Ampeg ST shematic

[fattimmy]

I picked up a late sixties ampeg solid state amp and am looking for a schematic so I can bring it back to life. It's an ST, not SVT. these were sold as ST-22L Gladiator, ST-25L Olympian, or ST-42L Colossus. All three models were the same head with different speaker cabs so a schematic for any one of those models would work. I appreciate any help in locating a schematic.

[Stevem]

Let me guess, it's blowing fuses right!

[teemuk]

This should do it.

[fattimmy]

Teemuk, thanks a bunch for the schematic. Exactly what I needed.

Steven, It's not blowing fuses. Bought it at an estate sale. Works but sounds like crap. I knew it needed work when I bought it but it was cheap and I'm comfortable doing the repairs. However, from your reply it sounds like fuses blowing is a common issue with these amps. Is it a design flaw? I haven't gotten into this amp yet. Wanted a schematic first.

[gui_tarzan]

Ewwww... silicon!!!

Just kidding, I have two old Kustom amps in my shop right now.

[fattimmy]

With a good surgeon silicone can look and feel just like the real thing

[Leo_Gnardo]

With a good surgeon silicone can look and feel just like the real thing

Good one! What about germanium? Das ist mir doch alles Wurst!

I thing what stevem is getting at, lots of old SS amps are on last legs if they didn't long ago blow all the smoke out of their wires. Fender had an awesomely awful series late 60's, and many other companies did monkey see monkey doo. OTOH some folks do like their old Kustom amps, the big Road bass amp, Peaveys & Acoustics etc. Some were good, some were crap. And there's always that over-enthusastic crustomer who thinks he found some rare and valuable "prize" at a garage sale. "You can fix it, you're a GENIUS!" An invitation to the next lower level of clinical depression. Whenever I tackle a transistor amp I know I'm in for hours and hours of time-wasting fun. Idiot's delight, that's what they call it.

If yours is more or less working, many of the same things that go wrong in older tube amps need looking after so you can get prime performance. Fresh filter caps, swap out leaky interstage caps, drifted/burnt resistors, clean up pots, switches, multipin connectors, fx loop jacks, yada yada yada and let's not forget the speakers too. Not many SS amp owners want to pay for all that because they know their amp doesn't have much $$ value should they decide to flip it on Ebay or Craigslist. Some decide to get the work done because they know their amp can sound very good, hang the expense & don't worry about resale value. I hope that's you.

The charm of tube amps is, they have to be simple. Not long into the transistor error, um, era design engineers discovered the endless coffee pot & started designing way too much stuff into an amp. Now you have some amps that are essentially computers with an amp chip, how many zillion transistors is that? Decaf, boys, deeeee-caf! Simple circuits yield clear tone, whether transistor or tube.

[fattimmy]

Leo, How right you are on so many points. I've seen alot of people, including myself, think they could pick up that sweet cheap non-working amp, fix and flip it. My biggest problem in this equation is I can never seem to let them go once I fix them. Repairing SS amps is absolutely a time sink and in no way as enjoyable as working on tube circuits. However, i have found a handful of SS amps that give me clean sounds that I really like. This one included. When it was first tried out (before purchase, it was presented at an estate sale of an instrument collector as working and was being played) the cleans were really good if you could ignore some hum and pops. I haven't even turned it on since buying it since I would prefer to go through it first and check all the things you mentioned. This isn't my first amp project by far or even my first solid state. Having said that I was quite impressed when I opened this up. My first thought was, could they have stuffed anything else in here. The parts count on this thing is quite impressive. I haven't sold an amp I bought yet and doubt I'll start with this one unless I dissapointed with it. I guess I'm just a glutton for punishment.

[JazzGuitarGimp]

I played one of those old Fenders all through high school. IIRC, it was the PFL2000 Power Speaker. Eight ten's with a SS dual 70 watt power amp built in. I got a guitar preamp head kit from southwest technical products corp to drive it. The Fender had a circuit breaker rather than a fuse, and when I played it loud, the breaker would pop. I used to have a friend sit behind the thing and keep his finger on that breaker... Fun times...

[Leo_Gnardo]

the cleans were really good if you could ignore some hum and pops. I haven't even turned it on since buying it since I would prefer to go through it first and check all the things you mentioned. This isn't my first amp project by far or even my first solid state. Having said that I was quite impressed when I opened this up.

That's all a good start. Some things to look for besides what I already mentioned: filter caps coming loose from the PC board (common in Peaveys and I'm sure others.) If the amp uses "can caps" the wires are oftensoldered to tabs bolted onto mating fittings atop the cap - the screws sometimes loosen. Of course look for signs of obvious cap failure, splits in the cover, goo leaking out etc. then replace 'em. Some brands had a reputation for poor connections to the circuit board: "Peavey disease" or "Acoustic syndrome" however any brand might have similar.

Loose hardware holding on input & output jacks, that can really wreak havoc. Story time. One fine day I was given a GK RB800 to fix. The circuit ground for the entire amp was made at one of the speaker outputs. The nut was loose, dodgy ground, every op amp blew its lid. A long time picking about 8 or 10 DIP op amps off the board, replacing them with sockets, and hoping I hadn't wasted my time. Lucky that's all that went wrong, after replacing all the op amps & tightening the speaker jack nut ground, it worked but what a long fezzle, yeesh!

So pry your prize open when you're in the mood, I think with your experience you'll be able to make it rock like it oughta. Let us know how it's going. For the curious, what amp is it?

[Stevem]

Omg, those first fender SS amp where some fun! Weather it be good or bad more of them would have lived to today if the CBS production managers would have told the assy workers on the floor to make the screws that hold the output transistors to the heat sink tight, and not just snugg!

Just with the names they gave some of those amps they where sure to be a flop!

[fattimmy]

Leo, are you in the hudson valley?

[Leo_Gnardo]

Leo, are you in the hudson valley?

Quite rightly so, yes indeed. Right down the street from Rip Van Winkle. Don't see him much, he sleeps a lot.

[fattimmy]

hey neighbor. I'm in Ulster county. Currently live in the gunks but house shopping and looking to move a little further north

[Leo_Gnardo]

hey neighbor. I'm in Ulster county. Currently live in the gunks but house shopping and looking to move a little further north

You got a lot of musically talented neighbors, I could list a couple dozen and leave out a hundred more.

And probably a lot more whose talents are annoying hence the planned move I guess.

I used to think Frost Valley would be a good place to go, Denning, Olivera etc but now it's become hipster heaven. Bozos who don't want any windmills in their neighborhood, they're so cool. They can afford all the 'lectric they want, right from the coal plant. But if I moved there my customers would complain even more about how far they have to drive. Dogpatch-on-Hudson is equally inconvenient to all travelers.