Separate filament transformers

I get a bit more hum/noise after the amps have been on for a couple of hours
and the power transformer has reached it's maximum temperature. Now I'm
speculating about separate transformers for filament as a way to reduce the
consumption from the main power transformer. I have found a 50va toroid
that's 80x35mm and I belive that I would be able to fit two of these in each
M-125 amp. They do not have any centertap but I found this description for
a workaround. Any comments ?

http://www.augustica.com/Tutorials/Tubes/mains_filament_wiring.htm

Not a bad idea, if you have the room. It will make the PT run cooler. You can heat up the tubes prior to powering up the PT providing you put the filament transformers on a separate switch.

And have the power for the PT gated through the power switch for the FT so you can't put anything on the plates until the FT is switched on.

Transformers and steel covers (90x38mm) are ordered.
The plan is to mount the transformers on the bottom, outside of the amps.

I would like to know if the hum/noise is mechanical (external) or through the speakers (internal)?

Your post is not clear on this. If external, your solution should do nicely. It remains to be seen if internal. Even if temperature-related, there are so many potential contributors as to single out the power-amplifier may not be the actual solution.

It's internal.

OK.

a) The filament transformers cannot hurt, will reduce the load on the PT, and may help. That they are shielded is a very good thing.
b) IF this does not work, look after the main filter caps. Heat is the constant enemy of capacitors, and they are also wearing parts, if only over a very long time. But as you describe the symptoms - coming on after everything reaches full operating temperature - as capacitors age, their internal resistance increases which, in turn makes them get hotter, which increase internal resistance.... you get the picture.

At the same time, adding filament transformers will remove heat to the outside of the chassis, and may even address this possibility.

You asked for comments:

I would have run an experiment using a simple (cheap) separate filament transformer before ordering anything. If that experiment worked, I might have tried isolating one filament at a time to locate the root cause.

You could provide dc to the filaments using an external supply as an experiment. This could confirm or eliminate the filaments as the source of your noise.

Have you run the amps with the "forward" output tubes removed? Then with the "rear" tubes removed? If there is a proximity issue this may show it.

It's quite obvious that it's heat related. The room where I listen to music gets ~8°C
warmer in the summer so I thougth I will fix this before then. The tube filament
draws close to 10A so a regulated supply isn't that easy to find. I have some two
channel 1083 supplys that's possible to run in parallel but 10 amps is a lot so I
dropped that idea. That would have to be an external solution as well and I didn't like that.

I'm going to be more thorough when locating the filament wiring this time and hopefully the
amps will be dead silent after this little side project is done.

If you take the time to convert to 12V then a switchmode powersupply would fit fine.
The ecc82/12AU7 can use 12 by rearranging the filaments ( need to cut traces on the board sorry),
the powertubes might be connected in series a simple wiring would do.

Perhaps powering the powertubes would be all needed ? Thats a few minutes of work to arrange.

Transformers are already at the DHL pick up spot