Acoustic 270/370 Power Amp Section

[JacklegBass]

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

Well you remembered that there was separate reverb electronics board, I had long since forgotten that.

 

[Analogeezer]

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.

I can't speak for converting it but when I first went from my 220 to a rackmount rig, I was using the Acoustic 220 as a preamp to the QSC power amp I had bought (Series I 1400). Then I bought a proper rackmmount preamp, a Peavey ProBass 1000.

After I had used the Peavey for a while I was really depressed about the way it sounded so I went BACK to using the Acoustic 220 preamp section and it sounded way better.

Of course sounding better than a Peavey Probass 1000 was a low bar LOL.

Analogeezer

 

[Analogeezer]

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.

You could have said that directly instead of "fell out of favor over 40 years ago".

You should have included the highlighted/bolded bit in your original response, yes that IS useful information.

So to further suss this out, was this changeover from the 370 models to 320 era or later than that?

What do you mean by "later direct coupled Acoustic power amp modules"? Later 1970's In the 1980's?

Analogeezer

 

[JacklegBass]

So to further suss this out, was this changeover from the 370 models to 320 era or later than that?

What do you mean by "later direct coupled Acoustic power amp modules"? Later 1970's In the 1980's?

I can't speak for him, but I think the best example of a direct coupled Acoustic amp would be one we've just been discussing in The Acoustic Club thread, the 120-series including the second version of the 126 chassis where they switched to a dual-polarity power supply and got rid of the output cap. I didn't pay much attention after Acoustic's 3rd generation product, so I'm not sure what the Wagner Group did with their products once they assumed control.

 

[byacey]

You could have said that directly instead of "fell out of favor over 40 years ago".

You should have included the highlighted/bolded bit in your original response, yes that IS useful information.

So to further suss this out, was this changeover from the 370 models to 320 era or later than that?

What do you mean by "later direct coupled Acoustic power amp modules"? Later 1970's In the 1980's?

Analogeezer

They did fall out of favour, design-wise. In manufacture, there was considerable savings in eliminating the relatively expensive speaker coupling capacitor. In retrospect, it's a good thing they had that cap, otherwise they would have had a rash of burnt up speakers when the amps failed, as they were prone to do.

Their power amps at that time were essentially direct coupled, except for the final speaker cap.

 

[agedhorse]

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!

Quite a few design choices that were more a default of the times combined with the speaker choices that were available. It's helpful that there were really no clear cut established standards at that time either, so like the Fender tonestack concept, which came first the chicken or the egg concepts apply as well.

thank you for sharing all that very useful information.



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

Yes, there is additional circuitry present.

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.

Where does this information come from??? The third capacitor is not just more power supply filtering, it's part of the preamp supply and is necessary for both the noise floor and the stability of the LV rail. It's part of a low frequency R-C-R-C network.

The power amps are similar but different as well, the 370 used current source biasing of the driver stage and an FET mute circuit, the lower tier output transistor emitters are directly referenced to ground on the 270 and referenced through .047 ohm resistors on the 370.

Will the power amp assembly work interchangeably between the 270 and 370? Maybe, but they are not identical, and it's essential to verify the differences before making assumptions.

 

[agedhorse]

You could have said that directly instead of "fell out of favor over 40 years ago".

You should have included the highlighted/bolded bit in your original response, yes that IS useful information.

So to further suss this out, was this changeover from the 370 models to 320 era or later than that?

What do you mean by "later direct coupled Acoustic power amp modules"? Later 1970's In the 1980's?

Analogeezer

That design DID fall out of favor over 40 years ago, even the original Acoustic designers recognized this and evolved along with everybody else. They held onto this design longer than most. They changed sometime in the late 1970's to early 1980's IIRC.

I don't see the problem with the information I provided, it's absolutely accurate.

I can't speak for him, but I think the best example of a direct coupled Acoustic amp would be one we've just been discussing in The Acoustic Club thread, the 120-series including the second version of the 126 chassis where they switched to a dual-polarity power supply and got rid of the output cap. I didn't pay much attention after Acoustic's 3rd generation product, so I'm not sure what the Wagner Group did with their products once they assumed control.

Yes, this sounds about right.

They did fall out of favour, design-wise. In manufacture, there was considerable savings in eliminating the relatively expensive speaker coupling capacitor. In retrospect, it's a good thing they had that cap, otherwise they would have had a rash of burnt up speakers when the amps failed, as they were prone to do.

Their power amps at that time were essentially direct coupled, except for the final speaker cap.

Agreed, it did serve as protection which is good. Amp designs were not as robust back then, and the choice of relay protection or output cap protection (along with only a single supply rail) was more a matter of choice when early Lin based amps were the state of the art.

 

[JacklegBass]

Yes, there is additional circuitry present.

To be clear, what the 270 has that the 370 lacks are the Accutronics reverb tank, the 17-28 PC board (reverb driver/preamp), and the 17-27 preamp board with the tremolo and the reverb/fuzz control. Looking at the OP's photo he appears to have all of these, unless I've missed something.

Where does this information come from??? The third capacitor is not just more power supply filtering, it's part of the preamp supply and is necessary for both the noise floor and the stability of the LV rail. It's part of a low frequency R-C-R-C network.

That's pretty much what I was getting at albeit without the details. The OP was asking whether he needed the 3rd capacitor, which Acoustic obviously deemed wasn't necessary with the 270 preamp board. Since that 3rd cap was drawn only as part of the 370 preamp V+ supply, I didn't think I was treading too far out on a limb in surmising it was due to differences in the two preamp boards.

The power amps are similar but different as well, the 370 used current source biasing of the driver stage and an FET mute circuit, the lower tier output transistor emitters are directly referenced to ground on the 270 and referenced through .047 ohm resistors on the 370.

Will the power amp assembly work interchangeably between the 270 and 370? Maybe, but they are not identical, and it's essential to verify the differences before making assumptions.

I assume by "mute circuit" you're referring to the AGC circuit defeat switch.

The schematics and parts lists I have for the 270 and 370 power amp sections are identical except for the AGC circuit. Both amps use the same 17-19 PC board, the 270 board simply isn't populated with the ACG parts. Both have board-mounted emitter resistors for the output devices. I'm not sure what 270 you may have encountered without them.

Russ Allee has referred to the 370's AGC as an "anti-overload AGC-like circuit" before later referring to it simply as an "AGC circuit". From his statement that the ACG "shouldn't be audible up to where actual clipping or AGC comes into play", I would extrapolate that the 270 and 370 power amps would be equivalent up to that point.

 

[agedhorse]

Ok, I looked again and what I remembered as a mute circuit is in fact a crude limiter circuit. There's an additional .47 ohm resistor in series with the lower tier outputs that bias the limiter but could also be used to generate an RC time constant for turn-on muting purposes (and may in fact also function this way as a mute circuit to reduce the plop at turn-on).

If this is all true, and the physical heatsink assembly is properly punched for the application, I don't see why it wouldn't work. I would recommend verifying that the version conforms to these assumptions however.

The Lin topology being discussed was originally developed in ~1956 at RCA. It's not a coincidence that Jack Sondermeyer (Peavey's principle engineer) also used this topology as he was also a transistor applications engineer at RCA but most (not all) of his designs were based on the bipolar power supply evolution. My father designed servo amplifiers around this topology in the early 1960's.

 

[Al-vis]

Thanks guys for all this information. I am really overwhelmed by all the knowledge that is shared here.