[Arran]

Have you tried swapping out the Rectifier tube? Internal intermittent rectifier tube plate shorts possible? Not usually the prob tho, without the tubes giving a spectacular light-show.The 25-50 cycle power- trans will work on 60 cycles, but generally always run hot in my past experiences. Whenever I run across these sets, I always replace the 25-50 cycle ac trans with similar type 60 cycle trans I rob from other cheap parts sets as they come avail. I would suspect the power trans in your case(s)?
I remember I bought a couple nice TV sets many yrs ago for cheap at a garage sale ( without close inspection) of the mains, that never worked right. After closer inspection, I had purchased 2 TVs that had 50 cycle ac trans mfd in Mexico,for Mexico 50 cycle pwr mains, and although they worked, the "picture" was never clear & audio was distorted.Their lifespan was short also on 60 hz mains! Both ended up with smoked power trans & blown fuses. I have run across several old Canadian model Philcos with the 25-40 cycle trans that were previously totally-fried, windings internally by someone previous plugging them in using modern day 60 cycles mains due to internal excess heat inside the trans. Limiting incoming ac line voltage keeps these 25-50 cycle trans from smoking, for awhile, but its better to rid them if possible and replace them with other vintage 60 cycle types when possible.

That makes no sense whatsoever, I'm sorry but there are hundreds of sets in Canada still with their original 25 cycle transformers that still operate perfectly with proper secondary voltages, most even state on the build plate, 25/60 cycle. Given that 25 cycle transformers have a larger core, and use a heavier gauge of wire, this means less core saturation and cooler operation, the physics say the opposite. We have not had 25 cycle power for domestic use since the late 1950s, only 60 cycle, the only sets I have personally seen with bad transformers were sets that experience a dead short or other high load condition in the power supply, most of those were 60 cycle sets by the way. If running 25 cycle sets on 60 cycle was a problem then we would see many, many more radios up here with smoked power transformers. If there is excessive B+ there has to be another cause, shorted primary or just a higher line voltage.
Best Regards
Arran