What makes a good audio output transistor?

Carbsandcaps

Carbsandcaps

Super Member
I’ve been working with a lot of T0-3 based amplifier setups lately and finding an inexpensive replacement for a T-03 output transistor is next to impossible. Going on maximum voltage and amperage alone can bring up some matches that at least won’t blow up and produce sound but at what quality.

For instance I’ve come across 2n3055 which are NPN, 100v and 15 amp but seem to be made for motor controllers and industrial applications but for $1 each I ordered 10 because I’m undoubtedly going to blow up a few as I go through this learning process. I assume these would work in an amp but would sound quality suffer? What other specs should I be looking for when looking for a replacement for a discontinued or overpriced transistor?
 
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Lots of amps ran 2N3055 transistors.

If you're looking for cheaper subs for a TO-3, consider maybe a TO3P package. Its a plastic rectangle but they can be made to fit in the cutout of a TO-3, just have to bend the legs around properly. 2SC5200 and whatever the PNP is (2SA1943?? something like that) are common.

solid state amps tend to have so much feedback that I have my doubts that the original application of the transistor matters all that much. If it was going to affect it enough to hear, a distortion check would easily show it. If it measures at or below rated distortion, its fine.
 
gadget73 said:
Lots of amps ran 2N3055 transistors.

If you're looking for cheaper subs for a TO-3, consider maybe a TO3P package. Its a plastic rectangle but they can be made to fit in the cutout of a TO-3, just have to bend the legs around properly. 2SC5200 and whatever the PNP is (2SA1943?? something like that) are common.

solid state amps tend to have so much feedback that I have my doubts that the original application of the transistor matters all that much. If it was going to affect it enough to hear, a distortion check would easily show it. If it measures at or below rated distortion, its fine.
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I am going to look into that. Maybe my next project I’ll try it. I was just reading online that the 2n3055 has too low a frequency ect but it seems to be a very common transistor that is till available and has a decent power rating. I guess what I’m getting at is in comparison with an output tube which can have a different base, different plate voltage, dissipation ect. That the transistor has more options.
 
gadget73 said:
Lots of amps ran 2N3055 transistors.

If you're looking for cheaper subs for a TO-3, consider maybe a TO3P package. Its a plastic rectangle but they can be made to fit in the cutout of a TO-3, just have to bend the legs around properly. 2SC5200 and whatever the PNP is (2SA1943?? something like that) are common.

solid state amps tend to have so much feedback that I have my doubts that the original application of the transistor matters all that much. If it was going to affect it enough to hear, a distortion check would easily show it. If it measures at or below rated distortion, it’s fine.
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I just looked into it. Looks like the to-3p is the replacement for the to-3 and the legs are properly spaced for it. I’ll definitely be going that way next time especially since the original mounting holes are kept intact. Thank you for the idea!
 
way back when (60's and early 70's) the RCA 2N3055 was considered the workhorse at 15A. A lot of units including Guitar amps used them. But be careful as max C to E voltage is only 60V.
 
Carbsandcaps said:
They almost certainly are Chinese junk. But I can get them from amazon in 2 days. They would just be for testing.
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Ya, but if one or more go up in smoke, that could take out components up stream. On-Semi from Mouser or Digikey would make me a lot more comfortable.
 
The problem is with Gain (Hfe) matching.

You need a gain of 40 to 60 for a good audio output Q and matched.

Most inexpensive power transistors only have between 10 and 30 gain.

2N3055 specs say 20 to 70 Hfe.

I recently replaced 3055's with 2SD188's.

The 3055's from the good channel were all matched at 50 Hfe gain.

Had to go through 40 of the 188's to find 4 that were very close to 50 Hfe.

Worked great. They all tested from 15 to 70 Hfe.
 
sregor said:
A good alternative in TO-3P style cases (also TO-247 or TO-218) is TIP35C and TIP36C.
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That’s funny I was just looking up some to-247 case transistors and I came across the tip35c for around $2 each on digikey. Think I’ll order a few and do some experimenting.
 
The 2N3055 was one of the first transistors suitable for power amps, and many companies used them from the early 60s into the early 70s. The original devices used a hometaxial fabrication process that made them quite robust but which limited their bandwidth. Fortunately, though, a properly-designed circuit could have very flat frequency response to 20 kHz and beyond. (One example is the AR AU amplifier.) Modern 2N3055s, as well as all modern power transistors, have much wider bandwidth, up into the MHz. This makes them prone to spontaneous ultrasonic oscillations that can damage tweeters, so if you substitute one of the newer transistors, you should watch out for this problem.
 
I'm sure electronically they would perform excellently. I work with mainly Yamaha.. Due to the Yamaha heatsinks used with TO-3's . You would need a little modification to use the existing TO-3 sockets. Trimming a corner off of the transistor or milling a relief in the heatsink.
 
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