Replacement power transistors for old JC-120?

[uvacom]

Cool. I don't think I can go any further than this without actually having the amp. Thank you so much, Steve - you obviously know your stuff and I appreciate you sharing your time and expertise to a stranger. That goes for everybody else who has helped, too.

One small, final question - are there any books published which focus on this kind of amplifier design? All I really have are basic semiconductor textbooks and The Art of Electronics. I'd really like to do a redesign of this circuit but clearly my knowledge is not yet up to the task.

[olddawg]

I would really check out the difamps. If they have drifted you will slam the next stage with dc. If they are duals you can use individual transistors but will need to match their gains with an hfe meter.

[Steve Conner]

Sure, check all the transistors before you blow, err, power it up again. If there's one bad component in there that you missed, it can take out all of your replacement parts as soon as you power it back up, which can be really demoralising. The input pair "diff amps" would be one such thing.

At least have a read through the forum, there are lots of tricks you can use like light bulb limiters and so on.

The best book I know of on the subject is Douglas Self's Audio Power Amplifier Design Handbook. I got my copy from Amazon. It's geared towards hi-fi and squishing that last .01% of distortion, though. For guitar specific designs, there's a book by Teemu Kyttala available free as a PDF: http://music-electronics-forum.com/t14457/

[R.G.]

Audio power transistors have improved by about four generations since the 70s. I've heard the MJ15024/5 recommended as bombproof TO3 super transistors that work in just about anything. There are also quality TO220 driver transistors in the MJ range.

Edit: The driver transistors I was thinking of are the MJE15030, 1, 2, and 3 (NPN, PNP, different voltage ratings)

I looked at the datasheets for the original parts, and they have remarkably high hfe for that kind of transistor. I don't know of anything like that with a guaranteed minimum gain of 120. The MJE15032/3 are only guaranteed to have a gain of 50, and likewise for the TIP parts.

Nota bene:

The audio power amplifier bipolar output transistor world was turned over by the release of Toshiba's sustained-beta devices, the 2SC3281 and 2SA1302 (if I remember the numbers correctly). These were notable for
- sustained gain; gain was about 70-100 all the way out to about 10A
- high frequency capability
- massively large SOA than predecessors
- inexpensive; about $4.00.

They are, of course, heavily counterfeited now, and no longer made by Toshiba.

However, Toshiba does make the successors, with different part numbers - one was 2SC5200, I think. Motorola, then ON Semi makes a line of second source and enhanced sustained beta devices as the MJL3281 and MJL1302, but the real hot tickets are On Semi's NJL4281DG and NJL4302DG, which include what has been missing from power transistors ever since they were invented - an internal thermal tracking diode.

They also make several similar if slightly lower power devices without tracking diodes, notably the NJ0281G and NJ0302G, which are only $0.93 in large quantity and only $1.40 each in ones at Mouser.

[Phostenix]

however, toshiba does make the successors, with different part numbers - one was 2sc5200, i think.

2sc5200 & 2sa1943

[Steve Conner]

Well, when I said that audio BJTs had improved by 4 generations, I neglected to mention that 2 of them (the 3281/1302, and the 5200/1943) came after the MJ15024/5 and MJ15032/3. :-(

Why did I not mention it? Well, weren't the original outputs TO3s? All of the wonderful sustained-beta transistors listed by RG are TO-3P, TO-247 or whatever. So they won't fit. The MJ's are about the best parts you can get in a TO-3, unless you can find some MJ21194s, and even they're not really "sustained-beta".

RG: I knew that Sanken tried the tracking diode thing, but their parts were "Until stock exhausted" on Farnell. So I assumed they gave up on it. I didn't know that OnSemi were still making power BJTs with thermal tracking diodes.

[Phostenix]

... came after the MJ15024/5 and MJ15032/3. :-(

A search for MJ15032 & MJ15033 at onsemi.com returns "Did you mean MJ15023?"

[Steve Conner]

I think I meant MJE15032 and MJE15033: a complementary pair of TO-220 driver transistors.
Edit: the first generation was the 2N3055 :-)

[Enzo]

MJ15024 is the Swiss Army TO3, and MJE1503x is the TO220 guy.

[Mars Amp Repair]

Say, I don't want to sound 'ignant' here, but I believe if I have a question there's gotta be others that have it, too.

Now I've been throught solid state amp design, albeit a lonnnnnnnng time ago, but I'm not familiar with the abreviations of 'SOA' or 'VAS'.
Would someone please enlighten me & the others who perhaps are not familiar.

I've found Steve's dissertation here very enlightening & remindful of those bonny by-gone days of college. Thanx, glen

[Phostenix]

SOA:

Semiconductor Safe Operating Area

[R.G.]

Well, when I said that audio BJTs had improved by 4 generations, I neglected to mention that 2 of them (the 3281/1302, and the 5200/1943) came after the MJ15024/5 and MJ15032/3. :-(

Why did I not mention it? Well, weren't the original outputs TO3s? All of the wonderful sustained-beta transistors listed by RG are TO-3P, TO-247 or whatever. So they won't fit. The MJ's are about the best parts you can get in a TO-3, unless you can find some MJ21194s, and even they're not really "sustained-beta".

You are right about that. Nobody makes a sustained-beta in a TO-3 can, which is a pity. Metal cases are much more reliable. Sigh.

When I have to replace TO-3s with plastic, I try to use spring clips like this in one of the TO-3 holes to mount the new device. There is often enough room to do that.

I think that spring clips are a superior way to mount plastic devices because they apply the correct pressure without any danger of cracking the device or being too loose, and give less problems with insulating the bolts through the heat sink.

RG: I knew that Sanken tried the tracking diode thing, but their parts were "Until stock exhausted" on Farnell. So I assumed they gave up on it. I didn't know that OnSemi were still making power BJTs with thermal tracking diodes.

Yeah, I was disappointed to see that notation, too; also surprised to find the ON Semi devices. I think they're fairly new.

I've spent some time thinking about why all power transistors don't come with thermal sensing diodes inside, and all I can come up with is that there was no MBA-recognizable need for improvement back when they were hot sales items and that these days there's not much money in discretes anymore. Anytime you say "power X" where X is any power device, you need to be worried about temperature and an on-chip sensor would be incredibly useful in many cases, even if the designer chose to ignore it sometimes.

Another of those mysteries, I guess.

[Steve Conner]

Well, I started out trying to help some guy fix a JC120, and ended up learning 5 things I never knew about power BJTs myself.

I was going to use the MJ15024/5 for my current project, but I think I'll get some of the NJ parts with the tracking diodes instead. Farnell have those, albeit as US stock, or it might even be possible to get small quantities straight from ONSemi. Good job I didn't drill the heatsinks yet!

SOA is indeed Safe Operating Area.

VAS stands for "Voltage Amplifier Stage". I'm not sure if it's a standard abbreviation, it may just be a Douglas Self-ism. In any case, the standard solid-state power amp has three stages: input pair at the beginning, output stage at the end, and the VAS is the single transistor in the middle, acting as a common-emitter amp, that provides all of the voltage gain.

[Mars Amp Repair]

Ahh...got it.
I am aware of the different stages of a power amp, but never saw them abreviated. Thanx for the SOA abreviation, too. Thanx, glen

[Enzo]

I'm with you Glen, I only recently have noted the VAS abbreviation. We always referred to it as the voltage amp - no abbreviation. I think someone referred to it as the VBE stage recently.