Peavey Bass Mark III Clip Light Question

[TigerAmps]

Hello All:

I have two Peavey Bass Mark III amps that both had blown outputs.
One amps has the 400BH power board and the other has the 400BG power board.
The 400BH board also had 2 broken fuse holders on the board which were replaced as well.
I changed the original power supply caps in both amps (5000uf and the 1000uf caps) due to age.
I replaced the blown outputs and fired them up into a JBL 18 inch subwoofer.

They both are quiet and work fine but the clip light seems to be hyper active.
With the pre and post volumes at 11:00 o'clock, the clip indicator light seems to
illuminate very frequently. I was playing a fender strat with humbuckers to test the
amps. No pedals or boost was used, just the guitar straight into the amp.
I was just testing the amp out; checking the bi-amp function, line out, etc.

Both amps are loud and there is no audible signs of clipping.
The hyperactive clipping is common to both amps.

Is this common for these amps?

[Jazz P Bass]

I would check to make sure that you have a solid +/ - 15 volt supply.
You can measure the voltages directly on the power board ic's. (TL074)
Pin 4: -15Vdc
Pin 8: +15Vdc

Also check the supplies for Vac ripple.
There should be none.

PEAVEY 400bh Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

[teemuk]

They both are quiet and work fine but the clip light seems to be hyper active.

You mean "limit" light?

Unlike many other manufacturers Peavey doesn't estimate when the power amplifier stage potentially clipping distorts, but instead they detect whether there is difference in input and output signals of the power stage. Difference is distortion. Any distortion. Like clipping distortion BUT ALSO slew rate distortion, frequency distortion, or in case of 400BH distortion inside the closed loop of the power amp (because the distortion is detected from a single gain stage of a complex power amp). The distortion detector will trigger to any of that so it's much more sensitive by design to react than a mere clipping estimator, which only triggers when signal exceeds a certain amplitude threshold. This one triggers whenever there is distortion.

Both amps are loud and there is no audible signs of clipping.

But there's a catch. The LED simply indicates that "Limiting" takes place. The limiter is activated by the same distortion detector so simultanously as the amplifier distorts the limiter decreases its gain. If the amp, for example, clipping distorted its gain was quickly reduced below clipping threshold. Since distortion is therefore only transient by nature and doesn't sustain long a good chance is that you don't perceive it.

Now, turn "Out" the limiter feature. The "Limit" indicator LED should stop going hyperactive (as the feature is no longer toggled) and you probably should now more distinctingly hear the amplifier also distorting by clipping as the internal limiter no longer reduces gain interactively whenever distortion gets detected. Now, sans limiting, you can experiment to find a level where clipping distortion is inaudible and when you turn "In" the limiter feature again you can see how much distortion normal human hearing simply ignores.

[TigerAmps]

I would check to make sure that you have a solid +/ - 15 volt supply.
You can measure the voltages directly on the power board ic's. (TL074)
Pin 4: -15Vdc
Pin 8: +15Vdc

Also check the supplies for Vac ripple.
There should be none.

PEAVEY 400bh Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

I think you meant pins 4 (VCC +) and 11 (VCC -) not pin 8; please let me know if that is correct.

I checked the voltages:

Pin 4: 14.7 V
Pin 11: -14.8 V

Not sure why they are off by 1/10th a volt or if that would make a difference.
Should they be identical voltages?

If there is distortion, would that be coming from the front board?

I did replace the tl074, but I did not get a chance to test it to see if it help or made a difference. I will check tonight and update tomorrow.

[MarkusBass]

I think you meant pins 4 (VCC +) and 11 (VCC -) not pin 8; please let me know if that is correct.
I checked the voltages:
Pin 4: 14.7 V
Pin 11: -14.8 V

Yes, TL074 is in DIP-14 package and you should check pins #4 and #11 (exactly as you did).
Isn't the problem related to the fact that:

I was playing a fender strat with humbuckers to test the amps.

and humbackers have stronger signal than singles? Can you test the amps with a guitar that has singles pickups?

Mark

[g1]

Teemu explained in post #3 why this is normal for this model, and how you can test it by defeating the DDT compressor.
The owners manual also explains why the light will be on very often, on page 4 of the pdf:
"one should not be concerned that the limit LED indicated compression virtually constantly during a performance since this is what it was designed to do".

[TigerAmps]

I understand now ... Strange "feature"!

I was thinking along the lines on a CS400/CS800 compressor light which is much less active unless really driving the amp.

Thank you to everyone who responded.

[Enzo]

And no, do not concern over 1/10 of a volt. If the voltages are within even 10% of each other I am quite happy.

never lose sight of the fact these are guitar amps, not precision laboratory instruments.

[TigerAmps]

I thought that perhaps the sensitivity of the circuit might have required a closer match between the +/- voltages.
This was the first time I worked on and played a Bass Mark III so I was unfamiliar and clueless!
The fact that both behaved the same indicated a "feature" or two screwed up amplifiers.

[g1]

Strange feature indeed. I would not be too keen on playing an amp where the compressor light was on constantly either.
But the chances of both amps having the exact same fault with no other issues seemed quite slim. It's always nice to have another amp of the exact same model to compare to. Especially for warranty issues when you have a brand new second one to compare to, and a complaint such as background noise or something.

[Enzo]

Bass tends to be very spiky. Try playing a guitar through it, just to see, or even music from a CD or MP3.

[Jazz P Bass]

If you hit that DDT circuit with a hard enough signal & the volume up loud, the led Will light.