Ampeg B2R intermittent

[lowell]

This amp loses volume and becomes very distorted after being played for 30-45 minutes. I've checked solder joints and continuity w/ my meter and all seems fine. Customer has the issue happen during band practice, I cannot get it to happen, however I cannot play it too loud here at the shop for that long. I've put it on a dummy load and put some signal through it for an hour w/ no issues arising. I opened it up and checked the bias. All power transistors are sitting at different points - they're not all in the specified 20-30mv range on the schem.

Q10: 9mv
Q11: 29mv
Q12: 11mv

Q14: 16mv
Q15: 20mv
Q16: 13mv

Seems odd that they are so far off from eachother. Anyone know if this is just inherent in this design? Maybe it's because the Mosfets are not matched? Maybe this isn't an issue and all power amps have this inconsistency.

Finally, could the above issue cause the amp to overheat and have low volume/distortion after being pushed for a while? Maybe Q11 and Q15 have thermal runnaway?

[lowell]

anyone know what the best tactic is to get an amp like this to act up? I've been running it on a dummy load (large wirewound power resistors) for an hour at a time while feeling the resistors. I set the amp to a volume where the resistors are getting hot but not TOO hot to the touch.

[MarkusBass]

Maybe it's because the Mosfets are not matched? Maybe this isn't an issue and all power amps have this inconsistency.

This isn't an issue. This is how MOSFETs are matched at the factory. It will just cause that some transistors may get hotter than the others.

Finally, could the above issue cause the amp to overheat and have low volume/distortion after being pushed for a while? Maybe Q11 and Q15 have thermal runnaway?

No. You have to check whether bias currents change over time and whether voltages on TP9 and TP10 change (only then thermal runaway is possible). There are several voltages specified on the schematic and I suggest verify whether they change over time or not.
BTW, I never do any global resoldering before I know the reason for the amp failure. Otherwise, you actually don't know what was the reason for the problems.

Mark

[Enzo]

It won't do the failure for you. SO verify that the problem really exists. First, has the customer tried a different amp?

Have the customer bring his instrument to the shop and see if it will happen with his axe.

Does he have active pickups?

If so, how old is the battery?


And believe me, more than once, when I asked someone that they replied, "Battery?"

[lowell]

I know, the battery issue is definitely crazy. When the guy brought it in the second time saying it's still doing it (I never found the issue the first time) he had recorded a video on his phone of the amp doing it. He said he tried mutliple cabs too. At this point he's coming in to get it fixed or not, probably going to sell it. Definitely could be his bass though... good thinking Enzo... I'll ask him to play another bass next time he rehearses OR when the issue happens that he should quickly plug into another amp altogether and see if it's the bass.

[gbono]

How about cables? Also when you bench test it do you use a sine wave as the source? I used to be involved in the testing of CE products and we used white/pink noise as well as gated sinusoidal waveforms - sometimes very revealing results.

[lowell]

Interesting, how were those more revealing? can you elaborate?

[gbono]

Okay so testing with sine waves will give you a "steady state" view of how the system performs. You usually have to keep this type of stimulus in what I call "small-medium" signal region, since the system will overheat. 99% of the time your system replicates non-sinusoidal signals - so.... Using noise, or better yet gated sinusoids, you can look at the large signal region of your system without damage - you can see how good or bad the non-linear processing (i.e. limiters) function for example.

[lowell]

Is this similar to pick attack at reasonably high volumes?

[gbono]

Sure a gated sinusoid at low frequencies sounds like the T-Rex walking in Jurasic Park - I guess you could say the 1.5-2kHz tone sounds like a guitar pick.

[lowell]

cool