Thermionic cathode heater filament conundrum

Question

An old CRT I have specifies 6.3 V (DC, I believe) on the heater filament, resulting in a 0.6 A filament current (spec).

But I measure the filament resistance as 1.8 Ohm.

At V = 6.3 V that would result in a 3.5 A filament current, about 6 times too high!

I'm tempted to measure the filament current with a 6 V DC battery pack but I'm afraid to burn the filament?

Any advice is appreciated!

If you try ... do it slowly. The filament must be "just" a little "red", ... if you see it ... — Antonio51, Commented Apr 7, 2022 at 18:09
Also at 6.3V, the supply was usually AC (though DC would work and it was designed to be compatible with 3 cell Lead Acid. — user16324, Commented Apr 7, 2022 at 19:39

vir · Accepted Answer · 2022-04-07 17:53:53Z

4

1.8 ohms is the cold resistance; it will increase as the filament warms to operating temperature.

answered Apr 7, 2022 at 17:53

vir

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\$\begingroup\$ Ah. That solves it. I didn't think the effect of temperarure would be that large. Accepted. \$\endgroup\$
– Gert
Commented Apr 7, 2022 at 19:23
\$\begingroup\$ It's probably >6x R increase at 1500'K \$\endgroup\$
– D.A.S.
Commented Apr 9, 2022 at 14:13

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