Bad tube?

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sheehanje

sheehanje

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I was playing tonight - and all of a sudden I was getting a lot of background noise on channels 2 + 3. Was driving me crazy. Changed guitars, cables, took everything out of the loop. Tried 60watt mode and back to 100. Same thing and I would get a whistle that didn't sound like normal feedback while the masters was at noon.

Tried channel 1, and it sounds distorted and the reverb on it sounds wonky (for lack of a better word).

I'm guessing preamp tube - but now it's figuring out which one. Does anyone have anything else they could think of besides a tube? How hard is it to replace them in the JP-2C? I know the Mark V was a pain - especially V1/V2 - and the JP-2C is even more compact.

Of course I have a show on Sunday.
 
Blah - just rolled all the tubes and still noisy as hell. Time to call Mesa.
 
I hope you get a resolve for this....

Check your reverb wire hookup. Even with the reverb turned off, I believe the return signal circuit may still be active but this is just an assumption. You could also dial out the reverb on each channel. This is assuming the reverb circuit is contributing to the noise you are experiencing. Last thing you can try is to unplug the reverb cables on the amp and see if it continues. If it is tube related, it will be there regardless.

I would have done the "pull and replace no change put old back" method with a known to be good preamp tube. Relocating the same preamp tubes may not resolve the problem if one is bad. Since you moved them around, you may need the "pull out leave out" method to determine which tube is contributing, go all the way until all preamp tubes are out. If you have same noise with no preamp tubes, it could be a power tube.

Good luck 8)
 
bandit2013 said:
I hope you get a resolve for this....

Check your reverb wire hookup. Even with the reverb turned off, I believe the return signal circuit may still be active but this is just an assumption. You could also dial out the reverb on each channel. This is assuming the reverb circuit is contributing to the noise you are experiencing. Last thing you can try is to unplug the reverb cables on the amp and see if it continues. If it is tube related, it will be there regardless.

I would have done the "pull and replace no change put old back" method with a known to be good preamp tube. Relocating the same preamp tubes may not resolve the problem if one is bad. Since you moved them around, you may need the "pull out leave out" method to determine which tube is contributing, go all the way until all preamp tubes are out. If you have same noise with no preamp tubes, it could be a power tube.

Good luck 8)
Click to expand...


I did replace one by one with a good preamp tube. I even tried it again with another different preamp tube and no luck.

Brought it to the local store where I bought it - they are going to fix it and they gave me a loaner for my gig - a Mark V 35. Holy crap - what a great little amp it is. I was expecting a downgrade but the thing has it going on in the sound department. Doesn't have all the bells and whisltes of the JP-2C - and is much different tonally - but Mark IV mode is freaking beast mode on this thing.

Seriously thinking of trading my Music Man Majesty for this amp - so I can have a backup as well of what some of the tones I've been missing since trading in the Mark V towards the JP-2C. I would have every tone I've ever desired between the 2 amps...
 
Subscribing. Mine has also developed a little noise in channels 2 & 3. I have a new tube but haven't rolled it in yet.
 
bandit2013 said:
Does your JP have the FX loop mod that Mesa is now using? Or is yours before the change?
Click to expand...

Mine came with the FX loop mod.
 
So it was a bad preamp tube, and in the spot I first suspected - V1.

Now I'm baffled why trying two different new preamp tubes didn't fix it. What is the likelihood of having 2 brand new tubes that fail and create the same exact symptoms?
 
Unfortunately, it is highly probable. Also if there is a voltage shift on the tube in the circuit it is used it may enhance the weakness of the tube assuming that technique is used in the JP as it is with many of the other Mesa amps that use the same preamp circuit for clean and high gain. I have found the Mesa tubes not to be that sensitive to change in operating point voltages as other preamp tubes. But that does not mean they are perfect. Tube may have been out of balance (triodes far from being matched, they should be close to each other in gain which could lead to some issues if they are not). Also, characteristic may have been in spec during the sorting process but may have changed while in use.
 

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