Acoustic 270/370 Power Amp Section

[Al-vis]

Hi all,


I just purchased an old acoustic 270. Unfortunately the power amp section is missing. (Dont ask me why)



It is almost impossible to find one. Especially here in Europe (I'm from Germany)



Maybe one of you has an old and broken acoustic 270 or acoustic 370 (imo they have the same power amp) with a working power amp section and want to sell it.



Thanks so much.

 

[agedhorse]

Too bad, usually the results of a failed hack repair attempt. They are actually very easy to repair for a qualified service tech.

There is also a reverb tank in that chassis that’s missing it’s electronics. Is this by chance a bastardization of several models?

 

[Al-vis]

Not that I know. I bought it like that and I have no idea whats happend. But one thing is certain: there was no qualified service technician involved.

Where should the reverb tank electronics be? I thought it's there. (check the photo)



I forgot to write that this is a 240V export model. I don't know if there are maybe more differences beside just the transformer. But I think it shouldn't.

 

[Analogeezer]

There is an Acoustic club for owners and afficiandos of ACC Acoustic gear; you might post in that thread as there are a lot of guys with tons of information.

If anybody on TB can find you the parts or give you advice, it would be those guys.

Best of luck!

Analogeezer

p.s. If the rest of the amp is in good shape, especially the controls on the front panel you could part it out for people who need/want that stuff.

 

[agedhorse]

It's mounted to the inside of the back panel, looks like the connectors are missing or possibly they are located on the bottom (can't tell from the photo).

The green bridge rectifier is just floating there not mounted to anything. It should be mounted to the chassis bottom, next to the blue output coupling cap.
The power amp is very much a legacy quasi-complementary single ended design, fell out of favor about 40 years ago. If you are lucky, you may be able to find a boneyard amp that has the necessary parts. The 240 version has a different power transformer along with (possibly) different power switch wiring.

Here's what's supposed to be in there:

 

[Al-vis]

That's a 370, right?

Thank you guys for your help.

I try my luck in the ACC Club.

 

[Analogeezer]

It's mounted to the inside of the back panel, looks like the connectors are missing or possibly they are located on the bottom (can't tell from the photo).

The green bridge rectifier is just floating there not mounted to anything. It should be mounted to the chassis bottom, next to the blue output coupling cap.
The power amp is very much a legacy quasi-complementary single ended design, fell out of favor about 40 years ago. If you are lucky, you may be able to find a boneyard amp that has the necessary parts. The 240 version has a different power transformer along with (possibly) different power switch wiring.

Well that's because it's an amp from nearly 50 years ago, and the basic design is even older.

It's a bit like someone with an old classic car having issues being told "your car has a carburetor; those fell out of favor over 35 years ago." In other words not really helpful to add that bit.

Analogeezer

 

[agedhorse]

It’s helpful because the power supply and output coupling cap is unique to that topology, and because of this any replacement will need to share that topology.

This means that the later direct coupled Acoustic power amp modules can not be retrofitted, it has to be the like part.

Sometimes, helpful is contained within the context of the historical information I provide.

 

[byacey]

There isn't any + / - rails available off the existing power supply, just a single rail voltage. This is why the speaker output needs to be coupled through the smaller of the two big electrolytic soup cans that you see.

 

[byacey]

It’s helpful because the power supply and output coupling cap is unique to that topology, and because of this any replacement will need to share that topology.

This means that the later direct coupled Acoustic power amp modules can not be retrofitted, it has to be the like part.

Sometimes, helpful is contained within the context of the historical information I provide.

I think maybe a 450 / 470 power amp section could be shoe-horned in there, but those models use the rear chassis panel for the heatsink. It would be more work than what it's worth.

 

[agedhorse]

I think maybe a 450 / 470 power amp section could be shoe-horned in there, but those models use the rear chassis panel for the heatsink. It would be more work than what it's worth.

Yeah, it would be cheaper to buy another used amp.

 

[JacklegBass]

Where should the reverb tank electronics be? I thought it's there. (check the photo)

Yes, that's it, the reverb assembly PC board sits between the graphic EQ PC board and the power transformer. In the pic below you can see the original fish paper insulator between those two boards which may or may not still be there on yours.

That's a 370, right?

Easiest way to tell the 270 and 370 chassis apart is that the 370 has three screw-terminal capacitors while the 270 only has two.

I try my luck in the ACC Club.

We're here. I do have a non-functioning 270 with missing output devices, but I intend to fix it rather than parting it out.

 

[agedhorse]

Looks like the reverb connections are in fact on the bottom. Hard to see in the other photo.

 

[JacklegBass]

Looks like the reverb connections are in fact on the bottom. Hard to see in the other photo.

Correct. Two phono plug connections, one on each end of the tank bottom, then coax to the reverb assembly board.

I take entirely too many pictures when I'm in there. lol

 

[agedhorse]

I haven't worked on one in years, but I am very familiar with the designs.

 

[ricardol]

I haven't worked on one in years, but I am very familiar with the designs.

These old Acoustic amps/ cabinets are still a reference as they were used and recorded by many great players such as Jaco and John Paul Jones... What is so unique in their design that results in such a distinct tone? Do you know any modern amp/ cabinet combination whose design could better emulate their tone? Thanks!

 

[Al-vis]

Yes, that's it, the reverb assembly PC board sits between the graphic EQ PC board and the power transformer. In the pic below you can see the original fish paper insulator between those two boards which may or may not still be there on yours.

thank you for sharing all that very useful information.

Easiest way to tell the 270 and 370 chassis apart is that the 370 has three screw-terminal capacitors while the 270 only has two.

Do I need something else beside the complete power amp from 370 to build it into the 270?
For example the third capacitor?

 

[DaveKirkpatrick]

Hi, I love the sound of the old Acoustic amps! Would there be an option of converting this amp into a preamp and use it to drive an external amplifier? No idea if it's a possibility, but it might be more affordable and involve sourcing fewer rare components. Another option might be to find an old Acoustic 300 power amp... but those things are rare as well.

 

[JacklegBass]

Do I need something else beside the complete power amp from 370 to build it into the 270?
For example the third capacitor?

No, you wouldn't need the 3rd capacitor. It's 1,000uF and just adds a little more power supply filtering to the 370. I'm guessing it was probably deemed unnecessary in the 270 which has additional preamp circuitry.

If you could find a working 370 power amp assembly then yes it would drop right in. The 370 PA is nearly identical to the 270 PA, with the 370 having just a couple of extra parts to complete an automatic gain control (AGC) feedback loop. I wouldn't hesitate to interchange them.

Of course would also need to screw the bridge rectifier back down to the bottom of the chassis.

 

[JacklegBass]

Would there be an option of converting this amp into a preamp and use it to drive an external amplifier? No idea if it's a possibility, but it might be more affordable and involve sourcing fewer rare components. Another option might be to find an old Acoustic 300 power amp... but those things are rare as well.

Good idea, I thought about this too. The question is whether the wire feeding the Line/Monitor Output jacks on the rear panel are being fed directly from the preamp board or instead are daisy-chained off the power amp board input in which case he'd need to fix that.