JP2c sick

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Satchur8

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Hi everyone in Boogie Land, it's my 1st time here and I'm desperately seeking Susan! LOL just kidding! I'm desperately seeking for advice if anyone has had this issue with the JP2c.
Bought this amp after going from Mesa Strategy 500 back in the 90s, then to Marshall JCM800, then to Marshall JVM410HJS, then back to Boogie JP2c. I really thought I had found the holy grail of tone, then 5 years later it's dying on me. I guess 5 years was a good run but I'm reluctant to sell it...but if I have to, I will....

It only happens in Channel 3, but not instantly and not all the time. Intermittent faults are the hardest to track I know. I bought it new in 2016 so the 5 year warranty expires in Dec. Unfortunately I can't get it to the distributor due to COVID restrictions as i don't trust couriers with my PreCiOus!
It seems to 'warm up' then play up. The sound totally sounds like the amp is farting. Literally the sound will flutter off then die to zero, ONLY IN CHANNEL 3 MODE, usually exacerbated if I flick on the Shred switch also happens faster. I did take it to a recommended repairer and he found nothing and still charged my $100 for time.He said it's not the power tubes, but i did change the preamp tubes. It seemed ok for a while and now it's back to haunt me! I spoke with the dealer and they say it's weird, because if anything were to go wrong in the amp it usually happens in the 1st 18months of use not 5 years later. Said it's definitely not the tubes ! Sad that I can only use 2 of the 3 channels now, but I prefer Ch 3 hi gain mostly to solo.
Luckily I'm not gigging till Covid settles....

Has anyone had a similar issue with the JP2c?
Thanks for any advice....
Joe
 
Hi Joe

Did you have any luck finding out what is causing this problem?
 
I can understand your frustration.
One primary thing to consider would be the preamp tubes. When they age, they do odd things. Most Mark amps will change plate voltages on the preamp tubes, usually V1 for different channels and often times will pull in or shunt bypass capacitors. The only time I had something similar was when I tried Tung Sol, Mullard, or any other brand from New Sensor 12AX7 tubes that share the same common coiled heater filaments. However, that effect was different, it was an increasing motor boating sound that grew louder and louder until I turned off the amp. If you opted for any of the new sensor 12AX7 variants, Svetlana, Tung Sol, Mullard, Sovtek, EHX, they all seemed to make flutter noises and motor boating oscillations when used in the JP2C. Just weird.

If it is not tube related, it could be a JFET on one of the bypass capacitors. Dirty connection in one of the ribbon cables. To me this sounds like a bypass cap is intermittently getting pulled in. (depends on the JFET circuit, mostly they are used to short out components or bypass them with a resistor to minimize the effect. Solder joint problem if it is due to change in thermals and only happens after the amp has been running for a some time. The JFET may be a long shot guess as would be a solder joint but those are things that tend to show up latter on and not within the first 18 months of use.
 
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