GK MB150s-III strange sounds !!

[Arbutt]

Hello all

Let me tell you the story of this one
Customer has 2 of them , One of them had a strange almost slight bad speaker fault, He has changed the speaker still the same. Bring both into me and i test both side by side The faulty one does have this problem and all tested with different basses and speakers inc my test speaker,
Di out checked ok,
Headphone out checked ok,
Gallien-krueger_MB150-III,_MB-150S-III_Preamp_Service_Manual.pdf
Checked the Return with a signal and the problem is there
gallien-krueger_mb150-iii_pwramp_sch.pdf
I know this is a strange fault and i have tried to explain to customer about finding it and i wouldn't worry about it, Nobody will hear it while on stage , But seems to be hell bent on getting it repaired and he adores the amp
2023_01_25_10_19_43.mp3
The fault
after about 1-2 min of power on you can hear a slight distorted decay on the note you have played. E and A string are the only ones that come up with the fault
E string 5th and 7th fret and A string 5th fret ( Audio clip is A string 5th fret )
I have spent time to sort of a couple of issues with the pre amp , most pots Testing i'm 95 % sure its the power amp
Unable to use the scope on this fault,
Before xmas i have a chat via email with a GK tech , at the time i was pretty sure it was the pre amp at fault. But i have changed my and now believe its the power amp. I have since lost the email , deleted on a monthly clear out

I do remember the tech saying about the driver transistors .
So my next plan was to change all drivers

Does anyone have any thoughts on what to do next

100% not the speaker or bass or vibration from the cabinet!!!!

[The Dude]

You suspect a power amp problem, but you only supplied a preamp schematic. Please link or upload the power amp schematic. Have you checked amp bias? Is it adjustable? I'm wondering if bias has drifted and you're too near cutoff. That might cause distortion when signal level drops below a certain point and transistors are turning off.

[Jon Snell]

You will be well advised NOT to change the driver transistors.
If they go faulty you have either no sound or DC on the speaker. Neither of which is your issue, up until now.
Check the quiescent bias current.
If R35 has a dirty spot, (pre sets often do) the bias voltage will be too low, causing crossover distortion.
Follow the service manual instruction to the letter!

[Mick Bailey]

You say it's 100% not vibration from the cabinet, but did you fully isolate the amp chassis from ANY vibration? I've had amps where I've had to run them in an adjacent room to the speaker before finally establishing there was a mechanical vibration issue. My only reservation about crossover distortion is it's not usually note specific, though the clip does sound like this so is worth checking anyhow.

[Arbutt]

You say it's 100% not vibration from the cabinet, but did you fully isolate the amp chassis from ANY vibration? I've had amps where I've had to run them in an adjacent room to the speaker before finally establishing there was a mechanical vibration issue. My only reservation about crossover distortion is it's not usually note specific, though the clip does sound like this so is worth checking anyhow.

Hi Mick Yes i fully isolated the cabinet from the combo which are terrible for vibration
I used my own test cabs . when i had them side by side , same setting and same instrument , test cabs etc, you could hear the problem on this amp but not on the other

Thanks

[Arbutt]

You suspect a power amp problem, but you only supplied a preamp schematic. Please link or upload the power amp schematic. Have you checked amp bias? Is it adjustable? I'm wondering if bias has drifted and you're too near cutoff. That might cause distortion when signal level drops below a certain point and transistors are turning off.

Thanks i have uploaded the power amp diagram
Bias i think i checked, But i have had this awhile now, So i better check again

[Arbutt]

You will be well advised NOT to change the driver transistors.
If they go faulty you have either no sound or DC on the speaker. Neither of which is your issue, up until now.
Check the quiescent bias current.
If R35 has a dirty spot, (pre sets often do) the bias voltage will be too low, causing crossover distortion.
Follow the service manual instruction to the letter!

Thanks jon
i do remember the GK tech saying he thinks it might be drivers failing . I wish i could find the damn email
Thanks again

[Jon Snell]

Thanks jon
i do remember the GK tech saying he thinks it might be drivers failing . I wish i could find the damn email
Thanks again

They either work or go noisy or fail.
They don't "fail" slowly.

[Mick Bailey]

Given the fault occurs after a couple of minutes suggests a thermal fault to me. Try a can of freeze spray to see if this pinpoints where the problem lies.

[Arbutt]

Given the fault occurs after a couple of minutes suggests a thermal fault to me. Try a can of freeze spray to see if this pinpoints where the problem lies.

Yes mick i tried some freezer and heat gun too Just too intermittent, Causing me too many issues delaying the other work i have in the workshop . In the end he has taken it back

[Arbutt]

They either work or go noisy or fail.
They don't "fail" slowly.

Ok maybe i should of said going noisy rather than failing

[Arbutt]

Thank you all for the help The post is now closed. The amp has been returned not repaired