i.e. the problem is that the cathode signal
has to drive any loading it sees due to the PSU, so a high impedance might be preferred.
Slainte, Jim

Yes, you're right on the nail - that's a common reason given by those
who have developed and use current sources - the high impedence. Here
is a useful reference on DHT filaments which refers exactly to this.

http://www.tentlabs.com/Products/Tub...ent/index.html

http://www.tentlabs.com/Products/Tub...ingmethods.pdf