such as 5U4, 6L6, 12AX7, etc. will be around from now on. The ones that may be hard to find are tubes such as OA2, 2050, etc. Some that are difficult to find now and are expensive are tubes such as 7868, 6973, 7199, and so on. The Russian and Chinese tubes do not work well as subs. I have developed a change-over kit to replace the 6973 tubes when the current supply runs out, and also a change-over for the 7199 on Wurlitzer amps. The construction of the Seeburg amps that use a 7199 does not lend itself well to alteration, so some- thing else may have to be done. The Russian 7199 tubes do not work well in Seeburg amps. Use an American made tube if you can, even though the cost is higher, it will last a long time.