Acoustic 270 voltage on output

[pontiacpete]

OK now I can make sense of the 25v on Q406-7 emitters. Again there's only a .1 resistor on that middle rail which has the 25v and the emitter of Q407. And there's only a 1ohm resistor be the Q407 base and emitter.. and so on.
Still perplexed about the zener issue.

Anyway went ahead tried running up the AC voltage without the limiter and got to 88vac with the main current draw at .64A.
Little on the chicken side to go further.

[g1]

How can there be 43.7v (according to the schematic) at the collector of Q404 when it's connected to a CR408 a 4.3v zener?
Am I reading this wrong or whats goin on?

The other side of that zener is close to the center point voltage (approx. 45V).

[pontiacpete]

ARe you saying that the anode of the zener is seeing ~45v through CR407?

Does this mean the base voltage on Q405 looks good?

[g1]

Yes, seems roughly correct.

[pontiacpete]

Thanks G1

I ran the variac up to a 100v and the main current draw is up to an 1A. (this is with still with one output transistor on each side)

With a 7A fuse should I it expect to see this amp draw up to 2.5A?
I guess that would the most it would draw with signal...

[g1]

1A with only 100V from the line seems excessive.
What voltage is across R422? And across R423?
You said Q411 emitter is .5V, but schematic shows .16V there?

[Enzo]

Entering late.

You seem confused by the voltage on the output with no load, so we need a load of SOME sort. But when you put a real load on it, it burns up. SO all we want is for the output cap to be able to charge, so put like a 1k resistor to ground from the output. A 1k dummy load if you like. That will allow the cap to charge, and eliminate the voltage reading there. But it won;t draw enough current to damage anything.

Where did you buy the 2N3055s? There are a ton of fakes out there. Also, original 2N3055 was a 60v part, only later ones had the 100v rating you need for this amp.

While we try to sort this thing out, you only need one pair of 2N3055s in it, not three pair. So you could put in ONLY Q407 and Q412 for example, then if it turns sour, only two parts are destroyed. You may already be doing this.

When using a variac, the idea is not to turn it up to say 100v and watch it get hot, the idea is to watch the current meter and if it starts to ramp up current faster than normal, STOP. Other than the turn-on thump and cap charge rush, I don;t expect this amp to draw much more than 30-40 watts at idle. 1A at 100v mains is 100 watts, and you are not even all the way up. And voltage readings taken while on 100v are not usually very helpful. When your bulb or something else it making the 90v power supply 50 instead, then it makes sense to expect 25v on the output bus before the speaker cap, but you CAN'T COUNT ON IT.

Sometimes it is helpful to run and amp cold during tests. SO if you short Q404 E to C, that will put the amp in the coldest bias setting. it will increase crossover distortion that way, but we are not even trying to get signal through it now, we are trying to make it not burn itself up. We can take the short off and bias the unit properly later.

Way back in post #1, I bet that CR409 was not really shorted, what with the 1 ohm parallel resistor. Unless the resistor was open.

From Q405 E all the way down, I see two 100 ohm, three 1 ohm, and two 0.1 ohm resistors, are they ALL healthy?

The fact that the voltages are high across R425,426 are high explains too much current through the outputs.

[J M Fahey]

Fully agree and add:

Output transistors were MJ15022

Thatīs what you need.
At least you can get good ones at Mouser.
Good 2N3055? not even at Mouser (long story)
I love 2N3055 and made about 8000 amps with them during 35 years, but the honeymoon is over, had to give them up between 2004 and 2008.
Now not even ST or ON, factory direct , can do the job.

[pontiacpete]

I got the 2n3055s from Newark. Should have done a search here before buying them. Better yet I should have just remembered what I read not that long ago by JM here:http://music-electronics-forum.com/t42565/. No wonder they were so cheap at $1.91 ea
Anyway thanks guys for jumping in on this. I probably have four good MJ15022s that came out of this amp to get it up and running while a new batch is on order.

[J M Fahey]

Problem is that even the original ones have been cheapened.
This coming straight "from the mouth of the horse", (the Avnet representative in Argentina) who normally wouldnīt even *talk* to me .... but thanks God is a Musician and comes often with weird ideas

Mind you, thanks to him I buy packs of ST 2N3055 , straight in the original Factory packing and pay only **67 cents** each (probably what Newark and others pay) but he told me:

"in the old days Factory competition was based on quality, people gladly paid U$2.50 (1980 dollars, more than twice as much today) for Motorola or RCA transistors because they went WAY over official spec compared to standard 1$ ones, it was easy to find 60% of what you bought would stand 100V or more instad of official 60V and so on, that explains Acoustic/Kustom/Peavey using them with +/-45 or single +90V rails and getting 100W RMS from a single pair if needed
.
But today itīs cutthroat price competition, so factories lowered price (my modern 67 cents would be 25 or 30 cents in 1980 dollars) , (good) transistors meet exact datasheet specs (60V under any condition, 70V with 100 ohms BE) and nothing beyond"

So modern amps can still be designed around them, they might do as replacements in, say, a NAD 30 or 40W per channel *home* Hi Fi amplifier....... but forget using them in the old workhorses; Guitar/Bass amps are abused by definition.

[pontiacpete]

So I'll hold on to rest of these and maybe use them in something less powerful.

All resistors are good now, had that one 1 ohmer opened up

I put 4 MJ15022s and the amp is already sounding like a healthy amp, on the limiter, though.
IT's at full AC voltage, 120v. The main ac current draw now at .28A, there's 60vdc at the big cap, 33v on the rail in the center of the transistors.
Now there 200mVdc on the base Q412. How does this look?

I have 2 more good MJ15022s, and thinking trying them in place. The amp is now making a good clear clean sound with the bass. This is great news after fussing for so long with the 3055s.

[J M Fahey]

Well, those obviously were fakes.

I save the leftovers , also repair "pulls" (where I replace all new ones to ensure matching) , to make low voltge supply regulators, they make killer LM78xx current boosters.
Itīs slightly annoying to perforate heatsinks with a TO3 pattern instead of a single hole "plastic case" transistor, but under, say, 40V and up to 5 or 7A *all* perform well.

[pontiacpete]

Replaced all the bad 2n3055s with MJ15022s and it's working great.
Thanks to everyone who contributed. Much appreciated.
pete