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Amp Repair

Amp Tubes





I got a call from a customer not long ago who told me he had picked up an AMI K. It worked okay, but didn't sound very good. Upon checking, he found a Seeburg amp in it! He wanted to know if I had the correct amp. Now here is a can of worms. AMi used a number of different amps starting with the J (1959) through the Continental 2 (1962). I sometimes think that they were caught off guard by Seeburg and Wurlitzer going to stereo jukes for 1959. All Seeburg's were stereo, but Wurlitzer made you pay extra to get the stereo setup.


I have seen AMi Js with the small gray mono amp used from the F up through the I. These were early production jukes, and I think they were just using up what was on hand. But the strangest thing I ever saw was a J with two of the gray amps mounted on the back door to make it a stereo box! There is no question that this was factory installed, the mounting frame had been in the box since new. No modifications had ever been made to it. Since each amp used a pair of 6L6 tubes, this gave the juke a lot of power; it could potentially have pushed over 60 watts. This is real wattage folks, not the fake watts that most solid state amps are rated. This juke probably could have shoved the speaker straight out the front grill.


The later production J used a smaller amp with a pair of 6973 tubes for each channel. Stereo was optional. On the mono machines, the pre-amp was mounted on the amp chassis. For stereo jukes, both output stages were mounted on the amp chassis, and there was a separate pre-amp chassis mounted beside the main one. They used basically the same setup through 1962.


The problem with their amps is the output transformers. They are somewhat smaller than they could have been, and they are prone to failure. Of course, poor (or no) maintenance through the years adds to this problem. A bad 6973 can cook a transformer in no time. When stereo first came out, I suppose the 6973 was a real boost. Here was a tube that could handle 12 watts, and was small enough to allow mounting 4 of them in about the same space as a pair of 6L6's. No one knew that they had a short life. One of the biggest problems is heater-cathode shorts. They were made by RCA initially and RCA has never been known for long-life tubes. They work great, but just don't last.


In an amp with grounded cathodes, this is not as big a problem. But they must be fed with a negative bias, somewhere between -25 and -35 volts. If this voltage approaches zero, the tube starts over conducting and fails very quickly. Sometimes it takes the output transformer with it. Many, many times I have gotten similar amps in for repair where someone changed all the capacitors, but failed to repair the bias power supply. Perhaps they didn't realize how important this is, but the amp shows up with fried output tubes, and sometimes a bad transformer.


Another topic I've touched on several times is the 6973 tubes. For all practical purposes, there aren't any left. The tube companies are grabbing up anything similar and re-labeling them. You're getting a tube that is rated as low as 8 watts instead of the original 12. Believe me, this can make a LARGE difference in how your amp sounds.


Another problem with the subs is that the internal connections are different. Remember this: no modifications are necessary to use subs in a Seeburg or Wurlitzer amp. To use subs in an AMi or Rock-Ola amp, you must connect a jumper wire between pins 1 and 8 on the sockets. Failing to do this will cause extremely low volume, and a tube life of only a few weeks!


Some Seeburg and Wurlitzer amps, and a very few Rock-Olas use 7199 tubes. These usually last a reasonable length of time, but there is no substitute for them. The Russians are making a tube they claim is a 7199, but it really isn't. They will introduce hum into your amp, noticeable at low volume. You will not be able to turn the hum up or down with the volume control, it will always be in the background. I've had limited success putting a small amount of DC on the filaments, but this doesn't work in every case. The bottom line is that it just isn't that great a tube. The domestically produced tubes that are left over are much better, but quite a bit more expensive. My suggestion is to bite the bullet and spend the extra money up front. They seem to last a long time, and once installed will give better performance with no hum.


Another Russian tube that can cause problems is the 6SN7. Here again, it's a hum problem. They seem to make a good 6L6 tube, though. I've used hundreds of them and have only found a couple of bad ones. The tubes to avoid are those produced in China. I've tried using them with poor results. Short life and harsh sound are what I've encountered.


I have noticed one thing over the years, about the time something gets to be a real problem, collectors just move on to something else. The late 50s and early 60s jukes that are so popular now are slowing up some. I don't know if it's price or availability, but there is a growing interest in the late 60s and 70s jukes. I suspect it's a little of both, coupled with a younger crowd getting into the hobby now.


The next big problem is going to be the IC's and transistors used in the 70s jukes. Most of them have been discontinued and this is not as simple a problem as changing an amp over to use a different tube or transformer. This problem may not be able to be solved. We're not talking fantasy here. Try having a discussion with some of the board techs over on the pinball side of the hobby. They've got their hands full with certain master display driver chips being discontinued. Tell me what good a pinball machine is when you can't read the score! They haven't taken the IC dilemma lying down. They're re-engineering and re-manufacturing several boards with modem-day components and keeping thousands of machines running. Jukers may soon find themselves in a similar situation. Time will tell.