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Acoustic 370 bass amp — help!

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  • #16
    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.
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

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    • #17
      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.

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      • #18
        Some guys reform them using a lamp limiter. Not sure what wattage bulb they use though.
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #20
          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 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?

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          • #21
            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.

            Click image for larger version  Name:	'    Reminder.gif Views:	0 Size:	154 Bytes ID:	928865
            https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/23517-acoustic-370-repair-procedure
            Who does not know and knows that he does not know - teach him Confucius)
            Who knows and does not know that he knows - wake him Confucius)

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            • #22
              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.
              "Yeah, well, you know, that's just, like, your opinion, man."

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              • #23
                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
                Who does not know and knows that he does not know - teach him Confucius)
                Who knows and does not know that he knows - wake him Confucius)

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                • #24
                  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?

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                  • #25
                    Welcome to the forum.
                    And a logical question
                    What is not clear ?
                    Explain it.
                    Who does not know and knows that he does not know - teach him Confucius)
                    Who knows and does not know that he knows - wake him Confucius)

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