leftside
Headphoneus Supremus
I just took a look on eBay. Most of the GEC KT88 on there are pretty burnt out, but Langrex has a nice pair. Expensive I know, but good luck finding a genuine NIB/NOS for cheaper:
https://www.ebay.co.uk/itm/274978443831?hash=item4005fe1237:g:I3YAAOSwCC1hYC~O
Like the metal base GZ34 these things last for ages. And like the metal base GZ34 they will typically test quite high forever, and then suddenly one day fail. That's why you should always look at the side getter flashings on the GEC KT88's. Bigger/more silver = better. Smaller/more brown = worse. Honestly some of those expensive KT88 on eBay are on their last legs.
https://www.ebay.co.uk/itm/274978443831?hash=item4005fe1237:g:I3YAAOSwCC1hYC~O
Like the metal base GZ34 these things last for ages. And like the metal base GZ34 they will typically test quite high forever, and then suddenly one day fail. That's why you should always look at the side getter flashings on the GEC KT88's. Bigger/more silver = better. Smaller/more brown = worse. Honestly some of those expensive KT88 on eBay are on their last legs.