Ampeg

[Simpson]

I have one Ampeg Micro cl with some distortion.

When i measure across R121 and R122 i get:
R121: +/- 3.3mv Q109
R122: 0v Q110

So i replace Q109 and Q110 and nothing change.

I take a look on the bias circuit and there is no sufficient voltage to turn on Q110.
I have decided to increase R112 to 910R and measure

R121: 80mv Q109
R122: 80mv Q110
Which i think is too much.

What i should do?

[Jon Snell]

The link does not work for schematic.
Please repost it.
Thank you

[Jazz P Bass]

Here is a zipped copy: AmpegMicroCL.zip

[Simpson]

Thank you.
Im having trouble uploading the file

[Jon Snell]

Try 820R maybe.
Was Q109 and Q110 faulty? Did you replace them with identical components?

[Enzo]

You are making changes to the AMOUNT of bias, while the problem you are trying to fix is an imbalance in output current. SO the result is you now have excess idle current. WHen you adjust that bias it affects both top and bottom halves of the circuit. It won't just affect only Q110.

Run a sine wave through the amp, what does the output look like on a scope? What kind of distortion.

Q110 does whatever Q108 tells it to do. And a shorted Q106 could affect things as well.

[Simpson]

Thanks for replying.

Try 820R maybe.
Was Q109 and Q110 faulty? Did you replace them with identical components?

With 820r has 22mv across the resistors. I also tried with a 750r but the distortion as come back a little.
I have replace them with identical components.

Run a sine wave through the amp, what does the output look like on a scope? What kind of distortion.

Q110 does whatever Q108 tells it to do. And a shorted Q106 could affect things as well.

The sound wave is not perfect. The distortion is obvious, and sound like a synth bass.
When the output is balanced, due to the increase in bias voltage , the amp starts to sound good, but pulling a lot of current.



The sound through the headphones is clean. So the problem is when i connect the load.

I notice that when i connect the speaker jack, the Vbe on Q109 goes up and the Vbe on Q110 goes down dropping under 0.6V.

[Enzo]

I asked what the distortion looked like, I was hoping you might describe the waveform. Or better yet, a photo. For instance, does the sine wave clip on one side only? or both sides? or is there a crossover notch? If we know this, it becomes a clue as to the distortion's source.

It seems to even out when you brute force a ton of current through it. Imagine your car idles rough and stalls a lot when stopped. If you jack the idle up to 3000 rpm, then it never stalls. But then the engine is running screaming hot. Your amp bias is an idle setting. Jack it up it just conceals the real issue.

SInce the phones and speaker are driven by the same circuit, plus you report that putting a speaker load on it upsets the DC voltages, it suggests the output cannot supply current well.

If it were mine, I would verify ALL the resistors in the output stage. R116, 120, 122, 118, 124. I would probably lift D103 to disable the Q106 current limiter.

[Simpson]

These are the pictures that i took, Enzo.

The distortion is more obvious when the volume is low.

I have check the resistors on circuit and they seem fine. Tomorow i will check the current limiter and diodes.

[nevetslab]

Is this driving a load, or open circuit? And, is this also with the higher output stage current you achieved by tweaking the bias circuit? You clearly have oscillation on the positive half of the waveform. As Enzo, suggested, remove the current limiter coupling diodes D102 & D103, to see if that has any change. Also, measure the voltage across the emitter resistors of Q107/Q108, being R115 & R116. They should be the same, or very close. As the gain stage for the output stage is asymmetrical (Q103), coupling the positive signal drive thru the bias xstr to the base of Q107, I'm wondering about Q103 being involved in this drive problem.

[Simpson]

Is this driving a load, or open circuit? And, is this also with the higher output stage current you achieved by tweaking the bias circuit?

It was with a 8ohm load and no tweaking.

I'm wondering about Q103 being involved in this drive problem.

I have replace Q103 and that distortion have gone. Now its sounding a lot better.


But the imbalance on the output still there. I have remove the diodes and check the transistors and resistors and they all seem fine.

[nevetslab]

It was with a 8ohm load and no tweaking.



I have replace Q103 and that distortion have gone. Now its sounding a lot better.


But the imbalance on the output still there. I have remove the diodes and check the transistors and resistors and they all seem fine.

What was the voltage across the emitter resistors of Q107/Q108 (R115, R116)? If the imbalance it there as well, and relates to the same imbalance on the output stage, you may need to replace that particular driver xstr.

[Simpson]

What was the voltage across the emitter resistors of Q107/Q108 (R115, R116)?

R115 - 0.58v
R116 - 0.42v

If the imbalance it there as well, and relates to the same imbalance on the output stage, you may need to replace that particular driver xstr.

unfortunately, They have tested ok. But just for sure i already have replace them, and the imbalance still there.

[nevetslab]

R115 - 0.58v, R116 - 0.42v unfortunately, They have tested ok. But just for sure i already have replace them, and the imbalance still there.

You might try trimming R111 to balance the difference seen across R115 & R116. Then, trim R113 or R134 to increase the bias current flowing thru the output stage Q109 & Q110. 3mV across R121/R122 would be 14mA, 6mA across them would be 28mA. When you had 80mV across them, you were running 364mA thru the output stage at idle, which we all agreed was way too much.