Acoustic amp-48-15 output transistor replacement.

[tubeamptech]

I've got an Acoustic 140 amp with one shorted 48-15 output transistor. (480015). I've checked some old threads, and it's a toss up between MJ2955, MJ15015, and MJ15024. Reading the data sheets, and old threads, I'm leaning towards the MJ15015. Am I on the right track? There may be some updates to these possible substitutions.

And yes, I'll be going through all the other components in the power amp to make sure every things good.

I had an Acoustic amp a few years ago with the same transistor short, and found the answer here (Applaud Now!). Just can't remember which one I used. Too many repairs, and not enough jamming!

[Enzo]

Please figure out what you need. MJ2955 is PNP, while MJ15024 and MJ15015 are NPN.

The cross to the 480015 is the 2N3055. I'd probably just go with MJ15003, I always stock them. MJ15024 would surely work but would be even more overkill than the 15003s. Whatever you sub, I would sub all of them, not just the one bad one. I want all my output transistors the same so they all share current equally and track thermally.

There is nothing wrong with MJ15015, I just never stock them. Pretty much anything I could do with a 15015 I can also do with 15003, so to me they are redundant. Many amps use MJ15003 so I stock them.


The nice thing about amps like this is just about any transistor you come up with that meets the basic specs is going to work.

[tubeamptech]

Sorry, I mistyped. All 4 outputs are NPN. I guess my main concern was the gain. I want to keep the replacements from from oscillating. or running away. I'll check out the 15003.

I'm pretty sure the old thread I read about a replacement for these transistors was from you, and have never heard back from the owner. Thanks again.

[Enzo]

I don't know. Whatever it was, if I said MJ2955 it was a mistake. Or I may have mentioned them in another context within that post.


Now there is a recent thread about some amp with the number 3155 on the power transistors, and those DID cross to MJ2955. But 3155 is the amp maker part number, not the transistor type. And not a 2N3055. Could that be the thread?

[tubeamptech]

The thread was back in 2010, so I can't remember specifics. It's just the replacement that was suggested worked fine, and the amp is still working.

I found an old Acoustic document which suggests replacing them with 2N3773.

Acoustic_transistor_notes.pdf

[gbono]

2N3055/3773 will work but I would use whatever is the cheapest - probably the MJ series. Check the 1000uF capacitor on the output, the power resistors on the PA board will need to have their solder joints checked, and either remove the edge connectors and solder all wires to the preamp board or use some tie wraps to keep the connectors from working free. I have also run across issues with the power supply filter capacitor - pretty simple design to work with though. I would be interested in seeing what "improvements" were made with the Guitar Center version of this re-brand but I haven't had a chance to repair any.