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Mesa 400+ problems

ariff

ariff

Nov 28, 2007
473
271
Lawrence KS
blackonblack1.bandcamp.com
#1
Hey everyone..
I have had a Mesa 400+ for a few years now and it has always been a great head. last night at a show it finally started giving me some problems. Here are the symptoms: Nothing plugged into the amp except for the Mesa 8x10 PH cab using the 4 ohm jack I get really mad static like the sound you get with a bad instrument cable.. But nothing is plugged into the amp. I tried both 4 ohm outputs and a different cable still the same. I switched over to the 8 ohm output and all was well and I played the show using the 8 ohm output.. Did not like doing that, but as they say the show must go on. Any thoughts on what might be the problem? I had the amp serviced, cleaned, gone through all tubes tested and what not about a year ago and the tech said it was in great shape. I'd take it to him, but he no longer lives here. The next closest tube amp tech is about a hr away.. Wondering if I can do anything to fix the problem..
Thanks!

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#4
Well I pulled all the tubes last night and cleaned the pins with Denatured alcohol, and coated the pins with deoxit.. Everything seems to be working good.. Of course it does when I am testing it. If it fails it will be at a show that's usually how it happens right:atoz: I did notice when looking at the tubes all warmed up that the 12AU7 was much brighter than the other 3 12AX7's is this normal?
 
#6
Some tubes will glow brighter than others. Nothing to be concerned about.

What you normally see glowing is the heater. The amount that you see it glowing will depend on the design of the tube and how the structure shields the heater winding inside the tube.

When using contact cleaners, it isn't always enough to coat the metal. Sometimes you need to scrub it with a brush and apply the cleaner more than once. It depends on how much oxidation is present.
 
#9
Not seeing oxidation on the tube pins and socket contacts is a good thing but that doesn't mean that it isn't there. So it never hurts to clean them.

In general, the jacks seem to be most problematic. Don't forget to clean the EQ in/out jack as well. Oxidiation there can really diminish your signal.
 
#10
Is it static or hum or buzzing type hum? If it only does it on the 4 ohm output and not the 8 ohm then that points to only a few possibilities. Both 4 and 8 are taken off the same output transformer so either it has a bad winding(unlikely), there is a bad connection from the 4 ohm tap of the transformer to the jack or circuit board where jacks are soldered, the jacks are dirty, contacts broken away from the circuit board, or have lost tension on their center conductor.

I would go back and verify you still have the static on the 4 ohm output only and go from there. It may be a fluke that you plugged in the 8 ohm and it worked. Static that comes and goes can be a preamp tube or weak pins in a tube socket. I've seen it numerous times. You can try wiggling or tapping the preamp tubes to see if you can get one to make the noise or try swapping one at a time and see if it goes away. Preamp tubes that make noise will not test bad on a tube tester.
 
#11
The sound was kinda like "white-noise" type sound when the stand-by switch is on and the unit is ready to play (even with no instrument). Like I said when I tested it at home of course I can't duplicate the issue. I am going to clean all the jacks tonight as well.. I'll also try tapping on the preamp tubes to see if they are going microphonic.
 
#12
ariff said:
The sound was kinda like "white-noise" type sound when the stand-by switch is on and the unit is ready to play (even with no instrument). Like I said when I tested it at home of course I can't duplicate the issue. I am going to clean all the jacks tonight as well.. I'll also try tapping on the preamp tubes to see if they are going microphonic.
Click to expand...

Remember to tap gently with a wooden stick. Orange stick or chopstick.
 
#14
OK all jacks have been cleaned and Deoxit applied.. She seems to be running well. I have not been able to reproduce the problem. I also cracked the bottom open and looked around for any visual issues like cracked solder joints. I did not see anything. I checked all the tubes by tapping on them with a pencil and they did not produce any microphonic sounds. I did notice there is one replacement tube in there Sovtek that does not have that blue glow to it. All the other Mesa tubes have a slight blue glow. I switched the tubes around and it followed the tube. Is this normal? This was the tube my old tech replaced in place of a bad Mesa tube. So it is a newer tube..
Thanks!
 
#19
Not all brands glow the same amount of blue. I've seen some that are difficult to see and some that don't glow blue at all. That tube may be good but if it is not matched to the other tubes it may not be getting biased the same as the others, especially being a different brand. But if it's working, sounds good, and has plenty of power I wouldn't worry about it.
 

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