Marshall DSL 100 - volume fades away and gradually comes back on

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.

Check the output valves.

Fading amp.

I had similar issues with my Marshall Valve state amp. It would play for about 5 minutes then fade away. I first replaced the valve to see if I could get in an easy fix. The amp still faded out (same issue).
I pulled the circuit board out and checked the back of the circuit board for bad solder joints. Under inspection, I saw nothing obvious, so while I was there I touched up most of the soldered connections with my soldering iron (around the valve connections and resistors) Just melt and set each one, add solder if you need.
Put it all back together and played it for a good hour without issue. The new valve (Rubi) was awesome from Artist Guitars in Sydney. Always make sure your amp is isolated from the power supply when you undertake any electrical work.

I am very new here and don't know how to answer a question to a specific problem so I will write about the most common problems that I have had over the decades doing guitar amp repairs. If you find that your amp is extremely low in volume and then suddenly comes back to normal- check out the effects return [or power amp in] jack. Especially in Marshall amps-these corrode and your signal dies. If there is loud noise such as hissing and crackling with no guitar patch cord in the input jack- the input jack is likely at fault. They normally have switching contacts that ground part of the input circuit when the 1/4 inch male plug is disconnected. When this in is corroded or too weak to make normal contact, you can get these symptoms. If your amp is a solid state Marshall or Fender and there is a random buzzing sound with no guitar signal connected- hit the side of the amp and listen for the buzz to go away or become louder-if it does, this usually means that the connections are loose at the two large filter capacitors. In one or more Marshall amps, they use a filter circuit in the low voltage power supple for the preamp tubes. If you encounter a dead Marshall[the model escapes me now] remove the circuit board and resolder the four connections at the bottom of the bridge rectifier. If you have any other amplifier repair questions you can email me direct at [email protected].