Constant current heating undervoltage

[phixphi]

Hi all!

I'm new to CCS heating. Apologies if the topic has already been discussed.
I've implemented a standard
LM317 scheme for heating 12AX7 series connected
150mA at 12.6V. It works, I do get an ideal 150mA
with an 8.2R sensor (measured across 1% HQ resistors which are in series before the LM's). But the voltage
wants to stay at 10.8V to 11.8V depending on a tube.
So far I have been using cheap Chinese ones (3pcs) for experiments
and a one TAD specimen.

Are my tubes crappy? I can't imagine having to adjust the voltage everytime I change the tube, or??

Thanks for any help

phixphi

[DF96]

ECC83/12AX7 is intended for series or parallel supply, so any genuine valve should work OK given 6.3/12.6V or 150/300mA. Given the low cathode current they usually run, there should be plenty of emission even when the cathode is a little cooler than intended. I would not worry about it.

[phixphi]

DF96, thanks for a reply. I understand that at less than a mA emission underheating is not an issue, but I'm trying to understand why it happens. The tube as the data says is intended for the both series/parallel connection, so the heaters should behave pretty much the same? Some data state 11.4V as the minimum heater voltage for 12AX7, so I'd naively expect that if you supply the nominal current, the voltage should automatically be in the allowed range?
Perhaps my tubes are low quality and not representative?
Or do I demand too much from tube designers/manufacturers?
Cheers,
phi

[DF96]

Modern valves are not made to the same standards as old ones. In addition, the change of resistance with temperature of the heater works in your favour with a voltage supply and against you for a current supply. CCS supply will amplify any heater resistance error.