why use audions in the constant current circuit?

[change]

i studied a constant current source circuit showes below,and i don't
understand why there are some audions in the circuit, what effect do
they take?who can tell me?
thanks
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[Jon]

Change,

The link doesn't work for me, so I can only speculate. If the audions,
also known as valves (veddy veddy British) and tubes (crass American)
are pentodes, pentodes have a constant current characteristic when the
plate voltage is above a certain value. Matter of fact, their
characteristics are very similar to FETs, which also exhibit this
characteristic, i.e., the current is relatively independent of the
Plate-Cathode (Drain-Source) voltage.
Regards,
Jon