Match Output Transistors?

[Bill Moore]

I have a Peavey Special 150 that a friend gave me years ago, (dead). He already had replaced some parts, and had given up, (blowing fuses). I repaired some open traces on the board, but it still had a draw on the power amp side. The original outputs were replaced, but with replacement numbers, I tested them all, and one is shorted, (they were incorrectly installed)! I'm not sure if I can find the replacement number of the dead one, and was considering replacing all, is that really necessary?
If I have a bad output tube in push-pull, I always replace them all, but I'm only wanting to get this project working, not rebuilt!

[DrGonz78]

When I have one failed output transistor failed in a set I typically will replace the entire set. Not only that I will replace the drivers and pre-drivers that associate with those output transistors. You want to get it just working again but you want it to keep working for many years to come. There was one case where I had replace 3 output transistors on one side of a PA system and left the working originals in place. That amp still works to this day and that was over 5 years ago. In that case I was feeling mighty frugal and got really lucky. I still recommend against trying to get lucky. 

[Bill Moore]

Thank you!
I guess I was trying to be cheap, but worried that mixing replacement part numbers wouldn't work. I do have the driver transistors, (one of those was also shorted), maybe I'll learn something from the experience.

[Enzo]

And also check every associated resistor for open.

Unlike 6L6s, these output transistors do not cost $20 apiece., typically a set of four costs less than a single 6L6.

The schematic calls for 8226 and 8227.   Those are short for 70484140 and 70474140, which are Peavey part numbers for MJ15052 and MJ15053.

PV will send you the schematic file and their semiconductor cross reference guide, both, if you ask customer service there.

Here in the shop, I will generally replace them with the sturdier MJ15024 and MJ15025.  The outputs work in parallel pairs, I would not mix types.  The drivers are not parallel to anything so sub them freely.

[Bill Moore]

Thank you also, Enzo!
Peavey was kind enough to e-mail the Schematic/Layout, and I found the cross reference on the web.
I measured for opens across the resistors, and lifted a leg of the diodes to check them. Maybe I'll check the resistors once more while the transistors are out of the circuit. My friend had given me a hand full of components with the chassis, and I changed the P/S caps, but the originals tested good. I still need to test the rest of the transistors, it really would be nice if they had sockets! I have been clipping an alligator clip to the leg I am lifting, trying not to overheat the things!
The rear chassis/heatsink was missing, I found one for an older Special 112 that I've modified. It doesn't have as many fins, but I think it will work well enough, (I doubt it will ever be asked to run at full power.

[DrGonz78]

Recently, I did a purchase order from Peavey and was a bit surprised at the cost of the MJ15024 & MJ15025 transistors, $10 each. I ended up buying them from Mouser for about $5-6 each.

[Bill Moore]

It could be that Peavey is the only source for the MJ 15052 MJ 15053, as a search doesn't yield much. I suppose there is another interchange I may have to pursue.

[Enzo]

Note, the guide calls out the part number as a MJ15052.  If you read the guide you will find other real world transistors use the same PV part number.   So MJ15003/4 would work.  PV sells parts that meet their specs, not necessarily the "real" industry part.  Read the part about how they made the part numbers.

[Bill Moore]

Thanks, I see now, use the "other" in house number for a guide. I ordered the outputs for about $14 shipped, need some time to look for the others.

[Bill Moore]

Well, I found parts, and replaced the outputs, as well as the other transistors in the power amp section. I installed the limiter, and evidently I lost my short, so I decided to connect a speaker. Now I get a hum, and it starts to "squawk", before I shut off power!
I suppose I need to look at the preamp section now!

[Bill Moore]

Finally got back to it, and tested all the preamp TO-92 transistors. At least one of each pair were faulty! I found the equivalents, installed them, and it lives! I removed the limiter and connected to A/C, and played my guitar through it, sounds OK.
I now need to modify the Special 112 rear cover/heat sink to fit, still debating whether to add more cooling fins, (the Special 130/150 are a lot larger). I can leave the footswitch jack, but need to lift the pre out, and power amp in, because of the spacing. I may just jumper the power amp jack connections, and forget about them. Since there are holes for speaker jacks, I'll use std. switchcraft and take advantage of them. They are marked 4 and 8 ohm, so I assume the 112 had a transformer output, parallel will have to do.