Is this one of those transistors that looks like a TO-5 welded to a small heatsink ?

https://www.ebay.com/itm/364778815801?_skw=40409+transistor&itmmeta=01JT2DBRD9SAA9TZ9G682QKNNS&hash=item54ee830139:g:qXsAAOSwxj5l7SZo&itmprp=enc%3AAQAKAAAA8FkggFvd1GGDu0w3yXCmi1ebO2I4P7B0y9cqe%2FLpuc%2Bcnd4BcCLtZN5aDO3gCTpR4j2Ux3uXDH%2FqkdI41qdvCdP7JuQwV7hz%2F1EnMO%2FJzzCaRnfkXHRkL1eRogNyyy3RDCu8EnNiZ3eK502CevyOHC6OLfQMxihEPEFIFiH9gAuzevo4y5AsbW%2FlRe9ZSgTfo0QHfFea29kNXL4m7n2NTNK9avTWxRByjIPHnmdu4%2FIjO5yJ6u4FCDPBPxezVT8ggb35hV1igphYxuPOTY0iE--sLBXmHuTQa9R%2Fjnm88p%2FOaxLeXtlppPy2WdJ2%2FSsg7g%3D%3D%7Ctkp%3ABk9SR96Gr83QZQ

Acoustic 470 or 450?

Quote from: J M Fahey on April 29, 2025, 04:09:26 PMBoth jacks shown are wrong for thatb use.
To boot they are *wired* wrong.
Also external speaker jack must disconnect internal speaker when used, amp can not drive two speakers at once.
Too busy now but tonight will upload proper jack and wiring.

both jacks are supposed to disconnect the internal speaker when a 1/4" plug is inserted into the jack. I never intended to drive more than one speaker. I agree that neither switch is insulated.  Nor will I argue that I may have wired these incorrectly. I look forward to seeing your solution!


thanks,

Tom.

Quote from: g1 on April 29, 2025, 03:02:13 PMAlso, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.

Usng a metallic uninsulated jack bolted to chassis shorts a great part of NFB , increases gain big time and makes "natural" noise unbearable.


Solution is usin g a properly insulated jack, properly wired.

Both jacks shown are wrong for thatb use.
To boot they are *wired* wrong.
Also external speaker jack must disconnect internal speaker when used, amp can not drive two speakers at once.
Too busy now but tonight will upload proper jack and wiring.

I am currently working on an old Acoustic 450 amplifier. I narrowed the problem down to a blown PNP predriver transistor. The model number is 480041. I cannot find any information on this transistor. What would be a good substitute? Thanks.

Yes, it must be insulated from the chassis. 
The shielded jack you posted must have been wired wrong or defective.  It should have acted the same as the other did when it was grounded to chassis.
Also, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.
An insulated jack mounted where the original speaker wire went through the chassis should not pick up any more noise than the original wires did in the same spot.

That is the wrong type of jack to use in that amp.
The ring should not be connected to the chassis, it is not at 0V.
You need a jack with a plastic insulated mounting ring that does not connect the ring to the chassis.

Yes, not much of a heatsync on that kustom.  I was being dumb trying to match the numbers on the schematic instead of doing the math myself.  Should have known better, but making transistor subs is still pretty intimidating for me. Won't make that mistake in the future.  Thanks again!

I started with this jack from switchcraft

https://www.switchcraft.com/1-4-mono-2-conductor-jack-w-nut-and-washer-tip-is-shunt/12a/

installation method is splice it into the speaker wires. I drilled a hole in the back of chassis to mount it. The shunt jack favors the internal speaker unless an external cab is plugged in. This worked as expected, but there was a lot of external noise. It could've been the ground connection as the jack is not isolated. I disconnected the jack from the chassis to provide an air gap with only the speaker wires as a path to ground. There was no change in the noise level. Therefore, EMI.

I next installed a shielded shunt jack.

https://www.switchcraft.com/1-4-shielded-jack-uses-12a-little-jax-jacks-2-conductor/cn12a/

This solved the noise problem, but it would only work with the internal speaker. Plugging an external cab into this jack effectively muted the amp. Thinking about this now, it may be that the shielding can shorts the signal to ground. I could isolate it from the chassis with a couple of nylon washers and some shrink wrap the threads where it passes through the chassis.


thanks,

Tom.