Late 90's Peavey Transtube Supreme repair (power transistor questions)...

[RJBAudio]

First of all, I'll add that I'm new to this forum but have been involved with music and electronics for about 20 years now and stumbled upon this site while trying to help a friend out with a broken amp.

First inspection of the amp revealed a blown 4 amp line fuse and I determined that one of the DC voltage rails was just about shorted to ground which led me to a faulty power transistor (shorted between the emitter and collector). This amp only uses two power transistors in a push-pull configuration in the output stage and the evidence of solder flux tells me that both have been replaced in the past. The current "bad" transistor has the part number "70473100" (PNP) and it's compliment which measures to be good is a "70483100" (NPN).

I noticed that some online parts sources dedicated to guitar amp parts carry the original or equivalent replacement parts but I came across a thread on this forum where someone planned to use some MJ15015/MJ15016 power transistors as a substitute in a different Peavey head. It seems that these alternative transistors might be a bit more robust and make it less likely that my friend will be bringing the amp back to me any time soon ("10" isn't loud enough for him most of the time) but since I can't find much info on the original parts (only the substitutes) I'm not sure how well the substitution will work or if it has any advantages.

On a side note, (although I don't think it is) is it likely that something else might have caused this transistor to blow and replacing it with a new one might cause the new one to fail as well?

So I guess my main question is... should I just buy the original replacement for the 70473100 PNP transistor or should I go with a new set of the MJ15015/16 substitutes?

[oc disorder]

Welcome to the forum RJBAudio yeah this is a great resource for repairing amps etc !
You may find this handy
Peavey factory semiconductor cross reference
I'm the newbie! And I got questions, many questions!
(the above is the URL !)
posted by Enzo.

I think that you will find the 70483100/70473100 are in fact MJ15015/MJ15016.
Its highly likely the driver transistors have been stressed and need checking too.
Also check the nearby diodes.
The MJ15024/MJ15025 would be an upgrade in "ruggedness" but shouldn't
be necessary.
I gather Peavey U.S. are very helpful and should be able to supply you with schematics and spares.

[Enzo]

PV wil in fact send you the schematics, and oc is right, the MJ15015 and the 70483100 are the same part.

You can buy parts right from Peavey or buy the generics. The MJ15015 won;t be any more robust, it will be the same part. The part didn;t fail because it wasn;t robust enough, it just failed. That amp's brother may work 40 years and never need a transistor. Chances are the outputs are all that is bad, but oc is also correct that you should check the drivers and any resistors associated with the output stages.

With that single pair of output transistors, I recommend replacing the bad one and the "good" one as well.

ANd yes, if you wanted to go heavier duty, the MJ15024/15025 pair would be it.

here's the guide, I'll save you the trouble of finding it in the other thread.

[RJBAudio]

Thanks guys... amp is fixed!!!

Once again, thank you both for all of the information.

Since there was evidence that both transistors (70473100/70483100 and there are only two in the output stage) had been replaced before and the PNP one was bad now I decided to go with the more "robust" option of the MJ-15024/25 set. I drove over to a local parts distributor (where I usually go for pinball machine electrical parts) and checked to see what they had and picked up a set of equivalent replacements to the MJ-15024/25 set (models NTE388/NTE68) which have much higher voltage/current ratings than the original set. I figured that since this part of the amp seemed to be the weak spot of the design and that my friend really pushes it pretty hard that I would spend a bit extra to make it a bit less prone to failing in that stage.

I checked as much as I could in the power amp section to make sure everything else was good. I got everything together and the amp works great now. I told my friend, who owns the amp, that it was fixed and he was very excited (I only charged him for the price of the parts and he responded with a very nice "You're the man!"). I gave him a rough analogy to help him understand the "upgrade" so to speak by saying that it is like having a car with a specific engine that has a transmission that has failed twice and now the car still has the same engine (power) but a new transmission that is less prone to failure under the same high-stress conditions. Afterwards, I kinda felt bad saying this because I had forgotten that his car isn't on the road right now due to a bad transmission.

On a side note I just picked up a 2,700 watt Carver Sunfire MkIV plate amplifier for my basement home theater subwoofers (15" TC Sounds woofer in sealed enclosure x2 in parallel) from a friend on another forum for $75 with one minor issue (the auto-on circuit requires a stronger signal than usual to turn the amp on, so I'll have to look into it but it is a pretty complex amplifier). The amp sounds great and there was no way that I could have found a better deal on an amp for this role (my brother took the old Ashley PA amp that I was using before).

So all in all it was a pretty good day for me and I really appreciate the help. This seems like a great forum and you'll probably be hearing from me again in the near future (I have an Acoustic 370 bass head that works but has some turn-on static noise that lingers at a lower level during operation... I've looked into it a bit a while back but didn't get very far).

Thanks again!

[Enzo]

RJB - please oh please don't get addicted to the NTE stuff. They are not the real parts. They are not made on the same lines nor by the same processes. In the case of one opposing pair of outputs, OK, but NEVER mix them with other real parts - for example in a powr amp with three outputs per side instead of just one. They won't share current properly nor will they track thermally.

But one very practical consideration is cost. Allied sells MJ15024 and MJ15025 for $3.86 each. They want $12.38 for the NTE388. I shudder to think what they sell them for over the counter.

I recently needed a power triac for a repair, and we wanted it as fast as possible. My local parts store said they had them, so I ran down ther. Only to find they didn't have them they had the NTE cross instead. The $6 from Allied or Mouser was going to cost me about $24 in NTE. I politely declined. WHat on earth could I charge my customer for a part I had to pay $24 for, and still keep a straight face?

DOn;t get caught up in specsmanship either. The stock parts are quite reliable and sturdy. I have replaced many many MJ15024 type parts in amplifiers too. There is no part made that can't fail or be made to fail.

In an amp like this, the power rails are what, about 50v? 55 maybe? That means the parts never see more than about 100v across them, so the difference between 200v and 250v part is meaningless. Likewise, the 100 watts into 4 ohms means they conduct about 4-5 amps at full out.

If those transistors were marginal, we'd see a steady stream of Bandits in the shop with blown outputs. (Bandit and SUpreme share the same board.)

There is also something in transistors called "safe operating area" which refers to the graphic representation of their voltage and current ratings. That 16 amp 250v part cannot do both at once. If it conducts 16 amps, it cannot go near 250v, and likewise if it is to handle 250v, it does so at substantially reduced current. SO either/or, not both.

I suspect what happened to this amp is that one output failed, and someone replaced it. They may not have considered the stress placed on the survivor, and it failed at some later point as a result. SO both were replaced. Quite possibly the second time through they again only replaced the bad one, meaning the replacement from the first repair was now the stressed survivor. That is why I suggested replacing BOTH.

Like all PV solid state amps, the power ratings are realistic, and the amps are designed to run all day at their rated power.

But I am glad you made it work and your friend is happy.

[RJBAudio]

I guess I should have been a bit more patient and ordered the parts from the net. I'm more of a jack of all trades, master of none (except maybe DIY "passive" speakers) type of guy and went by the recommendation of the guy that works the counter at the local parts distributor who considered the NTE parts equivalent to the part numbers that I supplied to him. I paid about a dollar more than what you quoted for that NTE part and I guess that all that I saved is shipping costs. I guess all is well that ends well but hopefully I'll know better the next time. I often order parts from Mouser and have browsed the Newark and Digikey sites but wasn't aware of Allied.

[Enzo]

Allied is a huge parts house, and I buy almost all of my Motorola transistors from them. Motorola is now On Semiconductor. Same parts.

Allied Electronics - Electronic Components Distribution

Your parts guy carries the NTE line of general replacement parts. he looks up what you want and finds the NTE equivalent. The stuff may have ratings that are close, so they are "equivalent." What is he going to say to you anyway, "These won;t work?" Hardly.

Look, it isn't like I think they will blow up. I am on a personal crusade to wean people from NTE. Other techs may not be so strident about it as I am. I just want techs to understand that NTE crosses your part number and then offers the part in their line that is the closest match to the real part. That is very different from being identical to it.

What is real dangerous is when folks find a part, look it up in the NTE guide and assume the specs for the NTE part are the specs for the real part. Your real part may have a rating of - making up some numbers - say 40v and 300ma. NTE crosses it to their part with an 80v rating and 500ma. Their part may work just fine, but you cannot assume that because their part is 80v, 500ma that your original part was too. SO it is always shaky to spec your real parts using the NTE book. yet I know plenty of folks who when faced with an unknown transistor, will look it up in NTE rather than finding a data sheet for the original part online somewhere.

I find that most any small signal transistor I want to cross comes back down the NTE123. Odd that a thousand different transistors all are the same as their one part, maybe? I find that any time I need a sub, I can find one just as well as NTE can from my stock of general transistor types.

In signal transistors it doesn;t matter a lot, but output transistors in high power amplifiers, like a Peavey CS800 or something, it can mean the difference between working or burning up.

ANd I do realize the circumstances for a hobbyist shop are different from a commercial repair shop like mine. I keep drawers full of all those parts across the room from me, the hobbyist often goes out and buys what he needs on a case by case basis.

[tube power]

Hi, I'm new here. I found this thread through a google search of this exact subject.

I am about to replace the 70483100 w 9804
and the 70473100 w 9803.

I am using mj15024
and mj15025.

Would the lower number, 24 go where the 03 went
and the higher, 25 go where the 04 went?

Does it make a difference?

Thank you.

[Steve Conner]

The 9803 and 9804 are the manufacturing date codes. 3rd and 4th week of 1998.

Now in the first post:
"70473100" (PNP) and... "70483100" (NPN). I checked that in the PV cross ref.

The MJ15025 is PNP and the MJ15024 is NPN.

So the MJ15025 replaces the 70473100, the MJ15024 replaces the 70483100.

Yes it makes a difference, if you get it wrong the amp will go up in smoke.

[tube power]

Thanks Steve.

Obviously I have no business even doing this, but I want to salvage the amp if I can,
and no one around here will even look at it for less than $100.
I've done a little soldering and know enough to keep one hand in my pocket.

[tube power]

I have one other question.
I've built a few simple circuits and modded some pedals.
Transistors always had 3 legs, these only have two.
What is the situation in reguard to polarity with these?

[Enzo]

Look at them closely, the two legs are off center. The transistor will only go in the holes one way and still have the screw holes line up. And that third lead is there, it just isn;t a piece of wire. The metal case of the transsistor is the third lead. The two screws complete the circuit to the rest of the board. Also that is why such transistors usually have an insulator between the transistor and the metal heat sink it is mounted on.

[tube power]

Waiting for the new ones to arrive from Hong Kong.
Had to desolder the old ones to see what you meant.
Now it is obvious.
Thanks.

[J M Fahey]

Hong Kong???

[Gtr_tech]

Oof....watch it there. There are tons of fake (counterfiet) power transistors out there right now. I wouldn't order from "hong kong phooey" or any other unknown dealer. Get the parts from a reputable parts dealer (Digi-Key, Mouser, Allied, Newark, etc) or you will find out the hard way about whats going on.

Read this:
Counterfeit Transistors