Some dumb Questions on Heater CCS supply

[jkeny]

Forgive the basic nature of these Qs but I'm building a LM317 CCS to power the filaments of two tubes (6AR8) which each take 0.3mA heater load.

Dumb Q1 - the tubes are connected in series - should the CCS be set for 600mA (2ohm R) or for 300mA (4ohm R)?

Dumb Q2 - I'm using a regulated 12V 2A SMPS to power this (for space reasons) - does it need a higher voltage to achieve the 300mA per tube? I'm concerned the regulation might cause a problem?

[dsavitsk]

Originally posted by jkeny
CCS to power the filaments of two tubes (6AR8)

Why?

Originally posted by jkeny
Dumb Q1 - the tubes are connected in series - should the CCS be set for 600mA (2ohm R) or for 300mA (4ohm R)?

300mA

Originally posted by jkeny
Dumb Q2 - I'm using a regulated 12V 2A SMPS to power this (for space reasons) - does it need a higher voltage

Yes. At least 2x 6.3 + the CCS dropout.

[jkeny]

Why? - The two tubes are a preamp addition to an existing power amp. Not enough current in the existing PS trafo & space limited.

300mA - thanks!

Aha, I feared this but thought I could parallel a 5V output from the SMPS along with the 12V. When I did this I found that it didn't give 17V but around 6.6V - something to do with the regulation I fear. OK I have a Dell laptop SMPS with 19.5V output which should do, I guess?