Acoustic 370 output distortion problem

[prulup]

Thanks
First question worth answering in about 2 Months, so ...
That amplifier is working fine.
90V single rail means, after supply dropping under load (15/20%) plus transistor losses (4 or 5V *each* side) that guaranteed output voltage, 20V RMS clearly written by output jack, is realistic.
It might reach 21 or 22V RMS on a good day, but that would be a Bonus.

So 20V RMS means 50W into 8 ohms (so your 70W is perfect), 100W into 4 (so measured 110W is fine) and 200W into 2 ohm, which is the standard output you can get with that supply and transistors.

In fact my own B200 from the 70īs and contemporary with similar Acoustic amps was 200W RMS into 2 ohm (4 x 12" 8 ohm speakers in parallel) , +/-42V rails, 4 selected 2N3055H transistors.

That Acoustic can *easily* drive 2 ohm loads, and was typically used with 2 x 4 ohm cabinets or the huge 2 ohm 4 x 15" 408:


In fact head is rated down to 1.6 ohm load, go figure, so itīs useless to try them to put out more into 8 or 4 ohm which are the wrong impedance.

Thanks J M, I will re-measure it with 4 and 2 ohms load and write you the precise numbers in the morning. However how do you bias the amp (safe area), how many mV acros the 1 ohm resistors? Thanks!

[prulup]

Hi guys,

so after the limitation circuit removed and changing the bias to 15mV across the 1R resistors I have these results:
no load - 24V
8 ohm - 85W
4 ohm - 125W
2,7 ohm - 195W

bias 15mV on each 1R resistors. Slight limitation on the sine wave. I guess it works fine... :-) Thank you for your help!

[J M Fahey]

Hi guys,

so after the limitation circuit removed and changing the bias to 15mV across the 1R resistors I have these results:
no load - 24V
8 ohm - 85W
4 ohm - 125W
2,7 ohm - 195W

bias 15mV on each 1R resistors. Slight limitation on the sine wave. I guess it works fine... :-) Thank you for your help!

Cool.
The cherry on the cake would be that you:
1) measure it on intended 2 ohm load (what the 408 cabinet provides) , wouldnīt be surprised at wall shaking 250W RMS or even better.
2) reconnect the limiter.
Far from "stealing power" , it actually means you can hit it hard all the time with very little distortion onstage; while without limiter and to get that clean sound you should use it around 100W average to accomodate transients.
Guess which of 2 options will sound louder in practice?
Which of course means that is a very healthy amplifier.

[J M Fahey]

Interesting fact about our trolls

Just read prulupīs answer, glad he solved it, and at the bottom of the page found this interesting piece of information:

Members who have read this thread: 29

Enzo, Dave H, Tom Phillips, Chuck H, J M Fahey, MangaS, prschmitt, Jazz P Bass, pontiacpete, mozwell, nickb, prulup, g1, nevetslab, nosaj, DrGonz78, Leo_Gnardo, nsubulysses, TigerAmps, Bloomfield, tdlunsfo, Drummer4gc, Helmholtz, yann

Notice that NONE of our 4 Trolls has even READ it, let alone answered, which CLEARLY MEANS THEY ARE NOT IN THIS TECH FORUM FOR TECH REASONS BUT SIMPLY TO SPREAD CHAOS.

This simple but revealing fact has convinced me that they must be banned at once, no contemplations, we are losing nothing and gaining a lot with such an action.

Seen through the microscope they clearly and unmistakeably look like Cancer cells and not regular tissue ones, what else do we need?.

Of course, "tomorrow" they will "read" each and every MEF Tech post thread and close it 5 seconds later, so system records their visit), just to throw a smokescreen ... too late for that, if done after the fact.

Just to increase sample size and not being accused of generalizing after "just one", a couple other samples, which prove my point:

* On "cocked wah sound":
Members who have read this thread: 27
nickb, old guy, DrGonz78, prschmitt, nosaj, J M Fahey, nevetslab, eschertron, tboy, galaxiex, olddawg, Leo_Gnardo, Chuck H, sgelectric, Helmholtz, Bloomfield, Jazz P Bass, Justin Thomas, Dave H, g1, pdf64, Pedro Vecino, R.G., Daver

* On "Marshall JTM60":
Members who have read this thread: 18
xtian, tubedood, 1ampman, Drummer4gc, Jazz P Bass, alexradium, Pulley, yann, pontiacpete, mozz, pdf64, nickb, nsubulysses, Pedro Vecino, Helmholtz, g1, Dave H, Randall

* On "first act":
Members who have read this thread: 26
Bloomfield, glebert, alexradium, mozz, jbltwin1, nickb, J M Fahey, dmartn149, Leo_Gnardo, nsubulysses, tdlunsfo, nevetslab, tboy, tubedood, g1, nosaj, galaxiex, Jazz P Bass, DrGonz78, myfoot, jomama, Helmholtz, Dave H, Justin Thomas

I rest my case .