Acoustic 370 bass amp — help!

[Smokestack52]

Hello! I have an Acoustic Control Corp. 370 bass amp that I have been meaning to repair for several years. The last few times I played through it (which has been a while), it was sounding just generally weaker and increasingly noisy. It never quit working — I just decided to retire it for a while. I followed a few posts here and elsewhere regarding this amp, and I suppose that’s why I’ve put off repairing it — it seemed that several techs were encountering more or less the same problem, namely that of having an entire new set of output transistors go up in smoke while bringing the repaired amp’s voltage up on a variac. From what I was able to gather, it seemed that the new devices (2N3055 power transistors, or current equivalents) were more efficient to the point of making this particular amp circuit unstable. Some techs even went so far as to say that completely modifying the amp’s topology was the only way to make it stable. Can anyone out there confirm or deny that this is the case with this amp?

As probably anyone knows who is reading this post, the Acoustic 370 is a classic bass amp — I would hate to think that at this point the amp is un-repairable. I have been working on and building tube guitar amps for 20+ years, but I have been doing so as a musician and a hobbyist — I am not an electrical engineer. This particular amp I find a little intimidating.
I have ordered a set of output transistors, as well as the transistors to replace the driver and bias ones. I have also sourced and ordered all the electrolytic caps I will need to do a complete cap job.
All I need is a little advice and encouragement that the amp will not just blow up after doing all this repair work...
Thanks!

[vintagekiki]

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/50421-acoustic-370-repair

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/914637-acoustic-370-troubleshooting
1)
https://www.ssguitar.com/index.php?topic=4500.0
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Not Acoustic, but it is a useful post on how to repair and adjust a solid state power amp step by step

[g1]

You have no reason to suspect your output transistors yet. They don't just go bad from age.
Yes, there can be issues with the differences of the new types. But if you don't have to replace any, you will not have to confront that issue.
There are plenty of other issues that could cause noise and reduced volume level. To go ahead and replace the power amp transistors 'just because' is looking to create problems that may not be there.

[Enzo]

More or less the same problem does not require replacing the output transistors. Beware trying to paint every problem with things the internet says most likely..

Your amp is working, just maybe getting a little tired. So it is NOT unstable, and not blowing up?

And I agree, transistors don't wear out like tubes.

Frankly when I see one of these fine old amps, with a complaint like that, my first reaction is those tiny electrolytic caps throughout the preamp. In this case most are 1uf, but also a couple 2.2uf.

[Smokestack52]

Thanks for prompt responses to my query! I guess I will try replacing electrolytics first and then report back!

[vintagekiki]

In this case most are 1uf, but also a couple 2.2uf.

[Drummer4gc]

If you (or someone else stumbling on this post) need to replace the output transistors, mj15015 work as replacements. I also have some of the old style hometaxial 2n3055 transistors I could let go of (they were pulled from an Acoustic 270).

[vintagekiki]

Each time when replace transistor(s) in the output stage, check (adjust) the quiescent current and half voltage on +C404.

[Smokestack52]

Just an update: I have ordered electrolytic replacement caps for the amp. Not easy to source those large can caps! Mouser doesn't stock them and the manufacturer will only fill an order of a few hundred pieces, apparently! I DID order a set of output transistors -- I did this before starting this thread -- but I will keep the original RCA 2N3055 transistors in the amp, based on the responses I am getting here. Initially, I was just following the advice someone posted about 10 years ago under the title 'Acoustic 370 Repair Procedure' -- where the OP went through and pretty much did the shotgun approach to repair...(replacing everything)
One question: is it okay to replace the small value electrolytics (1uf/35v, 2.2uf/35v) with tantalum caps? I could not find that small of a value in a standard axial electrolytic...

Again, once I receive all the parts and have installed them, I'll post another update on whether or not it takes care of the problem.
In the meantime, thanks for your responses!

[Enzo]

SO use radial caps instead of axial, we do that every day. Personally I'd avoid tantalums.

I VERY rarely have to replace thos giant screw terminal caps. However, if I do, I don't worrry about screw terminals. When these amps were made, that was the format of large value caps. But caps are much more versatile these days. SO plain old snap-in or just radial lead caps will work fine. They will be smaller so you'll have to mount them, then make solder connections.

I just checked Mouser, and they have several 1uf 50v axial leaded caps in stock.

https://www.mouser.com/Passive-Compo...uf+cap&FS=True

[Smokestack52]

So... when initially pursuing the shotgun repair approach to this amplifier (as per 'Acoustic 370 Repair Procedure' post here or elsewhere (I'm not sure)), I found all the large can-type screw terminal electrolytic capacitors. Two of them (7800uf/100v and 1000uf/100v) I sourced from a surplus seller, and they are NOS caps. Now, I know it is never ideal to use old electrolytic caps, but I have also read of many instances where people have done just that. I have read about people baking NOS caps to cause the crystallized electrolyte to turn into a gel again. Does anyone here have any experience doing this? I have also read about some techs setting up a slow-charge station to bring these caps up to voltage out of circuit S-L-O-W-L-Y while monitoring voltage drop across a large value, high wattage resistor...
I am wondering whether it would be worth it to do all this, or just keep the original screw terminal caps in the amp -- ? Enzo states above that these large caps rarely fail. If it were a tube amp, I would just go through and replace all the B+ electrolytics...

Also, Enzo (this addressed specifically to you): you said you wouldn't use tantalum caps in place of the lower value electroytics (1uf, 2.2uf)... on closer inspection, those caps in my amp appear to be tantalum caps already, and they appear original... is this possible? All of them test good for capacitance. I haven't tested them for leakage as I have the amp taken apart and have the output transistors removed.

Still awaiting the delivery of some thermal paste -- will re-install the old transistors and re-assemble after that.

[Enzo]

I personally dislike tants. They are perfectly good caps, but they have ZERO forgiveness for over voltage or ANY reverse voltage at all. yes, sure the originals could easily be tantalums. And I see no advantage really in a guitar amp.

If we are getting NOS big filters, how do we know they are in better shape than the existing old stock in the amp?

[The Dude]

............ If we are getting NOS big filters, how do we know they are in better shape than the existing old stock in the amp?

Ha. We don't! In fact, I would think a NOS "vintage" electrolytic would be in worse shape than what's in the amp, since it's likely been sitting longer without use.

[DrGonz78]

NOS = No’s. Well for for electrolytic caps.

[Jazz P Bass]

An over voltaged (even a small value) tantalum is a site to see.
Pop Goes The Weasel !