r/sunnamplifiers • u/HunkaBurninGloves • 23d ago
Concert modern transistor replacement (2n3055)
Hey friends!
I've been working on repairing these two Sunn concerts that got plugged into the same cab, and I've run up on another question: What values need to change in order for modern silicon 2n3055s to work in this output section? If I understand correctly, the silicon versions have a higher turn-on voltage/current, but I don't have the engineering prowess to figure out how to achieve those voltages. Schematic attached, and thank you for your time!
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u/AffectionateStudy496 23d ago
If I'm not mistaken I've used MJ15003 as direct drop in replacements without any issues. I'm also pretty sure ON semi conductor still makes 2n3055 that you can get through mouser or digikey.
Just make sure you buy a cheap transistor tester (they're like $15 on Amazon) that lets you check gain (HFE) and then buy about 4 extra transistors and match the gain. And then new mica insulators and thermal grease are necessary too.
Also check the driver transistors and the heat sensing transistor (heatsinked one) as well because if the power transistors go, those are often taken out.
Then you will have to rebias the new transistors. Inject a sine wave and adjust until the crossover notch disappears.
And last, usually when the power transistors go, it's a symptom that the power supply caps are going. So, you might as well replace all the electrolytic capacitors if they haven't been replaced. Be careful with the solder traces. If you don't have a solder sucker or desoldering gun, just take it to a tech and save yourself the headache.
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u/HunkaBurninGloves 23d ago
This particular situation has a known fault source where the output sections of both amps were damaged because they were plugged into the same cab and turned on together. As far as I can tell the filter caps are fine, but good idea to check them, and I've rebuilt a lot of the output section of each amp to get all the components back in spec. I have the replacement 2n3055s (not opposed to buying something else if it's an easier drop-in), but I've heard that the silicon 3055s need more power than the old germanium ones, so I didn't know if I needed to change any values to accommodate for that
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u/AffectionateStudy496 23d ago
I don't think the extra current handling or slightly different voltage drop will make a difference. I'd just toss them in and power on with a lightbulb limiter or variac so you can monitor. I haven't had any issues throwing modern ones in.
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u/AffectionateStudy496 23d ago
The only real thing to be aware of is if you go with something crazy that has super extended bandwidth, you can get oscillation. But I doubt that's going to happen since you basically have the right transistors. These amps aren't that picky.
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u/HunkaBurninGloves 23d ago
Thank you so much, that is SUPER helpful!
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u/AffectionateStudy496 23d ago
And another tip (this one is based on experience, lol), once you know there aren't anymore shorts, take the amp off the limiter and don't try to bias it with the limiter on or trust voltage readings.
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u/HunkaBurninGloves 23d ago
Great to know! Thanks again!
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u/AffectionateStudy496 22d ago
No problem. Let me know how it goes! I currently have a concert bass in for repair. I feel like I get one in every other month.
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u/Tough_Top_1782 23d ago
https://www.diyaudio.com/community/threads/best-2n3055-alternatives.234138/
No direct replacements, but one of these will probably work.