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More Mesa Boogie problems

GeneralElectric

GeneralElectric

Dec 26, 2007
10,314
1,375
36
NY, NY
#1
Howdy,

So I gave this Mesa Boogie 400+ that came with JJs and which I then replaced with Mesa tubes. Now I turn on the head and it plays fine. Then after playing a few notes I get this horrible blaring feedback much louder than the bass I was playing.

I had the preamp at 6 and the master volume at less than 2.

Turning off quick to avoid the horrible blaring, I turn it back on and its on standby. Turning it off stand by it works fine for another 2 minutes before it happens again. I put it on standby and it stops.

I turned the master volume to 0, and both channel volumes to 0. I take it off standby... BLLLLARRRRRRRGGGGGGHHHHH!

So that experiment didn't work.

Fuse is good and these are new tubes taken fresh from the box. There is no burnt metal smell.

HELP!!!!!:help:

I have 2 shows on the 11th and I need this amp ready since this is my only bass amp.:bawl:
 
#2
Are the tubes properly biased for the head? What type of cab are you using? Are you using the right ohmage for the speaker outputs? Are you running anything through the effects loop? If not, make sure that the effects loop know is on zero.
 
#3
These were new Mesa 6L6 tubes, and new Mesa 12AX7 preamp tubes. I don't know how to check if they're properly biased or not.

I'm using 2 8 ohm cabs plugged into the 4 ohm speaker jack outs.

Nothing is being run through the effects loop and the slave and effects knobs in the back of the amp are both at 0.
 
#9
adam precision said:
Mesa tubes don't need to biasing. They are selected for there amps. Sounds like one of you preamp tubes has gone microphonic (which will cause a feedback type sound). Try swapping back the old jj preamp valves. Hope this helps.
Click to expand...

Right, but if the previous owner set the bias for the previous JJ tubes, the bias will be off. I'd take it to a tech, let them know you put Mesa tubes back in.
 
#11
Take your amp to Dave's Sound in Whippany, NJ. I had problems with a D-180 I bought and Dave did a great job and the price was quite reasonable .
We all want to fix our own stuff whenever possible, but sometimes it's just better to let someone with the schematic and some experience have a whack at it.

Peace
 
#17
I'd suggest setting your volume knobs the opposite way...low on the preamp, and higher on the master.

As a quick and easy check, I'd swap the JJ tubes back in (start with the preamp tubes). If it goes away, then you can start swapping the Mesa tubes back in one at a time.
 
#18
adam precision said:
Mesa tubes don't need to biasing. They are selected for there amps. Sounds like one of you preamp tubes has gone microphonic (which will cause a feedback type sound). Try swapping back the old jj preamp valves. Hope this helps.
Click to expand...

Wrong. Mesa may adverstise that their factory preset bias negates the need for adjustemnt, but this is false. Several people have had problems swapping output tubes that matched Mesa's ratings which all magically cleared up once the amp was properly biased.

GeneralElectric said:
Also, the power tubes glow blue when I turn them on. Could this also be a problem?
Click to expand...

Maybe. Maybe not. Generally blue glows aren't bad, but that's not a hard and fast rule.

I'll try swapping out the preamp tubes in the morning.
Click to expand...

Swapping preamp tubes is not a big deal, but swapping power tubes is. Your amp had problems BEFORE you swapped tubes. It needed SERVICE, not you messing with it.

Benjamin Strange said:
90% of the time a tube amp is experiencing problems, it's a tube that is at fault.
Click to expand...

Wrong. You've said it before, and I've corrected you before. While modern tubes may not equal the quality of old stock counterparts, they are nowhere near this unrelaible. On the contrary, 90% of porblems with tube amps are NOT the tubes themselves.
 

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