Offered is a quartet of Xf2 O getter type dark brown based EL34s in good working conditions, made in UK by Mullard Blackburn between 1969 and 1972, whose scores on a calibrated Metrix U61B tube tester are listed here (from the Mullard datasheet: 100% = 100mA on the anode when g1 voltage is -12.2V and both anode and g2 voltages are 250V):
Tube #1, factory code Xf2B9I3, made in 1969 and labeled "Philips UK": 98%
Tube #2, factory code Xf2B1A1, made in 1971 and labeled "Telefunken": 93%
Tube #3, factory code Xf2B2J2, made in 1972 (rubbed away brand): 95%
Tube #4, factory code Xf2B9J3, made in 1969 (rubbed away brand): 96%
The tests have been performed on other seven relevant points, always getting very good figures and suggesting a 1-4 and 2-3 coupling.
A real world test has been also performed on an Hampton PA280 amplifier in order to verify the absence of noise, the full power output and the long term stability, even in an application that is beyond the recommended limits (555V on anode and g2, that is more than enough to destroy the majority part of both contemporary and actual EL34 variants).
Due to its sonic properties and its robustess, the Mullard Xf2 EL34 O getter variant manufactured between 1968 and 1973, with welded plate and interconnected g3 rods, is the unmatched benchmark for many modern clones.