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resistor or a zener diode.
Tube purists might  use valves as a constant current source for
audiophile quality valve amplifiers  but I am a pragmatic practical
minded person instead of a tube purist audiophile and I prefer simple
set ups that work well.
 
For the Hammond preamps and the Leslie amplifiers  and guitar amplifier
applications  the constant current source circuitry  is overkill and
totally unnecessary because the conventional method of supplying the
voltage to the valve plates with the plate resistors has worked well for
many decades. This is a case of ''if it aint broke don't fix it''
similar to my recent experiment with DC filtering for the AO28 heater
filaments .
All the best.
Kon

David Anderson wrote:
Google it. There is tons of discussion of the Constant-Current Source  
on tube DIY sites. Christoph is correct in his description, but you  
can actually implement a CCS using a pentode rather than solid state  
devices depending on how much of a tube purist you are. They got a  
lot of attention as an upgrade to the Foreplay preamps.
 Try searching 'Camille C4S'.