Since I'm fixing a piece of audio equipment, I thought I'd post details here.
This is the chassis of a Bush tabletop radio (AC34) which was bought by my father in the mid-fifties. I remember it in our house as a kid. Then, later it got cast out to be used as a barn radio where it had a hard life. Birds roosted on it, spiders nested within, the veneer peeled away in the damp & it just gave up. I discovered it in a pile of junk in the early eighties, rescued it, had the veneer replaced, and fixed it up. Over the next 30 years it had periodic use (it has a shortwave band which can still be quite interesting to listen to), but I remember that it suddenly died around 10 years ago. I've been meaning to look at it since then &, now that I'm in "repair mode", its turn has come.
The ravages of time have taken their toll. But it has a common fault: the aged waxed paper capacitors have started to leak (electrically) & the output stage grid has slowly been pulled towards the anode potential of the previous stage. Presumably, the current increased and, finally the output transformer could take no more & the primary went open circuit. I've just unwound the transformer to investigate but, with 3000 turns of 0.1mm diameter wire on the primary, it's not something I care to rewind! Fortunately, I've found a modern equivalent &, after replacing all of the waxed capacitors with polypropylene types, and a few high-value resistors too, it works once more!
But we're not out of the woods yet: the radio still has a horrendous powerline hum, even using smoothed HT from my lab supply. Investigation showed that there's considerable leakage between the heater & grids in the output tube/valve. I guess that the mica insulators have accumulated a thin metal film from the excessive current flow. The tubes are all Mullard U-series types with heaters designed to be series connected. With approx. 50V AC on the heater, even a couple of Mohms leakage resistance feeds considerable hum into the audio circuit & the tube will have to be replaced.
I just had the crazy idea of zapping away the metallic deposit by putting a few kV between heater/cathode & the other pins. I'd need several Watts to vaporize this, so it would be a pretty lethal setup... It's probably fortunate that I don't have a suitable EHT supply to hand!
Maybe I'll post an update when I've replaced the UL41...