Ampeg BA115HP transistor fried. Need advice on what to replace

[Steve Fundy]

Aloha all. I was working on an Ampeg BA115HP earlier which came in sounding pretty crappy. I was in the midst of adjusting bias and turned the bias pot a bit too far CCW and POW. Blew fuse and at least 1 MOSFET. So now I need to buy some parts. Advice on what all I should replace? All 4 MOSFETS, but then diodes? Drivers? Thermistor? I usually work on tube amps and have considerably less experience with solid state amps..any advice appreciated.
Mahalo, Steve

[nsubulysses]

It's probably unlikely that anything else is shorted other than output transistors. If you have a light bulb limiter you could unsolder the output transistors and turn the amp on, on the current limitier, to see if it lights the lamp still.

Diodes can be checked just like diodes in any other amp so use a VOM on resistance setting to see if they conduct both or neither ways. Or use a DMM on diode setting.

[Steve Fundy]

Ok I like this solution. I do have a light bulb limiter. I figured I could test the diodes as per the usual way, but I wasn't sure about the drivers. I think I only shorted one of the four MOSFETS but I guess it's smart to replace all 4? They are cheap. Thanks for the reply.
Steve

[nsubulysses]

People say to replace them in sets so they have better chance of being evenly matched because they'll likely be from the same production batch. And yes, they are cheap anyway.

Drivers are just regular old transistors and can be checked same way you check diodes check from B to C and B to E in one multimeter probe polarity and then B to C and B to E with probes switched. Should conduct one way and not the other. May need to pull them from circuit for reliable results

You might also want to check that no .33 ohm 10W emitter resistors are shorted as well since some of the output mosfets have gone.

[Enzo]

My vote is any time an output transistor dies, I pretty much automatically replace the driver feeding it. I figure even if it didn;t kill the driver, the shorted output could easily have stressed or damaged the driver. And for the few pennies a driver costs, it is worth it for peace of mind.

The 0.33 ohm resistors are more likely to open under stress than to short.

[Steve Fundy]

My vote is any time an output transistor dies, I pretty much automatically replace the driver feeding it. I figure even if it didn;t kill the driver, the shorted output could easily have stressed or damaged the driver. And for the few pennies a driver costs, it is worth it for peace of mind.

The 0.33 ohm resistors are more likely to open under stress than to short.

Yeah I was thinking along the same lines. I didn't think about the .33 ohm resistors, but a good idea. Thanks

[nsubulysses]

Enzo, I think you said in the past somewhere that it is not good for certain amps to turn the amp on without output transistors connected. I sometimes do this just as a quick flip on flip off test in a current limited scenario and haven't blown stuff up because of it. But, unless I am mistaken, what is the scenario ??

Maybe you just meant not to hook it up to mains and flip the switch or are there certain output topologies that this could not be a good?

in this case I recommended to do this because I did that when I had an ampeg BA115 with shorted output devices a few weeks ago. I didn't replace the driver transistors because they were fine but I trust Enzo much more than myself so maybe that will be my new way

[Enzo]

I don't like to do that with the BJT outputs, because the circuit is expecting the voltage drop of the BE junction in the outputs. In the case of these MOSFETs, it is a different story. Go ahead.


As to just a quick flip, electricity moves a hell of a lot faster than your switch finger. You can burn something out in an instant. If you think something has to only be on for a second to be safe, then don't. That is why we use bulb limiters and variacs with ammeter.

[J M Fahey]

Oh, I play Russian Roulette safely!!!
If hammer falls on an empty chamber, I hear a metallic click, so no big deal.
And if I hear a BANG! , I move my head away from the muzzle

[nsubulysses]

i would not trust myself to be faster than an electron obviously . I trust the current limiter to protect but I know it's not full proof (fool proof ?)

When you say you dont like to do that with bjt because of expected voltage drop are you saying that In the same way of turning on a tube amp with no tubes, because without PS voltages pulled down by current draw it may exceed max V for some components ?

[Enzo]

No, not at all. By voltage drops I mean the BE junction of the output transistor is in parallel with part of the driver circuit, and that junction is part of the circuit. When you remove it, the driver now has to carry all currents and in some amps that can damage the driver. Nothing at all to do with power supply levels shifting. The voltage drop across the junction is to what I refer. If you prefer not to think of it that way, just think the current path the heavy duty output represents will be missing.

[nsubulysses]

Ah, I see. thanks for explaining .