Ampeg BA115 very loud humming sound

[evil79]

I've got an ampeg ba115, and i just replaced the power transistors and everything worked fine for a few weeks, and now when i turn it on, there is a very loud humming sound, so loud i have to immediately turn off the amp. i thought it would've been the filter caps, so i replaced them, but its still doing the same thing. Any advice?

[The Dude]

Disconnect the speaker. Clip a voltmeter to the speaker leads. Turn on the amp and see if you measure any DC voltage on the output.

[evil79]

I did what you said. On the black terminal there is -.8mV, on the red there is -37.3Vdc. That's a lot huh? What should I do?

[The Dude]

I'm not sure what you referenced to for those readings, but to clarify we're looking for one reading. Put the ground of your meter on the speaker neg wire and the hot of your meter on the speaker pos wire and report back your DC voltage reading. I'm assuming you used chassis ground for a reference, but let's be sure what you have across the actual speaker wires.

[evil79]

I was using chassis ground. Ok, so with meter clipped to both speaker leads like you said, reading is -37VDC.

[Jazz P Bass]

Ampeg BA115 Schematic

Your output section is blown again.
When those mosfets go they really stress the rest of the section.
Here is what I would recommend.
Check & replace any shorted output mosfets.
Replace the zener diodes & the 47 ohm resistors on the output gates.
Replace the drivers MJE340 & 350
Replace the bias transistor Q3.
Check all of the resistors around thses parts.

[The Dude]

Your output section is blown again.
When those mosfets go they really stress the rest of the section.
Here is what I would recommend.
Check & replace any shorted output mosfets.
Replace the zener diodes & the 47 ohm resistors on the output gates.
Replace the drivers MJE340 & 350
Replace the bias transistor Q3.
Check all of the resistors around thses parts.

... and when you get it working again. Be sure to check the bias. I have yet to see one of these come biased correctly from the factory and it's even more critical if you've changed any parts. There is a thread on how to bias your amp HERE.

[evil79]

ok, I've got all those parts on me. How would i check the resistors? Pull them and measure resistance? I've measured some fried resistors before and they still seem to be in tolerance.

[The Dude]

ok, I've got all those parts on me. How would i check the resistors? Pull them and measure resistance? I've measured some fried resistors before and they still seem to be in tolerance.

Just unsolder one leg and lift it up.

[evil79]

So I replaced all the parts that you mentioned. I haven't plugged it into the speaker yet, but when I tried to bias it, nothing happened. It just stayed at 4.3mV at all turns of the bias pot. And then I measured volts on the speaker wires and there's still like -37VDC on there. What else could it be?

[The Dude]

Can you tell us what voltages you have on the outputs (referenced to ground). Also, don't hook up the speaker until the DC on the output is gone.

[DrGonz78]

I don't think this was mentioned before, excuse me if it was, but did you test the amp with a current limiter? A light bulb limiter or more elaborate current limiter is very important in this situation. It would be wise to have one on hand and build one if not. You have insulators between the backs of the mosfets? Perhaps you are using the old insulators and those are worn down? Do not plug this amp directly into the wall until you confirm the light bulb test, just my 2 cents.

[evil79]

I'll get back to you on the voltages. yes, I'm using the same insulators. I guess that's not good. I guess I should get some new ones. I've been meaning to build a light bulb limiter, but not sure how. Does anyone have a diagram or how-to?

[Enzo]

Google "light bulb limiter" and you will see more about one than you could read in a lifetime.

[Jazz P Bass]

What voltage (dc) do you have on the bases of Q1 & Q4?
And pin 7 of U3.