Merry Christmas Martin,
some of the biggest problems with "Stahlroehren" like EF12 and VF14 is that the Dumet wire used for the connection Fernico seal-socket is only copperplated steel that corrodes over the years and flunks off, mostly cracking and braking. Most likely for VF14 is the filament short when the plate voltage jumps 35-38V to 90V and back. Sometimes those tubes can live for quite some time, but most likely they will go bad fast.
Similar problems have the early AC701, when the Dumet wire was only tinned, after revision 2 (Index/serial number ends with 3) a three piece lead was incorporated, that replaced the dumet part outside of the tube with NI-steel wire.
There is no way to fix them, I saw people smearing nickel or silverpaint to get conductivity, but it won't really work that well.
Hope that helps,