Did they fail suddenly or decline fast?And this is the one to avoid at all costs even though the 15 Euro price tag sounds enticing.
I learned many years ago to avoid Sovtek rectifier tubes.
they never last.
I bought 10 and did test all that they funcrion and output voltages were within few volts. I have put in use two but they only have less than 100 hours of use. Just few days ago I measured one output and I think it was about the same they initially were. Obviously they are 5Y3 copies? I have 5Y3s from many brands and they have output voltage differences which are spread farther each others and those my Sovtek fit inside them quite well.