No schematic available that I could find, unfortunately. Found the pinout of this particular transistors here: http://www.theremin.us/144/142transistors.htm

I made some notes when testing the transistors with the diode test function on my dmm. They all tested the same:

source to drain - .133v (both directions)
source to gate  - .733v (no reading with leads reversed)
gate to drain   - .733v (no reading with leads reversed)

Something concerns me, and I'm not sure if it's related to the problem, but the amp uses a Mallory can type capacitor with the values, (2x) 20uf, 450v, (2x) 100uf, 50v. The cap is getting extremely hot. At least as hot as the 6L6, which are running with nearly 500v on the plate. I have these values on hand so recapping will not be a problem, but wondering if these things could be related.

I have an Alamo Paragon Bass amp (ss pre, tube poweramp) with no output. Traced the issue back to a faulty preamp. It consists of four to-92 JFET transistors (TIS58). Can't find any data about these guys. Also, I've never encountered JFET transistors, only bipolar. No idea how to test them to determine whether or not they are bad.

By the way, the Ampeg BT-15 and Jordan Entertainer are working beautifully. Thanks guys!  :dbtu:

Finally received the components for this amp.

Replaced most of the transistors. Bad ones, and ones I was on the fence about.

Powered up through a 60w current limiter, and it works... Sortof. After a few seconds, the bulb begins to glow, and the amp hums badly, and the output begins to distort. The bulb gets brighter, (still orange-ish) and I notice a tiny bit of smoke coming from the board. Quickly shut it off and tried to feel which component was getting hot, but I couldn't tell.

I replaced the "mystery" resistor with a 15ohm. Roly, I suppose your right. As worthless as this amp is, the experience is definitely worth something!

Alot of my older amps (pre 1970's) have orange/brown gunk on the boards. Always figured it was flux.

Wondering if I can use these to replace to TO-66, as I have them on hand:

http://www.mouser.com/Search/ProductDetail.aspx?R=TIP29Cvirtualkey51120000virtualkey511-TIP29C
http://www.mouser.com/Search/ProductDetail.aspx?R=TIP32Cvirtualkey51120000virtualkey511-TIP32C

I guess I should come clean about the EL panel. The fuse to the panel was blown so I shorted it with a wire. I actually had no idea the panel was supposed to light up until about two years after owning it. The panel and amp worked fine, but the second time I used it "loud" with the panel functioning, the amp "died". I don't know if these things could be related... 

Anyway, here are some pictures. The TO220's that I ordered to fix the peavey (I had leftovers) might be a useable replacement for the to66 drivers, I haven't checked...

JM, thanks for all of the advice... I will have a chance to work on this again later this evening and will post my findings.

Thanks for all of the replies!

I did test all of the transistors from C to B, C to E (all tested open) and E to B, and also doing these tests with the leads reversed. All tested as "probably maybe okay".  Also, there are no bad solder joints or broken copper traces. I spent about 2 hours one evening giving the board a good long look, and removed any residue, and making sure no solder had melted, or shifted somewhere it shouldn't be. This amp gets very hot. Actually, I've read alot of literature on this amp, and someone working for Ampeg described this amp as getting "hot enough to fry and egg on". I'm actually surprised it's lasted this long.

Anyway...

I built the limiter. the 200vac winding is fine, the electroluminescent panel lights up as it should. When the 56vac leads are connected to the board, there is almost a complete short and the bulb glows as bright as it can (60w, sorry Roly). This is with all output transistors in place.

When the output transistors are removed, the limiter bulb glows, but much less bright. Also, R830 begins to cook itself. When the amp died originally, R645 (33ohm) was toast. This amp has no fuse, actually. I'm not sure why the fuse holder was removed, but if I get the amp functioning, it will most definitely be added.

Edit: V+ rails are -50.0vdc and 50.1vdc with limiter in place.

Also, the schematic calls for 2500uF 80v caps. Whoever recapped it used 2200uF 200v caps. I've heard of going higher in capacitance, but not lower. Besides storing way too much voltage, should I be concerned?

I guess it's worth mentioning that most transistors measure about .6 or .7 v from collector to base with the diode test function, which seemed a little high. These are very old transistors, though.

I get an earful about shotgun repairs from you guys. What are my options, besides replacing all semicons on the board? The little TO66 guys (2n3767, 2n3741) ain't cheap... about 8 bucks a pop.

So, a little backstory on this amp before I discuss the issue(s).

This amp has been played alot since we bought it. It's been on month long tours, played every night at (almost) max volume. It's always been great, and never needed any work besides a cap job and some new pots. One night at practice, something failed, and the amp started humming, and no sound came out.

I took it home, tested the output transistors, they were bad. replaced them and the 33ohm resistor that blew, and the 10ohm that looked a little crispy. I turned it on, and the same thing happened, taking out the output transistors. Keep in mind I did this awhile back, before I had any experience with solid state gear (stupid). Anyway, it's been sitting in a corner ever since.

I started tinkering around with it last night. I tested every transistor on the board, as well as the TO66 transistors, and every one of them tested okay. The 300ohm resistors are okay, as well as the .47ohm. The power section has all new caps. Transformer is fine, no shorted diodes anywhere.

Is it possible that a transistor can test okay, but fail when under a working load? That's the only conclusion I can come to at this point.

Wow Roly, thanks for that super informative post. It answered alot of questions for me. I didn't know that Darlingtons doubled the gain, so that is very good to know.

I guess I am "shotgunning" this amp. Some of the rectifier diodes crumbled and one of the transistors fell into pieces when i removed the heatsink. some resistors also seem a bit "crumbly". I'm not sure if its due to age, cheap components, or a combination.. I figured I'd replace all of the critical components. If it fails again it will probably just go in the trash, it's simply not worth the labor, hah.

sorry to revive this. just getting around to replacing the dead transistors. I haven't had *too* much trouble locating modern equivalents, but I'm having a difficult time with the 2N5227. (http://alltransistors.com/transistor.php?transistor=4648)

I'm wondering if this is an appropriate replacement: http://www.mouser.com/ProductDetail/Central-Semiconductor/MPSA64/?qs=sGAEpiMZZMutXGli8Ay4kHKRdIvtrnJvR850nOB1k%252bo%3d

Really, all of the to-92's in this amp are hard to cross reference. So, to avoid asking this question EVERY time I'm stuck, what are the most critical parameters when it comes to replacing old transistors?

Okay, thanks for the advice. I replaced the dead components, and the amp works great now. Sounds great, too. Reminds me alot of a Twin.

When the amp is powered on, i get about 50mV of DC voltage form the output. This reading diminishes the longer the amp has been on. When I turn the amp off, I get about 8vdc, but only for a moment. Is this normal? I'm assuming that is the "thump" that comes from powering the amp on and off.

Sorry for the stupid questions! I don't mess with much solid state gear. 

Enzo, I didn't replace the 13886 diodes. I only replaced the diodes because, well, they're old. I've seen diodes fail in old equipment like this, and they typically short which causes a slew of other problems. Just playing it safe.

Enzo, you were right. The resistor that I assumed was a 680 is actually a 15. the other is 10.

Roly, I've put a red slash through everything that failed/burned. Any idea as to what could have triggered this failure?

I replaced all of the diodes with in4007, except for the P-600D. Didn't have any on hand, and they tested fine.

I tested the 10w .33ohm power resistors in circuit, and they tested fine. Should I test them out of the circuit? Seems like if they burned through, there would be some obvious damage to the board.

Thanks for the replies!

Ok, so Peavey responded with the schematic, which I've attached.

This is the youtube "tutorial" I used as a guide to test these output transistors:
http://www.youtube.com/watch?v=REW2lF0sb74

All of the TO3's passed the tests in the video. If there are more thorough tests to be conducted, let me know. I tested all four OUT of the circuit, at least three times a piece.