I've got this Peavey Century power module which I managed to break (as usual)
http://peavey.com/forum/viewtopic.php?f ... 07#p218907
In the above thread we mention all the components between the 60W and 120W module are identical, except for the number of power transistors and corresponding emitter resistors. The 60W does 8 ohms. the 120W does 4 ohms
Looking back at it, Peavey use 4 2N3055's in the 120W and 2 2N3772's in the 60W
The 3055's are 15A 115W devices, the 3772's are 20A 150W devices, they are running them from a split 74V supply for 37V a side. Why the overkill, is it overkill? Or have the spec's changed over time and the devices can now handle more current? 37V @ 15A is 2.5 ohms
I'm thinking, if I have blown one of the transistors, which is what I suspect, I can just replace them with either a pair of 2N3055's or a pair of TIP3055's (plastic package) and still be OK to run them at 4 ohm. It's not I want it louder it's just I want to run a pair of 8 ohm speakers with it. Chime in if my vague math skills have let me down again I can double up on TIP3055's and get a couple of extra resistors if necessary.
Peavey Century Tranistor Substitutions
Re: Peavey Century Tranistor Substitutions
yeah well, if you have the cross ref, you also know both those types are part number 70484140, which is also an MJ15003. If I had a blown output or two in one of these amps, I'd just replace them all with MJ15003 and walk away, and not look back.
Go look at a data sheet for 2N3772 or 2N3055. here is a 2N3055 for you:
http://www.mouser.com/ds/2/389/CD00000895-490764.pdf
Scroll down to the graph labeled safe operating area. All power transistors have this, and similar results. Your 60v 15A transistor is good for 15A OR 60v, but not both at once. The chart shows you that to draw 15A through the part, it is OK up to about 8v. And to run 60v across it, the part is good only for about 200ma, 0.2A. Now that is DC, but it tells the tale. And the tale is this is not overkill at all.
In the 60w version, it says 22v RMS across 8 ohms at the output. 22vRMS means 31-32v peak of DC either side. That is a little under 4A through the 8 ohms, and thus through the output transistor. 15A at 37v never flows here. The power supply cannot source 15A anyway, and output can't swing all the way to the rails anyway. And that 3.88A at 31v is right at the edge of that SOA (safe operating area). You can't double the current through them safely. To get the 120w version, you really do need twice the transistors.
Note the 120w version is rated at 20v across 4 ohms, so basically the difference is simply twice the current output.
Go look at a data sheet for 2N3772 or 2N3055. here is a 2N3055 for you:
http://www.mouser.com/ds/2/389/CD00000895-490764.pdf
Scroll down to the graph labeled safe operating area. All power transistors have this, and similar results. Your 60v 15A transistor is good for 15A OR 60v, but not both at once. The chart shows you that to draw 15A through the part, it is OK up to about 8v. And to run 60v across it, the part is good only for about 200ma, 0.2A. Now that is DC, but it tells the tale. And the tale is this is not overkill at all.
In the 60w version, it says 22v RMS across 8 ohms at the output. 22vRMS means 31-32v peak of DC either side. That is a little under 4A through the 8 ohms, and thus through the output transistor. 15A at 37v never flows here. The power supply cannot source 15A anyway, and output can't swing all the way to the rails anyway. And that 3.88A at 31v is right at the edge of that SOA (safe operating area). You can't double the current through them safely. To get the 120w version, you really do need twice the transistors.
Note the 120w version is rated at 20v across 4 ohms, so basically the difference is simply twice the current output.
Re: Peavey Century Tranistor Substitutions
So the Motorola part is better? I ask because I hate having to order stuff, so if it can be got at Radioshack my life is easy. The curse of no longer having access to a parts room 
Aw dang I see what you mean, the MJ15003 has higher limits, which moves the SOA range, even at 40V it's good for 8 or 9A. Maybe I should order a bunch for my S500D and just get it running too. Cheers
While your on, your opinion of NTE? We were using them in my old shop with their cross references, but since then I've read people complaining about them being crap or substandard?
Aw dang I see what you mean, the MJ15003 has higher limits, which moves the SOA range, even at 40V it's good for 8 or 9A. Maybe I should order a bunch for my S500D and just get it running too. Cheers
While your on, your opinion of NTE? We were using them in my old shop with their cross references, but since then I've read people complaining about them being crap or substandard?
Re: Peavey Century Tranistor Substitutions
The 2Nxxxx parts might have been Moto as well. The MJ15003 is a stronger part, as you mention. You don't want to mix types, you want all the outputs to be the same type. The Moto part and the RCA part may be equivalent, but they are made by different processes. Especially in parallel transistor situations, it is important the transistors are the same type so they share current well. Otherwise one may turn on a little sooner than the other, leaving the other off and the early one hogging the current.
NTE sucks. TV repair shops and the like love them because they can just stock the NTE series and then some part or other in the line will be close enough to work in thousands of places. A small number of NTE types are supposed to be close enough for thousands of real parts. Well close is close, but real parts are even closer.
I am not saying they are bad transistors, they probably work within their specs like any other part. But if you look up some 40v 100ma transistor and a 80v 500ma transistor and find they cross to the same NTE number, doesn't that make you a little suspicious? Check this out, 2N3055 crosses to NTE130. Newark sells 2N3055 for $1.30. NTE130 they sell for $10.29. WHy should I pay nine extra dollars for this part? That is a lot to pay just to get a transistor wrapped in a poly bag.
I said we don't want to mix types, so if you wanted, you could replace all the outputs with NTE130, then they would all be alike. If you stick one NTE130 in with three other 2N3055s, then I am not confident they will share current well. The point is the NTE is what they would have you use from their line to replace your 2N3055, however it is NOT a 2N3055.
If you already have a shop full of NTE that is one thing, but if you have to order them, then why oh why not just order the real parts instead?
NTE sucks. TV repair shops and the like love them because they can just stock the NTE series and then some part or other in the line will be close enough to work in thousands of places. A small number of NTE types are supposed to be close enough for thousands of real parts. Well close is close, but real parts are even closer.
I am not saying they are bad transistors, they probably work within their specs like any other part. But if you look up some 40v 100ma transistor and a 80v 500ma transistor and find they cross to the same NTE number, doesn't that make you a little suspicious? Check this out, 2N3055 crosses to NTE130. Newark sells 2N3055 for $1.30. NTE130 they sell for $10.29. WHy should I pay nine extra dollars for this part? That is a lot to pay just to get a transistor wrapped in a poly bag.
I said we don't want to mix types, so if you wanted, you could replace all the outputs with NTE130, then they would all be alike. If you stick one NTE130 in with three other 2N3055s, then I am not confident they will share current well. The point is the NTE is what they would have you use from their line to replace your 2N3055, however it is NOT a 2N3055.
If you already have a shop full of NTE that is one thing, but if you have to order them, then why oh why not just order the real parts instead?
Re: Peavey Century Tranistor Substitutions
Thanks Enzo, I finally got around to it, and ordered 10 of the MJ15024 (after you said you only kept the 15003 for warranty repair and the 15024 was better still), two of which went back in the Century and it's running just fine now.
6 of the remaining 8 are going into my old H&H S-500D, which blew a channel, and didn't appreciate me replacing the single blown transistor with a 2N3055 which was the only part I could get locally at the time. That's a project for when I have lots of time to solder and sleeve all the legs.
6 of the remaining 8 are going into my old H&H S-500D, which blew a channel, and didn't appreciate me replacing the single blown transistor with a 2N3055 which was the only part I could get locally at the time. That's a project for when I have lots of time to solder and sleeve all the legs.