Acoustic 370 output power vs 470

[nsubulysses]

Hey, I have posted this thread on the Acoustic board too but am also posting it here because this one seems more active. Hope that is alright.

This is puzzling to me can anyone comment??

I have an Acoustic 370 which I have had for about 5 years. I believe it works just as it should. It was recapped (not by me) when I got it, except for the 3 big electrolytics. Checking them recently they meter ok and also provide the voltages they should so to me they seem fine. It is very loud to my ear and puts out a clean sine wave up to about 155W. Here is a pic of it putting 24.9V into a 4 ohm load. This thing hangs with other relatively loud amps no problem. I have a reissue Model T and a V4.



Another 370 I got recently I somehow broke when I put a 20K sine wave into it. Not sure if this was coincidence. 3 of the RCA 48-15 output transistors were blown. IIRC, everything else checked ok. I think that was all I replaced. I got some old RCA 2N3055s from a friend. Other ppl on the net said these were a legal substitution and they are equivalent. Not sure if this is 100% true but it seems to perform basically the same as my other 370 as far as scope and voltage checks. I "matched" the 2N3055s with a Heathkit IT-18 transistor checker. I bought extra 2N3055s and only used the ones that tested the closest to the 48-15 transistors. It still puts outs a very even sine wave but it only goes up to 24.2V -- about 146W at clipping. It put out 146W for an hour and a half. I have not played it through a cab at full volume yet.


The thing is I'm working on this 470 now and it is putting out 166W at clipping into a 4 ohm load. This seems strange. I have looked at the manuals and 470 is rated at 170W RMS into 4 ohm load. This does that. Ok, that's not the strange thing. 370s in the manual claim 205W RMS into 4 ohm load. The two I have appear to me to work normally but cannot get to this output.

Could this be because of using a 1K sine wave, or because the 470 actually has more clean power but not more total power, or some other reason? I thought 270s and 370s were considered 'heavy duty' and 470s were 'medium duty.' I had a 470 which I sold to get a 370. I wanted more volume and low end and I felt like I got that.

Here is the 470 putting 25.8V into 4 ohms -- 166W. Am I being misled by something or does anything come to mind that I should be checking if my 370s actually are putting out about 75% power??

[nsubulysses]

Oh guess this is my first post. I have consulted this forum for years and gained lots of insight into my troubleshooting. Thanks!

Also maybe this should be moved to the guitar tech section whoops

[g1]

Check your reference specs. as far as load impedance.
From elsewhere in the forum archives: " ACC rated the 370 (per service manual) at 205W @ 3.2 ohms 5% distortion and 325W @ 1.6 ohms at 5% distortion. That would be 25 and 23 Vrms respectively. I believe they referenced 5% distortion as just clipping of the output waveform."

[The Dude]

This is not really an answer to your question and I understand your curiosity as to why the expected output is not there. That said, doubling the power only provides an increase in output by 3dB. So, you're talking about a dB or so difference. Even if you were to get the output to its rated spec, It would be barely, if at all, noticeable. If everything is working correctly, I wouldn't worry horribly about it. It's not uncommon for manufacturers to source parts from different vendors throughout the build history of an amp for cost saving, parts availability, or whatever else. My best guess is that there were some minor parts changes from the time the amp was designed and spec'd and the time it was built. Not an uncommon practice at all.

[J M Fahey]

Both use roughly the same power supply rail voltage so driving the same load they should put out about the same ... which they do.
But the 370 is the more powerful because it can *easily* drive a 2 ohm load (in fact itīs guaranteed down to 1.6 ohms, go figure).

By the way, 205W*3.2/4=164W
You got 155W? Cool, WELL within spec

[Enzo]

Both models have the same power supply voltage, 92v, single sided. So with 45vDC either way, the most your output signal could be is maybe 30v of signal. The 470 has two pairs of output transistors, the 370 has three pairs. In my mind, that means both amps put out the same voltage, but the 370 likely can handle higher currents, and so lower impedance loads. Your 25v or so of output signal sounds good to me.

Note the 470 has a 2000uf output cap, while the 370 has a 3900uf output cap. That larger cap also would help pass higher currents.


By the way, a sine wave at full power is a tough job for an amp, they are made to amplify music. Sine waves give them no rest between notes. MY choices for burn-in are either high power playing music or reduced power with a sine wave.


Oop, Juan beat me to it.

[nsubulysses]

What's great is that I read Juan's answer and it kind of didn't click, but then I read Enzo's, understood, and then realized Juan's answer was basically the same. Thank you all for your responses.

I was quite surprised to see the 470 putting out that much AC. One reason it got me wondering if the 370s were maybe lacking power was because of other threads saying to not mix output transistors. One 370 has all originals but the other has RCA 48-15s and also a few RCA 2N3055 (not RCA 2N3055H either). I have read somewhere or another these are equivalent but there are also many things printed about 370 output transistors and what works and what doesn't. That's also kind of the reason I let it rock a sine wave for so long to see if a problem would eventually come up. I will heed your warning and try real music for future though thanks. Any reason you think to wonder if this transistor mix will eventually fail? Seemed rock solid for that 1.5 hrs.

[Enzo]

48-15, also seen as 480015, or was it 4800015, I am too lazy to look, is just an Acoustic house part number for the 2N3055. Back in the 1960s, the 2N3055 was THE power transistor for most everything.


If I were rebuilding one of these today, I'd probably stuff it with MJ15003. The warning not to mix is not about 2N3055 and house numbered 2N3055. It is about mixing the 2N3055 and something else, like my MJ15003. When you mix types, the transistors are made on different processes and gave different characteristics. What CAN result is that an odd transistor could turn on at a slightly lower potential than the others, and so it hogs current and the others sit idle. It tries to handle the entire output, and then fails. And going the other way, an odd transistor might turn on at a lightly higher level than the others, and so it never turns on, and might as well not even be there, and so the amp is working on fewer outputs.

Certain power amps of the 1980s came with either Motorola transistors or RCA transistors. The parts had equivalent specs, but for the reasons I just described, you needed all of the transistors in your amp to be one or the other, no mixing.

[Enzo]

Wild guess, are you an old Cream fan or something?

[Leo_Gnardo]

If the 370's output is clamping, not clipping, don't forget there's a limiter built into the 370's power amp. You would see a test sine wave rise to a certain level then go no further, not clipping as expected. I've seen some of these amps otherwise healthy but clamping prematurely with a fraction of a watt output due to parts drift, others with a couple dozen watts, then others where the limiter doesn't work at all and they clip as you would expect. IIRC there's a 6.8 Megohm resistor that likes to drift in the limiter circuit, and by replacing it, and varying its value, you can select a limiter clamp level that's just about slap on the clipping point. Have a look at the Q315 FET gizmo at the bottom of the power amp schematic ke7vhn posted here on MEF 7-12-2010.

http://music-electronics-forum.com/a...c-370-2-1-.pdf

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Wild guess, are you an old Cream fan or something?

I'm sure we'll have an answer soon from our OP but for the moment, with nsub as the prefix, here's my best guess:

USS Ulysses S. Grant (SSBN-631) - Wikipedia, the free encyclopedia

[J M Fahey]

Iīve used 2N3055 in over 8000 amplifiers, go I enjoyed or suffered all variants along the way.
The "good" ones are incredibly robust RCA 2N3055H (Hometaxial) , a process which is no longer used because it doubles cost.
The Motorola version was 2N3055A (obviously obsolete and unavailable).
ALL modern 2N3055 , even if the real ones by a good manufacturer (IN , ST , etc.) are NOT the same, at all.

Not because they are intrinsically "bad" but because they use a very different process, calles Epitaxial (so they "should" be really labelled 2N3055E)
Much faster and "Hi Fi", as in 3mHz bandwidth compared to 800 KHz, but half the safe operating area, which is what matters for us.

So I suggest you replace all *at least* with MJ15015 , better by MJ15003 as suggested by Enzo and even better by MJ1500x , the NPN ones of course.

A little more expensive, nothing to write home about , and in fact cheap insurance.

So replace all (itīs just 6 transistors after all) and sleep like a baby.

In the old days 2N3055 was all really available and it limited you to +/-45V tops (and that with hand picked ones) and only way to gtet big power was to lower load impedance.

FWIW my first successful product in the early 70's was a Bass amp, ... 400W RMS into ... 1 ohm !!! (yes, not a typo).

Lots of selected 2N3055H in open air heatsinks (Musicians jokingly called them "the transistor factory ) , driving 2 large 4x12" speaker cabinets , each one having 4 x 8 ohms speakers in parallel for 2 ohms box impedance.

And of course 2 cabinets gave a total 1 ohm impedance.

The companion guitar amp was 200W RMS into 2 x 4 ohms cabinets, so 2 ohms total load.

There was no other way to go beyond 100W , at least not with standard available parts.

And Acoustic was under the same constraints.

[Enzo]

I'm sure we'll have an answer soon from our OP but for the moment, with nsub as the prefix, here's my best guess:

Your guess is as good as mine. I was reminded of early Cream songs NSU and Tales of Brave Ulysses.

[nsubulysses]

You guys are great

I used to love this old anti-nostalgia band. Guess I still do. Lotta good amps on the cover. This song is kind of anti beatles so don't play if any beatles fans are around. We all know Beatles fans are the most militant. Even more so than Black Sabbath fans

[johnk_10]

48-15, also seen as 480015, or was it 4800015, I am too lazy to look, is just an Acoustic house part number for the 2N3055. Back in the 1960s, the 2N3055 was THE power transistor for most everything.


If I were rebuilding one of these today, I'd probably stuff it with MJ15003. The warning not to mix is not about 2N3055 and house numbered 2N3055. It is about mixing the 2N3055 and something else, like my MJ15003. When you mix types, the transistors are made on different processes and gave different characteristics. What CAN result is that an odd transistor could turn on at a slightly lower potential than the others, and so it hogs current and the others sit idle. It tries to handle the entire output, and then fails. And going the other way, an odd transistor might turn on at a lightly higher level than the others, and so it never turns on, and might as well not even be there, and so the amp is working on fewer outputs.

Certain power amps of the 1980s came with either Motorola transistors or RCA transistors. The parts had equivalent specs, but for the reasons I just described, you needed all of the transistors in your amp to be one or the other, no mixing.

I just replaced all of the output transistors on a 370 using MJ15003G's and I can tell you, they DID NOT work properly in mine at all. it has a weird fuzzy oscialltion with the MJ15003G's. right now it's working and sounds great with 5 of the original 48-15's and 3 old 2N3773's, but i'm going to try 8 new 2N3055HOM's in it when they get here.

[g1]

it has a weird fuzzy oscialltion with the MJ15003G's.

Like crossover distortion? And if so, did you try adjusting bias (by pot or resistor adjustment) ?