Marshall 50 w head starting to hum

[diablo175]

Having only played solid states, valvestates (and a VTM) this is the first all tube amp head I've had and while it's served me very well thus far, I started noticing an audible hum a few months back that has gotten progressively worse. This morning it was the loudest and worst I've heard yet. What should I expect in addressing this? Replacing the pre-amp valves or the power amp valves? The amp sounds fine and still does all that I ask of it. Concerned it's gonna crap out at an inopportune time.

[tomteriffic]

I'd put my money on filter caps.  Tap all of the tubes with a pencil (eraser end) or fingertip if you have good calluses and see if you get any loud bonks or thumps.  Swap the tubes around and see if it decreases or goes away or moves with the tube.  If you have a tube rectifier in it, swap it with a known good one.  But I'd still put my money on filter caps if the problem is getting progressively worse.

[crunchee]

I'm leaning towards Tom's explanation, but you gotta ask yourself:  How old is the amp, and has it ever been serviced?  If so, when?  And did they install fresh filter caps at the time?  Has the amp ever had the filter caps replaced?  Preamp tubes are easy to check, especially if you have good spare preamp tubes handy, 'cause they don't need anything special for installation...just put 'em in, as long as they're a 12AX7/ECC83 type.

[gtone]

It's only humming because it doesn't know the words... (ducks and runs)

[tbonesullivan]

Does it have a detachable power cable? It could be a grounding issue.

The other responses are also likely culprits. If it's not a new amp and you can't remember the last time it went in for servicing, it might be time.

[jettster]

If you have an effects loop you might be able to do some diagnostics by plugging a guitar into the effects loop receive to see if you still have the hum. That's an easy way to test the power tubes. Also, are you using any pedals? If so try bypassing those too.

[Disturber]

Try swapping the rectefier tube first. If it's gone bad it can cause hum. And perhaps use some contact spray on all the tube sockets. 

I had some weird hum in my 1973 50 watter recently. Turned out it was the standby switch. Sprayed it with some contact spray and everything went back to normal. But I've had problems with rectefier tubes more than once.