Bias on Ampeg BA115 too low cause occasional random-ish noise?

[Kenrod]

Here's a little more info and another side-by-side comparison:
I turned the master volume the rest of the way up. A small increase in volume.
I turned gain on up to "8" and still not nearly as loud as it should be, not even close.
I turn on a Peavey Microbass amp (20 watts into an 8" speaker) and turn the one volume knob all the way up......just as loud as the Ampeg. The Ampeg sounds "better", probably due to the 15" speaker, but it's not appreciably louder

[g1]

if these amps did not have a preamp gain level, but only a master volume (like a lot of amps), I still wouldn't have to turn the volume up to "10"

Just wanted to clear this up, whether relevant or not. Amps with only one volume control are controlling the preamp gain, not the master. The master is hardwired to "10" internally.

[Kenrod]

Just wanted to clear this up, whether relevant or not. Amps with only one volume control are controlling the preamp gain, not the master. The master is hardwired to "10" internally.

Nice to know, especially for someone like me; still trying to grasp all the basics of operation, but hopefully point was made in my example, even if not technically accurate.

[Kenrod]

While I'm waiting on parts to arrive (that I may or may not really need) i'm looking at the JFET sitting right between the last known good test voltage at TP5 and IC5 (U5).

At this point, I wonder if all the signal is getting through to IC5. I'm going over JFET operation theory to understand what could be happening there. I see that it's an N-channel device and seems to have a neg voltage applied at the gate, and that's tied back to the headphone jack sleeve.
I also have to believe there might be something about that limiter circuit, clamping the signal (as mentioned early on by Enzo). Unfortunately, my limited knowledge is preventing mw from really knowing what I might do about it.

This is where you guys can feel free to jump back in.

TIA

[g1]

That fet will be a mute to mute the power amp when using headphones.
When turned on with no phones, you should measure a very low resistance from drain to source. Alternatively you could jumper a wire from d to s and see if it makes any difference.

[Kenrod]

That fet will be a mute to mute the power amp when using headphones.
When turned on with no phones, you should measure a very low resistance from drain to source. Alternatively you could jumper a wire from d to s and see if it makes any difference.

ah, that makes sense. I can make that quick check, but likely puts me back to the IC5 or something about that entire limiter circuit at fault.

[52 Bill]

Q9 is the input mute, Q6 is the gain mute for U5. Q9 needs to be low resistance and Q6 needs to be high resistance in order for the signal to be un-attenuated.

Measure the dc voltage at the gate of Q9. It should be zero with no headphones plugged into the jack. If it is anything other than zero it will cause Q9 to reduce the signal at the input of U5.

Measure the voltage at the gate of Q6. It should be -16 vdc after a short turn on delay. If the -16 volts is missing or dramatically lower, Q16 will cause the gain of U5 to be reduced.

[Kenrod]

Q9 is the input mute, Q6 is the gain mute for U5. Q9 needs to be low resistance and Q6 needs to be high resistance in order for the signal to be un-attenuated.

Measure the dc voltage at the gate of Q9. It should be zero with no headphones plugged into the jack. If it is anything other than zero it will cause Q9 to reduce the signal at the input of U5.

Measure the voltage at the gate of Q6. It should be -16 vdc after a short turn on delay. If the -16 volts is missing or dramatically lower, Q16 will cause the gain of U5 to be reduced.

Woohoo, now that's what I'm talking about. I can't wait to get home now and try it, although I'm almost always ready to leave work and get to the hobbies.

[g1]

I'd still like to know if anyone thinks that the spike on the scope (post #35) could be caused by the limiter in normal operation?

[Kenrod]

0 volts on Gate of Q9
-15.6V on Gate of Q6

[Kenrod]

I changed out IC5.
No change.

[52 Bill]

You have the correct gate voltages, but you can try replacing Q6 and Q9. To test the circuit before replacing them, just remove Q6. Any difference?

Now remove Q9 and short the connection where the drain and source were connected. Any difference? I guess you can leave Q9 in circuit for this test.

[Kenrod]

Pulled Q6, no improvement
Shorted drain to source of Q9, no improvement.

[Kenrod]

I should also add that after I pulled Q6, I now a small negative spike on the waveform. Not nearly as large as the positive spike, but visible on the scope anyway.
Don't know if that is telling or not.

I'm beginning to wonder if there is anything on the output side of IC5 loading it down.
Forgive me if this sounds crazy, but since it's a dual op-amp and the output of side A feeds the input of side B, is it premature to ignore the limiter portion since the A side output isn't being squashed? Is the limiter going to squash the input to side A if "needed"?

Just a thought.

[52 Bill]

I should also add that after I pulled Q6, I now a small negative spike on the waveform. Not nearly as large as the positive spike, but visible on the scope anyway.
Don't know if that is telling or not.

Removing Q6 has reduced the feedback resistance to some extent, so there are going to be changes to the signal. This test was only to prove that the two fets were not causing the loss of signal. They aren't so put them back in and continue the search.

I'm beginning to wonder if there is anything on the output side of IC5 loading it down.
Forgive me if this sounds crazy, but since it's a dual op-amp and the output of side A feeds the input of side B, is it premature to ignore the limiter portion since the A side output isn't being squashed? Is the limiter going to squash the input to side A if "needed"?

I don't understand what you are asking here. U5A is part of the limiter circuit. Signal from the output is sent to the network of diodes that feed the led in the optoisolator OC1. The resistor side of OC1 is part of the feedback loop of U5A, which controls the gain of the stage.

Have you checked all of the diodes and resistors in that part of the circuit? What are the two 50 volt supplies measuring at. I would guess that if they are grossly mismatched, the balance of the circuit would be thrown off.