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:

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.

Recently purchased this head from a guy on Craigslist for $50... Once I got home, I plugged it in, and then... nothing. There is a "thump" when you plug the cab into the amp, though. Checked the output and I am getting almost 40vdc from the speaker output. Cool.

Two resistors were completely fried, and one of the driver transistors was actually melted. I tested the four to-3 output transistors, and they checked out OK. The driver transistors seemed bad. at least one had a short between collector and emitter/base. I tested all the smaller to-92 transistors on the board, and wasn't able to come to any real conclusion, but am going to assume they are BAD. In any case, I ordered all new transistors, except for the output ones.

In the picture, the fried resistor to the left (680 ohm) reads -42v on one side, and 21 on the other. I assume this is a symptom of a dead transistor. The other fried resistor is a 100 ohm.

Anyway, just want to make sure I am on the right track here. No schematic available, only a few pictures online. I've e-mailed peavey for a manual.

Hello all, (obviously) my first time posting here. I've been building and repairing tube amps for some time, but solid state technology is uncharted territory for me.

I recently acquired an old Jordan Entertainer amp. There is very little information available online regarding this amp. In fact, the schematic inside of the amp is actually wrong. The amp was working for awhile, until it was jostled while playing, and made a loud hum. I found that the rectifier diodes were physically damaged. i replaced them, and the amp worked for awhile. I was playing, and the same thing happened, this time taking a resistor with it. this resistor is wired in series with a diode, connecting the base of two output transistors. the schematic inside of the amp does not show this resistor, only the diode. the resistor was so completely burned, that it couldn't be identified. i replaced the resistor with a 22k and replaced the diode as well.

so now, the amp powers on again, and stays on with no issues, but no sound. only the slightest bit of 60hz hum, which is not noticeable until your ear is placed right on the driver. with the volume maxed out, the hum gets only SLIGHTLY louder. the amp has spring reverb, and you can actually HEAR sound coming from the springs/tank itself. So, SOMETHING is working, but it isn't the output section.

Seems it would be obvious to replace the output transistors, but before I do that I want to make sure there isn't something else I might be missing?

EDIT: I suppose I should clarify, when I say "the same thing happened" I'm referring to the symptom (hum), and not the physically damaged rectifier diodes.