Mesa F-50 General Health Problem...

[leonwhite73]

Hiya there Chaps,

Just wondering if any of you tech types could shed any light on this... OK, so my F-50 failed at a gig - the sound was cracking/cutting out then there was a bright flash at the back of the amp while it was switched on (inside the grill) followed by a burning smell - I presumed this was a valve popping so I replaced the full set of valves in the amp (4x AX12s & 2x 6L6s). Amp is now working, but since I've put the new valves in the amp just doesn't seem to play the same? The volume is definately not as loud on the clean channel, and the tone seems to 'clipping' rather than 'ringing' if you know what I mean - I've also noticed the right hand side 6L6 is glowing blue... don't know if this is normal or the valves need 'playing in', just the amp really doesn't seem to sound as good as before?

If anybody could shed some light on this I'd be grateful thanks kids!

Leon White.

 

[mikeyp]

Not long ago I had a very similar situation. I had a power tube go and one of the, I believe they're called screen grid resisters, got so hot the solder liquefied and it fell from the pcb. The amp did work but had low volume also. The one that fell out on me was located right between the power tubes on a small pcb. If you look through the back grill you'll see the small board, but you may not be able to tell if the resister is there or not because it on the bottom of it. If it fell completely out you may be able to shake the amp and hear it rolling around. I soldered mine back in and worked fine, but since it was still under warranty I took it in and they replace the resister for me. Good luck.

 

[leonwhite73]

Hi Mikeyp,

Thanks for your post... I've taken the Amp to my local Tube Amp specialist and first thing he mentioned was a 'Screen grid resistor' so that sounds like a similar problem that happened on your amp - they are gonna have a look at the amp next week for me so I'll post again when I've got it fixed.
The guy at the repair shop did advise changing the power tubes every 12 months if you use the amp a lot (I gig a lot with mine) to avoid it happening again...

Thanks!
Leon White.

 

[leonwhite73]

Hi Peeps,

I've now got my F-50 back and it's working great - Exactly the problem Mikeyp suggested - The Screen Grid resistors needed replacing on the circuit board next to the valves, it wasn't an expensive job, cost £36 pounds.

Thanks!
Leon White.

 

[Hobo]

I just got my F-50 head back from the tech. Same problem, screen grid resistors.
Mesa solders them to the bottom of the PCB board so that they are hanging upside down. If they get hot the solder can melt and the resistors can fall out. My tech installed the new ones on top of the board so they won't fall out.

 

[leonwhite73]

Hiya Hobo,

Yep, exactly what the guy who repaired my amp said he had done. This seems like a common fault on these amps - As I said in my earlier post, I'm gonna make sure I replace the 2 power amp valves on a regular basis.

Thanks )
Leon.

 

[mikeyp]

Yeah one of mine is now on top also. I was wondering if there were a tube problem, and the resister can't fall out, what will fry next. Maybe it's a good idea to have them soldered from the bottom? I too will replace tubes more often.

 

[seakayak]

I just discovered this as well on my F-50, I'll have to replace a resistor.

There is a picture and discussion about this exact problem here: http://www.amp-fix.com/biasing%5b1%5d.htm