Ironsounds

[Les1]

My iron sounds 18 watt developed a loud humm on both channels, what do I look for ?!

[Daviedawg]

Hi Les. Welcome to the hell that is hum.
Not a lot to go on there. How old is the amp? Did the hum start all at once or gradually become noticeable. My initial thoughts relate to a failure of one of the power supply smoothing caps. But if it is a recent build that is less likely. If it appeared suddenly that is also less likely.
So on the assumption that it is twenty years or more on the go I suggest you replace the power supply caps in their entirety. It will need doing some time soon in any case if you use it a lot.
If it does not fit those symptoms it could be a ground short somewhere which you will need to hunt for. Chances are a chopstick exercise will show up other noises related to the issue. Since it is both channels it limits where in the circuit you need to hunt.
That's how I would approach it at first. Ask again if you need more help with the exercise.

Dd

[Les1]

Yeah, it’s 18 years old and the loud hum cam on suddenly

[Daviedawg]

Ok. The route I would suggest is to replace the smoothing caps in the power supply first. But meantime you could chopstick the power supply and output stage hunting for a failed joint or component. Moving wires and tapping components to see if you generate noise as a clue to what if anything has failed. Tap the circuit board as well.
Chopsticking (in case you have not done it before) - Live amp upside down giving access to all of circuits, using a non conductive prod such as a wooden or plastic stick (and not a lead pencil) move wires and tap components looking for a noise response or change in hum as a clue to where the issue is. Keep one hand "behind your back" as there are lots of volts and some amps around.
You can find appropriate circuits to suit your version to refer to in the downloads section on the left side of this page.

Dd

[Iain T]

As the hum is on both channels you need to look at common areas of the amp after both channels. You're looking at the Phase Inverter, power stage and, of course power supply as your initial steps in diagnosing the issues.

If you have spare tubes you can try swapping them out to eliminate a very simple fix although it's probably not the most likely failure point - more likely a symptom if replacing tubes fixed things.

There are a lot of great videos online from the likes of Psionic, Rift Amps, etc. on diagnosing this kind of issue and the general approaches to take usually from a real-world perspective fixing a customer's amp. I find that stuff to be incredibly useful.