Friday, June 26, 2015

Rare Polytone 102

Ok, yeah I'm a tube amp guy but occasionally I come across an old Polytone that I say yes to. I'm not gonna lie to you, I'm not the first guy anyone should take their solid state amp to and 99.9% of solid state amps out there I will simply say 'no' to. Plus it's worthy of my ugly amplifier photo pile!



However these are just really well made amps with a simple enough circuit. Still a pain to get around in but not like a Line 6 or some other digital horror show.

The nice thing about these old Polytone amps is the power supply/amplifier is separate from the preamp. You can just pull the thing out and work it from there.



A few connectors and it pops right out. The worst thing about these Polytone amps is they are filled with fiberglass insulation for vibration dampening:



Digging out the power amp feels like laying insulation in the attic with my dad in the 70's. Remember Carter and the drive to make our homes more energy efficient? Good thing. Fiberglass feels icky on the fingers!

These amps have a fun preamp. They have the strangest tremolo in the history of tremolos. They can go below 40BPM. Not particularly useful unless you have some hipster noise project. I like it. What were they thinking back in the 70's?



This amp boast a single 15" speaker and 2x8" speakers wired through a crossover cap. Interesting design. Big, clean 120 watts of jazz here. And yes, they were designed with the jazz player in mind though I discourage plugging a chorus pedal into one just cause, well, these sound pretty great and like I heard in a music store once: (Jazz player) "Hey man, what's a good chorus pedal here?" (Sax player under his breath) "The broken one".

The owner of this fine beast also owns the Magnatone 2x12 on this page. He uses them together, not loud, but just for a big sound. That's an excellent combo.

So what was the problem with this amp? It had a few issues. Like I said I'm not the best with these solid state amps but I'm persistent. It hummed like a power station and didn't pass audio. First thought was filter caps. Changed them and hum went away but no audio still. About 5 minutes into that 'pssssst!' and the LM391 driver I.C. caught on fire. Had to order one and replace the socket too. Did that and voila, audio. Really, really terrible audio, but audio nonetheless. Checked the power supply, fixed a ground issue but still really poor audio. Logic tells me one side of the power amp isn't working. Sounds a lot like a push pull tube amp with one tube out, common problem with Fender Blues and Hot Rod Deville/ Deluxe amps (resistor failure!). So I pull the power amp again and inspect and my gut was right. One of the driver transistors had 2 broken legs from a previous repair. They were soldered but poorly soldered and came undone. Re-soldered and now it's loud and clean.

Best thing about a project like this is if I see another one soon I'll know what to do better next time. Plus I needed to order parts for this and now I have spares. This was a fun project. I'd work on another one!

I should add that I used to own a single 12" Polytone amp years ago. One of the best upright bass amps I've ever played. Nothing sounded quite right for that instrument in the mix, and it could throw the sound out there and spread it around nicely. My bassist likes to use the bow, long tones. This would heat that amp up quite a bit so twice in about 5 years I needed to re-solder the .3 ohm 10 watt resistors back in place! 2nd time I found a permanent solution but can't quite remember what that was now!

J

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