Acoustic 370 bass amp — help!

[vintagekiki]

Welcome to the forum.
And a logical question
What is not clear ?
Explain it.

[Smokestack52]

First off, let me say thank you for all the technical advice and recommendations.

Secondly, well...
I was going to launch into a big tirade defending my posts and my decision to join this forum to seek helpful advice...
Instead, I just have to ask --
WTF?

[vintagekiki]

This is the basic pain of most musicians "faithful" followers of various portals. Instead of doing what they know (to play), they deal with the age of the built-in components, types and colors of guitar and amps wiring strings, various modes, sound analysis of youtube clips ... The gallery of questions asked is inexhaustible.
For all my xyz engineering work, almost all users inquired to replacement elco caps, and when the output tubes need to be changed it is like having their teeth extracted without anesthesia

[The Dude]

As others have already said in the thread, I see no reason whatsoever to replace good parts. Replacing those transistors isn't gaining anything. The amp has been working for years with them installed.

[vintagekiki]

A370 operates at a fairly high voltage (Vc= +90V) This means that when selecting output and driver transistors, the transistors are first selected on the VCE, and only then on the hfe.
In principle, due to the mains voltage tolerance, the VCE should meet 120V. I used checked transistors on VCE from 2N3055 to MJ15003.
As far as hfe is concerned, it is not so critical, it is important to satisfy the condition that hfe is in mutual range of +/- 10% (so that the transistors open equally)

During first start-up (protection resistor in +VCC) first set offset (UC404 = +45V) and then quiet current (25mA per output transistor)
The amplifier adjusted in this way satisfies 90% of the needs. Who loves tuning"in small intestines" can use gallery of instruments, they will get only visual satisfaction, the difference is almost imperceptible to the ear.


https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/23517-acoustic-370-repair-procedure

[Smokestack52]

I have tested all the original power transistors using one of those open source M-Tester kits -- I don't know how useful these tests are for big transistors such as these. None are open or shorted, but neither are they matched all that closely. The original transistors average hFE is around 24 (but as I said -- they are all over the place). The set of new NTE transistors I purchased are all fairly well matched and average around 40 hFE. Is the general consensus here that I should still stick with the old ones? Would the gain or frequency range of the new transistors make this amp become unstable?
Here's a quote from another forum regarding this same amp that gives me cause to worry a little bit:

"Several years ago I had the Acoustic 370 or model 371 (same amp but in combination with 301 bass cabinet) for repair. Like by several other amps with triplet quasi-complementary super ß buffer in the output also here was unwanted oscillation the main problem - read more from post #20 about
In this case, I had made short work: I reduced the upper half to a normal darlington topology and the lower half to a Sziklai Darlington. The amplifier is running fine since performing this steps.
The challenge to achieve stable operation with a super beta push pull quasi complementary follower was too large (because too expensive) for me."

And another quote from the same thread:

"i rebuilt one of these beasts for a friend..... 3 TIMES!!!! and each time, i'd be bringing it up slow on a variac, and when i got to 60 volts WHAM!, the ammeter needle would peg, and bye-bye to another whole set of parts..... i thought i was making some kind of mistake, until i talked to other amp techs, and found they had all the same experiences. glad to know there's a cure for it. i didn't have a schematic available at the time. i had to build my own by following wires and traces. one of the things that did seem odd to me at the time were the output devices being paralleled with no separate emitter resistors. bipolars hog current, and this alone is it's own set of accidents waiting to happen.... i've offered to rebuild my friend's amp (after about 20 years of it being a 65lb preamp) with a fully complementary design, that's a whole lot more stable and MJL4281/4302 output devices, which are a whole lot "beefier" and much more linear. of course it won't have the same "quasi" sound, and won't behave badly when clipped (which isn't so bad since there's no tweeters to fry). with it being a bass amp i can probably get away with OVERcompensating it."

I'm wondering whether to stick with the old, definitely unmatched set of transistors or install the new ones...Advice?

[vintagekiki]

So... no experience re-forming these caps then?

About reforming electrolytic capacitors

Reforming Old Electrolytic Capacitors - YouTube

Restoring life to a dead electrolytic capacitor.

reforming electrolytic capacitors - diyAudio

Reforming Electrolytic Capacitors

Reforming Electrolytic Capacitors

Reforming Caps

Rap on Replacing Electrolytic Capacitors

1)

How to Reform Electrolytic Filter Capacitors on tube radio or amp. - YouTube

Electrolytic Capacitor Reforming - Why and How To - YouTube

Build an Electrolytic Restorer, October 1970 Popular Electronics - RF Cafe

Electrolytic Capacitor Reforming Circuit under Meters Circuits -14179- : Next.gr

capacitor "forming" - diyAudio

[g1]

Some guys reform them using a lamp limiter. Not sure what wattage bulb they use though.

[Smokestack52]

So... no experience re-forming these caps then? I've had electrolytics blow up on me in old tube gear, so I am aware of the dangers. Literally, blow up... no, I was not injured, but I could have been...

No, I don't know that the old electrolytic caps were bad, necessarily. But based on what the amp was sounding like, and based on my experience servicing tube guitar amps, it seemed prudent to replace. The values of the big caps in this amp, though, make replacement difficult -- getting exact replacements is difficult, if not impossible. Enzo mentioned using smaller modern caps, making solder connections, and figuring out how to mount in chassis... I was hoping to keep it original looking inside, but in the grand scheme of things that's not important.

[vintagekiki]

Every electrolytic so and "vintage" (that it would not be surprise) before use should be previously formatted, measured and then build in, because they are sitting longer as a stock on the warehouse shelf or in artisan drawers.

[Jazz P Bass]

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

[DrGonz78]

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

[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.

[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?

[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.