SWAPPING SUBSTITUTE PARTS
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 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.