Marshall MG250dfx's sound fades out

Did you try a new 9 volt battery?

Well without testing a bunch of parts. I would say some component in the clean preamp has changed. It may have happened over time and now you are noticeing it. without checking all the parts it would be hard to tell. Get it to a service center. Or you may send it in to Pro Line Music
215-736-8055

It sounds like you bought a defected amp. You should contact Marshall and tell them the problem. You should have some kind of warranty.

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Since you just purchased this, verify that your guitar and cables are good on another amp and if they are good, then exercise your warranty on the Marshall.

Connectors and cables for guitars are chronic problems so verify they are good to avoid looking foolish.

Play around with your placement and where you are compared to the amp. I have a line 6 spider jam and I was getting pissed everytime I went into guitar center b/c theirs had more bass than mine. Then I realized that I put the amp beside me when I play but I play infront and over it when I go there. Have you ever noticed how much gain you lose when you take it outside compared to in your room. I would just look up the owners manual and see if you are missing anything. Check the fuse in the back too. If you clean isnt "that" clean and your overdrive is sucking then maybe the fuse is on its way out. Only $1 problem though. good luck

I do NOT believe it is possible to do this to the straight volume with Marshalls. The controls only vary the overdrive features and turning them on and off.

Try carefully cleaning your cooling fan with qtips for the blades and a vacuumn. See if that helps the fan. If not it will need to be replaced. If you are experienced with amp repairs have a look for loose parts on the circuit board. If you can safely disassemble it, look at the soldered side for bad solder joints. From another poster on here with the same problem I have learned that resoldering R107 and R96 helped. Make sure they aren't burned out or have changed in value.They are in the FDD circuit. Have a look at your owners manual and it will make sense. The schematic is available here:http://www.ampix.org/displayimage.php?album=search&cat=0&pos=0 If it doesn't come up use the search function on that site. Do not attempt to service this yourself unless you are absolutely sure you won't hurt yourself or the amp.

IF this is a vacuum tube unit, these symptoms are that of a tube that the heater is intermittent, that is it cools off due to a break and then liights again.

Open the back and observe the heaters in the tubes. DO NOT be fooled by some that have TWO heaters in them. (use a mirror if necessary) A single one that comes and goes can cause the problem and the voiration can make it come and go. This behavior in a tube is NOT somthing readily spotted in a tube checker. The delay in fading and coming back would be the delay in warm up following a heater going out and cooling.

There may be thermal sensor that protects the power amp that is sensing over temperature and turning off the power amplifier stage.

Try cooling with a fan and see if it lasts longer. If you are obstructing the cooling in any way, try to avoid that.

As a test, leave on for an hour and THEN try driving sound. If it works for a while, then driving is what is overheating it.

VERIFY that you are NOT running too low impedance speakers!!!

Most amps will drive down to 4 ohms. That is a single 4 ohm speaker or two 8 ohms in parallel MAX.