AMI Model AA jukebox monoblocks?

[gadget73]

thats just how the math works out based on the stuff in the datasheet. It will be slightly different in the amp since voltages and such aren't quite identical but it should be reasonably close. sqrt 17.5 * 5000 = 295.8


in reality the plate to plate power doesn't matter that much. Its the power to the speaker that matters. I only mentioned it because what you'll see in the datasheet for power output is not going to be what you can measure at the speaker.


split, inverted, and re-combined is pretty much exactly how it works. The tube ahead of the 6L6 stage is the phase inverter. This one is a paraphase type so it uses 2 triodes, but that doesn't really matter. One input, two outputs 180 degrees apart. In the earlier days, a transformer was actually used for this, but you can get it done in a number of ways.

Unbalanced input comes in at pin 1 grid of the 6SL7. Triodes invert phase just because of how they work, so a 180 inverted output appears at pin 2. That signal is fed to the grid of the upper 6L6, and back to the pin 4 grid of the 6SL7 through some resistors to knock the signal down. The output is inverted again so the signal ends up in-phase with the original input signal. That goes to the lower 6L6 grid so signals from grid to grid on the 6L6 are 180 out of phase. The output transformer essentially re-combines them to a single output.

 

[ruraldrew]

I'm having a hard time finding a replacement for the .0033 uf capacitor shown here. It's shown as polar so that means it has to be electrolytic, right? Do they even make electrolytics that small?

 

[gadget73]

not polar, its a paper cap. Sub it with a film cap.

 

[ruraldrew]

not polar, its a paper cap. Sub it with a film cap.

Isn't that the symbol for a polar cap? I thought non polar was two flat lines and polar was one flat and one curved?

I think it's this one?

 

[gadget73]

Old schematic markings were sometimes not consistent. If it doesn't have a + or - mark, its not polar. I wouldn't put too much stock in the curved plate thing. That looks like a typical plastic shelled paper cap. The line marks outer foil, not negative.

Either way, you can sub a film cap for an electrolytic. The only real penalty is size and cost. For values under 1uf there isn't much need for an electrolytic though. Pretty sure an 0.0033 600v 'lytic never existed anyway.

 

[ruraldrew]

Old schematic markings were sometimes not consistent. If it doesn't have a + or - mark, its not polar. I wouldn't put too much stock in the curved plate thing. That looks like a typical plastic shelled paper cap. The line marks outer foil, not negative.

Either way, you can sub a film cap for an electrolytic. The only real penalty is size and cost. For values under 1uf there isn't much need for an electrolytic though. Pretty sure an 0.0033 600v 'lytic never existed anyway.

Thanks. Are all these brown waxy looking capacitors going to be nonpolar, too, despite what the schematic says on some? The largest value on them is .1 uf.

 

[gadget73]

yeah those are non-polar paper caps.

Band indicates outside foil. Modern caps don't mark that, but if you have a scope or an amp with a decently sensitive input its easy to work out which is what. Arguments are endless whether it makes any real difference, but I do it just because its easy and isn't likely to hurt anything. Basically you hook the amp or scope to each end of the cap and hold it in the middle. Reverse the leads. Whichever config has either a bigger squiggle on the screen or a louder hum has the + lead connected to the outer foil. I just mark the ends with sharpie and put the new cap in the same way the old came out. That will normally go to whatever is the lowest impedance part of the circuit. Ground if its grounded, or plate if its between tubes. The idea is that it provides some shielding for whatever the more sensitive part of the circuit is.

 

[ruraldrew]

Got the caps and bias resistors ordered for both amps. Now I need to find some decent sounding 6L6s for hifi. I've only ever used them in guitar amps.

 

[gadget73]

they all work fine, this one can run any 6L6 available though. Metal, 6L6G, GA, GB, GC, 5881, etc.

 

[ruraldrew]

they all work fine, this one can run any 6L6 available though. Metal, 6L6G, GA, GB, GC, 5881, etc.

After some looking around I decided to try some new Tung Sol 7581a's.

 

[gadget73]

those should last a very long time with as gently as this will run them vs their max ratings.

 

[ruraldrew]

Other tube amps I've worked on had a bare heavy gauge wire grounded on both ends that ran the length of the chassis, and pretty much all the component grounds were to that wire. I don't seem to have that here, would there be a benefit to doing the grounds that way?

What about converting from the two prong to a three prong power cable?

 

[gadget73]

the better way is to ground it at one end only and use that as the buss. Thats more inclined to give you a quiet amp, but re-working an existing one that has a bunch of direct to chassis grounds is kind of a pain in the neck. Personally I wouldn't go re-designing it.

The 3 prong cable is another one of those endless debates. Sometimes it creates ground loop hum, sometimes not. It will make it safer if your outlets are proper though.