Acoustic 370 bass amp — help!

[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]

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!

[vintagekiki]

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

[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]

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!

[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.

[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.

[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

[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!