Cooked Acoustic 370

[Enzo]

If you have suitable transistors in it, and it is working, then for sure... stop fixing it.

If I installed some 15024s, I'd probably tweak the bias too.

[Phostenix]

If you have suitable transistors in it, and it is working, then for sure... stop fixing it.

I'm not fixing it, I'm tinkering with it.

All I'm going to do with it at this point is check & record various voltages for my own records & do some load tests. I have another one of these right behind it & I'm curious how they will compare. I also want to make sure that I'm convinced that it's rock solid before I sell it & "put my name on it".

If I installed some 15024s, I'd probably tweak the bias too.

Just out of curiosity, how would you tweak the bias on this amp? And, thanks for your help, as always.

[Enzo]

Q305,310 are the drivers, well pre-drivers. Q304 is the bias transistor that maintains the space between their bases. Basically by varying R319 you control how hard Q304 conducts. Raising R319 makes the transistor conduct harder, which makes the amp run at cooler bias. Look for the similarity in any adjustable bias circuit. They all work the same. A trim pot in place of the resistor.

[Steve Conner]

If I installed some 15024s, I'd probably tweak the bias too.

Hmm. +1. I have seen one of my homebuilt amps oscillate because it wasn't getting enough bias. They fall into a region where neither output transistor is really conducting, and that puts extra phase lag into the loop. The amplitude and shape of the oscillations in the scope shot tends to agree with this explanation.

I had assumed the bias was adjustable and had been adjusted correctly.

[Enzo]

Of course I don't know if that is a factor in his case, but it is worth checking. As a rule I have not had stability problems with these output device subs.

[Phostenix]

Interesting theory. I've got the 2N3055H's all mounted up at this point (thermal greased), so I'm not going to remove them, but I can do some experimenting on the other 370 I have when I get there.

Also, I was looking at my pile of xstrs last night and realized that something I posted was incorrect. The new MJ15024's that I have are OK as far as beta gain numbers go. It was the new On Semi 2N3055AG's that I had in it for first startup (guinea pigs in case I had missed something & they smoked) that had the high 150-300 gain numbers. Those new 2N3055AG's didn't oscillate, though.

What do you guys think is the best way to measure bias current for the outputs in this amp? R322 & R323? It looks like that would give me the sum of all 8 output devices, though. Is there a better place to measure? I realize that adjusting for minimum THD would be preferred, but without a bias adjustment pot it's hard to check that.

With the 2N3005H's in, I've got 11.1mV across R323 & 11.6mV across R322.

[Enzo]

Personally I never measure the output current, it doesn;t tell me anything about crossover distortion, which is one major reasin for setting the bias. For that reason, I dislike when schematics say "adjust for xxx mv across R1234." That is the solid state equivalent to telling a tube amp tech to set the bias for -55 volts.

Lift one resistor and you can tack in a trim pot

[Phostenix]

I got the other 370 I have working today. For its intial startup, I put in 4 MJ15024's (2 on each side Q305 & Q306 and Q311 & Q312) and got a very similar oscillation as I did with the 1st amp. I didn't modify the bias, just tried them. New On Semi 2N3055AG's work fine without oscillation. I think I'll order some MJ15015's just to satisfy my curiosity.

[Phostenix]

Here's a pic of the oscillation. 0.5V/Div and 0.1uS/Div.

[Phostenix]

Dropping in a full complement of MJ15015's did not produce any oscillation (no bias adjustment).

[gbono]

I thought I would add this into the discussion - the original thread is on the Unofficial Acoustic site here 370 Bias Wrong in Trouble-Shooting, Repair and Restoration Forum

TO3 4815 Output transistor could be marked 4815, 480015, 60085. It is a SELECTED RCA 2N3055 HOMETAXIAL type. RCA made a change about 1976 to an EPI BASE type. Motorola did also make the change. If you use any 2N3055 type it may FAIL. We did a 100% Vce and SOA test on each device. We did not get 100% yield. The newer parts will need additional compensation to have a stable amplifier.

Good Hunting.
Best Regards

Duke Aguiar

[Phostenix]

I thought I would add this into the discussion - the original thread is on the Unofficial Acoustic site here 370 Bias Wrong in Trouble-Shooting, Repair and Restoration Forum

Since I'm repairing these amps to resell them, I went with the NOS 2N3055H's so I won't have to deal with people being nervous about them not having the "original" transistors. If I were going to keep & play a 370, I'd put the MJ15015's in it. The spec sheet says they are intended to be replacements for 2N3055's with a higher SOA. They were rock solid in mine to full output.

[gbono]

I wonder why the MJ15024 were unstable and the 15015 worked fine? I'll ask On if the 15015/15024 are made from the same process/geometry.

[Phostenix]

I'm guessing it has to do with fT the current-gain-bandwidth product. fT is .8 MHz for the MJ15015 (& the 2N3055) and 4 MHz for the MJ15024.


From the MJ15015 datasheet:

These PowerBase complementary transistors are designed for
high power audio, stepping motor and other linear applications. These
devices can also be used in power switching circuits such as relay or
solenoid drivers, dc−to−dc converters, inverters, or for inductive loads
requiring higher safe operating area than the 2N3055.
Features
• Current−Gain − Bandwidth−Product @ IC = 1.0 Adc
fT = 0.8 MHz (Min) − NPN
= 2.2 MHz (Min) − PNP


The MJ15015 datasheet includes the 2N3055 in the title of the Electrical Characteristics chart. It sure looks to me like the MJ15015's are a close match to the old 2N3055's except that the MJ15015 can handle more voltage & has a bigger SOA.

I will say that the 10 MJ15015's I bought have much higher DC gain than the NOS 2N3055's I bought. On my old Heathkit tester, the 2N3055's grouped mostly in the 50's. The MJ15015's I got grouped mostly in the 180-200 range. I don't think that matters as much in this amp as fT, though. At least that's my guess based on the limited testing I've done with these 2 amps. BTW, NOS 2N3773's worked without modification, too.

[gbono]

Makes sense though the maximum collector current for the two devices is 15A and 16A - makes me suspect that both transistors would have similar active areas. (The output capacitances Ccb for both these parts are 500 and 600 pF max.) I have pretty good access to On Semi Applications Engineering and I'll ask about the difference in processing or geometry when I get a chance.