The
problem with the 866 tube is that when it is turned off for long periods
of time the mercury collects in different places on the tube envelope. When
you store the tube horizontally this doesn't help as it collects on the
side of the envelope 
The
specs call for a filament voltage of 2.5 and a amperage of 5.0,
I usually bring the filaments up slow with a variac and let the tube cook
for at least an hour. High voltage has not been applied at this time.
The
large transformer is the 5 volt filament transformer, rated at 5 amps. I
wired the filaments in series as I did not have a spare 2.5 volt transformer.
The smaller transformer being used is rated at 600 volts with a center tap.
No
switches were used on the test jig, I just plugged in the transfomers ONE
at a TIME, Plug in the filaments FIRST. Always
keep your insurance policies updated. 
Be
careful, when bread boarding a circuit like this, terminals are exposed.
Keep your cat off da work bench.
Interesting but dangerous.