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AMI Model AA jukebox monoblocks?

R

ruraldrew

New Member
I happen to have two AMI model AA jukebox amps. These would have been in the Model A "mother of plastic" jukebox, so each is single channel, and the jukebox would have played 78's. They need some rebuilding or recapping at least.

Since I have two of them, I was thinking about trying to set them up as monoblocks. I don't think these are rare enough that it's a big loss to jukebox collectors. Tube complement is: 6SJ7, 6SL7GT, 2x 6L6, 5U4G.

6SL7GT is listed as the phase inverter, so all the preamping must be happening in the 6SJ7?

I don't really expect them to be "hi-fi" per se, but thought they might be fun to keep in the garage or something. I'm guessing there was only one preamp stage here, rather than a separate phono stage before the preamp? So maybe the best route is to just remove the 6sj7 stage so I can run these as power amps with a separate preamp. That should also remove any 78rpm eq curve stuff built in to the preamp. Thoughts?
 
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got a schematic? That would make all of this a whole lot easier. Fair guess that you can feed signal into the 6SL7 and make it a power amp, but no guarantees.
 
gadget73 said:
got a schematic? That would make all of this a whole lot easier. Fair guess that you can feed signal into the 6SL7 and make it a power amp, but no guarantees.
Click to expand...

afC9V4z_d.jpg


Might get better resolution out of the link:
http://www.verntisdale.com/schem/ami-a.jpg
 
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http://www.verntisdale.com/schem/ami-a.jpg

I would rebuild it "as is" and try it with your outboard preamp .
set the tone controls to "flat".

these amps typically used a crystal cartridge with 1 volt output which requires an input Z
of many megohms. notice the 6.8 meg resistor. You could jumper around that resistor.
 
604man said:
http://www.verntisdale.com/schem/ami-a.jpg

I would rebuild it "as is" and try it with your outboard preamp .
set the tone controls to "flat".

these amps typically used a crystal cartridge with 1 volt output which requires an input Z
of many megohms. notice the 6.8 meg resistor. You could jumper around that resistor.
Click to expand...

I guess I could try, with the expectation of probably turning the bass pot down a good bit. Don't circuits designed for 78s usually try to up the bass a lot?
 
ruraldrew said:
I guess I could try, with the expectation of probably turning the bass pot down a good bit. Don't circuits designed for 78s usually try to up the bass a lot?
Click to expand...

I always found un restored 78 RPM jukes weak in the bass dept, and I generally increase
the coupling caps values by a factor of 10, .05uf to .47uf. But rebuild it stock and give it a listen. try tacking larger values on the fly with clip leads.


I increase power supply filter caps to 220 uf and leave the cap coming off the 5u4 at 47 uf,
as to high a value will destroy the 5u4 .

these old amps can sound remarkable restored / upgraded , don not cut holes in the
chassis as this will reduce their collector value.
 
century tek said:
Where did you hear that? They use the same parameters as everything else. What you might be thinking about was probably before they implemented the standard RIAA curve.
Click to expand...

Their were all kinds of EQ curves for 78's including flat.....LOL!
 
Crystal cartridges don't need any EQ, since the cartridge itself tends to have an EQ curve built in. A juke amp is likely to boost upper bass, and be weak on low bass, to give it the warm boomy jukebox tone, but not go crazy with acoustic feedback and rumble. On my Seeburg amp from 1947, there are tone controls which you can use to select how much bass and treble boost your music gets.
 
If it was a ceramic cartridge, those are mostly self-equalizing. This doesn't appear to have any obvious phono EQ going on. You can probably feed in direct to the phono input, but I'd likely bypass all of that and feed input to pin 1 of the 6SL7. Ditch the volume controls, run a 470k from grid to ground and just connect your source to the grid. Maybe pop in a 0.01uf cap between the grid and your input connection if you like.

I would probably get rid of the relay setup. If I read that right, it puts higher voltage to the heaters until the output tubes begin to conduct, then it drops back to normal. Likely useful for fast warmup, but its not good for the life of the tubes.
 
fKQXiFe_d.jpg

ajTscc4_d.jpg


It looks like there are multiple taps for the "external speaker" on the output transformer? I see a 16, an 11, and what might be an 8? The output for the internal jukebox speaker doesn't seem to be labeled. If I want to run a typical 8 ohm speaker should I just connect to the tap labeled 8?
 
8 to ground, yes, looks like that lands on terminal #3 on the speaker connection panel. If I don't miss my guess, that should have had a metal strap on the middle terminal that could be moved to any of the other 3, and your speakers would connect to the external screws. Looks like the speaker screws on the amp won't do anything unless you jumper out the remote volume control plug, and have the proper jumper on the external speaker terminals.
 
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