Ampeg B3 (3158) problem

[spy]

I'm trying to repair an Ampeg B3 that pops/crackles at high volumes. The amp plays just fine at low levels but when you crank it up, levels at 10 is starting to act weird. It has two outputs, a High 50W 8Ohm and a Low 100W 4Ohm. The problem is audible only at the "High" output. But, on my bench with a signal generator, oscillator and a series bulb it starts to behave weird at both outputs. If I turn the power on with the volume and gain at 0 it's ok, when I start to turn the levels higher is pretty ok and I have a nice sine wave at the output, when I turn them even higher the series light bulb glows as the amp draws a lot of current. It stays in that condition even if Ι lower both gain and volume. I'm not sure if it's the right explanation but I think the amp is motorboating. Let me know what do you think before starting to change el. caps.

[Jazz P Bass]

I don't know if running an amp at full power is advisable with the lamp limiter in series.
You are literally starving the amp.
How does the amp behave without it?

[spy]

I haven't seen this before, every amp I tested with a lamp limiter didn't react like this. Everytime I set the input lower the lamp started to glow less, this amp remains the same until I close the power switch.

Just tested it without the lamp limiter and the Low output behave very nice at full output, without distorting the sine wave entirely after the maximum input signal before clean output, clean 50W at 8Ohms, I suppose because the limiter? It has a VTL5C7. The High Freq output is starting to distort very heavy after the 50W at 8Ohm. It stays clean with a nice sine wave on the scope but after it reaches full output the wave starts to distort. There are two VTL5C7 on the PCB, I suppose one of each channel. What about a defective VTL?

If anyone has a schematic it will be very helpful. I only have the version with one output stage.

[spy]

After measuring around I found out that the problem is not at the output stage! The signal is distorting at the preamplifier, more or less at different levels of the Drive pot.

[spy]

If anyone would like to help here are the schematics of the common B3. The only difference with mine is the output stage but the preamp is exactly the same.
Ampeg B3-preamp
Ampeg B3-power

After high input levels, when the signal is clipping I can see on the waveform an oscillation only at the negative side. It's visible right after IC3 (TP6) output. Before this point clipping seems normal but after R37 the negative side seems weird. There are some small spikes on the clipping line.

Any help? Does it have to do with the PSU? I have changed IC1 and IC3 with new ones but the problem is still there.

[Jazz P Bass]

One question comes to mind.
Why are you clipping the input?
I would measure the voltages listed in the test points.
TP: 4, 5 , 6 ,7 ..
Observe the conditions laid out in the test points.
Look for a TP that is abnormal (or not)
Maybe you are exceeding the capablities of the amp.

[spy]

I started wrong the whole thread. The amp pops and crackles when the Gain and Drive is at high levels. It doesn't matter if the master is too low or too high. That's why I started looking on the preamp. I'm sorry that I haven't mention it earlier. When the input is not clipping the amp behaves just fine!

[Enzo]

Have we ditched the light bulb yet? Does it do it without the bulb?

Are both power rails up to the same voltage? And both free of ripple? And whatever stage the distortion starts at, check the power again right at the IC power pins. Still full voltage and clean?

If you are clipping the input, then expect poor results. WHat happens when you play at typical levels through the amp?

If TP6 is the first place you get this distortion, note that that op amp is the heart of the graphic EQ, so there is a whole passel of circuitry that could be involved.

[Ramsay]

If the amp is motorboating it means wrong conection of the output transformer.
The conections are reversed.

[Enzo]

Hi Ramsay. That would be true on a tube amp. This one is a solid state model.

[spy]

Yes it do it without the bulb, too. Both power rails are the same, like 16.20V the positive and -15.90V the negative without any ripple.
There are two 22u electrolytics there, no problem to change them. They are doing the last filtering on the preamp stage with two 10 Ohm resistors.

The problem isn't normal, it can't be. It's very audible and annoying when you play bass and have the Gain and Drive a little bit high say 2-3 o' clock. It doesn't matter where the master volume is. That's why I test the circuit right after clipping the input stage. Nothing happens when you are not crank it up the preamp levels.

There are two broken slide pot tips on the EQ but these are broken long time ago the problem appeared. I don't have replacements at the moment and the amp is needed asap.

By the way, it's also visible at the input of IC3 (TP6) at pin 12! But not at TP5! Is there a simple way to bypass the EQ section?

[Enzo]

OK< so TP5 looks OK, but TP 6 has the issue?

Does thruning the graphic EQ on and off make a difference?

UIs there any DC offset on any output pin of any of the op amps? I mean in the signal path, not any that drive LEDs or something.

Connect the scope to the output of each stage and play the bass into the amp or have it played for you. At the reasonable levels you describe, is clipping there at the out of the first stage? Or does it take a couple stages?

[spy]

Without any signal at the input, Gain and Drive at 10 and Bass, Middle, Treble at 5, I have some mV at some outputs of IC1. The larger one is at pin8 (TP2) 40mV and pin14 (TP3) 18mV. Are these normal?

I'll try to describe one more time the problem with a bass connected. The Master volume or the EQ (on/off) don't affect the problem. If the master is at 1 or 10 it's the same. If the EQ is on or off it's the same. The problem appears when you crank up the Gain, it starts after 5 and it sounds like the speaker is blown. When you set the Gain for example at 3 or 4 the amp sounds great even if the master and/or Drive are at 10! The Drive pot affects the problem only at which level you have to set the Gain so the problem starts to be audible.

I don't know if it helps but the noise appears only at the negative side of the waveform.

[Enzo]

That is why I suggested tracing the signal stage by stage through the amp to see where it adds in this distortion.


No, a few millivolts don;t alarm me.

[spy]

It appears exactly after R37 (or at the input of IC3 pin12), not before, I don't know why but over there starts to be visible.