EL34's seem to be really fragile!
Only the junky ones! The good ones aren't fragile at all, and last for years.
The old Sinos were the worst of the worst. Very prone to failure.
The Winged Cs that the HXDAs were factory equipped with are in fact SED Winged Cs made in the St. Petersburg Svetlana factory. A different company called New Sensor bought the rights to the Svetlana name marketing, at some point, so SED changed the name to Winged C to be able to market them independently.
The ones currently marked Svetlana aren't as good, in my opinion, they aren't as rich sounding on the low end.
The Winged Cs are in fact very good tubes, and still available on the tube market, though at somewhat higher prices than other recent tubes. If you get them, be sure not to buy from certain vendors who are selling factory seconds. The tube store has an explanation that they only bought firsts, so I'd buy from them.
I can't speak to the Russian Mullard-labeled EL34s. Obviously, they aren't made by Mullard, England, a company that only exists now in name. However, if you want to get very spendy, the original English Mullard EL-34s are available NOS if you look hard enough. Millard often rebranded their tubes for other manufacturers, so many are branded RCA, Phillips, Siemens, Valvo, GE, etc.
The NOS Mullard EL34s last for a very long time, and often have fewer problems than new tubes. Back in the day, players expected their tubes to last a long time. The idea of changing tubes every six months or every year was unheard-of. You ran your tubes until they stopped working or sounded horrible, and that took quite some time!
As an aside, I had 1 1/2 years on my Winged Cs in my HXDA50, and they still sounded fresh. I ran the amp loud. On my current HXDA30, it's only a few months old, but so far, so good. I run it wide open as well.
