Mesa 400+ problems

I would agree with starting with the effects loop. Linking the send and return with an instrument cable is a quick and easy test.

 

The effects return (or power amp in) are the first place I would look. The switch to the 8 ohm tap was just coincidence. You could have a preamp tube going a little flaky till fully warmed up or even a dirty or "loose contacts" in a tube socket.

 

cleaning the fx send/return, and all your input and output jacks is a good idea.

 

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.

 

deoxit on a 1/4" cable, and insert/remove several times
follow up with deoxit on a qtip

let it fully dry before use.

 

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.

 

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.

 

Remember to tap gently with a wooden stick. Orange stick or chopstick.

 

If it is an output (large tube) it should have the blue glow in operation. Observe it when playing some hard struck notes. It could have just a different beam former in the tube or not getting proper bias.

 

If you don't see much of the blue when playing some hard struck notes it just has a different internal beam former.

 

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.