Hi,
I retubed recently an Audio Research VT200 with KT88 EH without any issues. I used to have also good experience in the past with Classic 60 amplifier (i had 2) with Svetlana SV-6550c and the other one with 6550 EH.
I heard also that the Svetlana (SED) KT88 are working very nicely as well, you should be able to plug into your VT50 with no issue
I retubed recently an Audio Research VT200 with KT88 EH without any issues. I used to have also good experience in the past with Classic 60 amplifier (i had 2) with Svetlana SV-6550c and the other one with 6550 EH.
I heard also that the Svetlana (SED) KT88 are working very nicely as well, you should be able to plug into your VT50 with no issue