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cyclecamper 2nd January 2013 04:47 AM

New big amp project
This will be kond of a long-term project. I bought an old Sound City 120 chassis, missing tubes and preamp tube covers and knobs and a few broken pots. It uses 6 EL34 output tubes. It has really large Partridge brand transformers; I understand the transformers are worth about $400-$600. I got the chassis on ebay for 258.17 The shipping is going to change it from a great deal to an OK deal. I'm going to see where in Florida it is and whether I can get a friend to hold it until next time I'm in that area.

The chassis and faceplate are a bit rough, so I'd really like to strip it and have the chassis plated. The model has a reputation for noisy tone pots. They way it has an 'active' tone control system, but I have yet to see exactly what that means in the circuit. Sone 'tone gurus' take the chassis and keep the splitter and 6 EL34 output section intact, but change the preamp and tone circuits to a HIWATT circuit and upgrade the turret board, resistors, and caps. Done well, I could end up with the equivalent of a hand-built HIWATT with more power.

firechief 2nd January 2013 09:52 PM

Just found this schematic - Prowess Amplifiers - Misc - Schematics - Sound City 120
Looks like a fun project.

cyclecamper 2nd January 2013 11:28 PM

I've uncovered a bit more history too, teaching myself what I could probably have read if I could afford the right books. No wonder the HiWatt look so much like the Sound City. The original HI-WATT amps were Sound City amps that were rewired. There is so much in common mechanically...that's another reason such conversions are still popular today.

There are some lovely ideas in these amps IMHO. There's a very practical aluminum heat radiation reflector / insulator / convection diverter just behind the power tubes. Some also have a similar plate bolted onto the adjacent transformer to keep it cool. Those details, together with the way the brits build guitar amps with the chassis at the bottom, look good to me.

I'm very inclined to add a similar reflector behind the row of power tubes in the Fender Super Twin I converted from combo to head. And a similar aluminum reflector/insulator bolted onto the transformer too. It just may need some small changes to make sure it doesn't ever trap rising heat against the chassis with the way Fender builds their amps upside-down with the chassis at the top. If I had it to do again, I might even have buit the Fender head cabinet with and angled top and split the angle of the faceplate. That would probalby make a big difference in keeping the head cool. I might even consider some kind of flip-out legs at the back; fenders will all cool better with the back of the amp raised, and using the fender tilt-back legs (does anybody??) on a combo or with a head bolted down on those slide-out arms just makes the heat trap much worse!

Passinwind 2nd January 2013 11:50 PM


Originally Posted by cyclecamper (
using the fender tilt-back legs (does anybody??)

Absolutely, still see it quite often. Even more often I see them using tilt-back stands though. Too bad Fender didn't just use vertically oriented drivers to begin with...:cool:

cyclecamper 3rd January 2013 05:41 AM

Yes, the tilt-back legs point the speakers nicely, but they turn the head cabinet into a hot-air balloon.

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