Tube Heater circuits, what am I misunderstanding?

[suncrush]

According to the data sheet 12A_7 tubes use a 12.6 V, 150 mA heater circuit.  That maths out to a resistance of 84 ohms.

I checked the resistance across pins 4 and 5 of the the two 12AU7's I have.  One returned 12 ohms, the other 16 ohms, which would draw up to 1 amp at 12.6 V.

So now, I'm totally confused.  Why don't the tubes match the data sheet?  One is a new JJ, the other is a NOS Sylvania.

There's clearly something going on here that I don't know, since this is my first tube-based project, and I'm hoping you guys can assist.

Thanks!

[merlinb]

According to the data sheet 12A_7 tubes use a 12.6 V, 150 mA heater circuit.  That maths out to a resistance of 84 ohms.

I checked the resistance across pins 4 and 5 of the the two 12AU7's I have.  One returned 12 ohms, the other 16 ohms, which would draw up to 1 amp at 12.6 V.

The resistance of heaters and light bulb filaments will increase considerably as they warm up.

True, when you first apply power you will get a surge current of up to 1A which falls as it heats up. This is why light bulbs usually burn out at the moment you switch them on. Tube heaters are a lot more robust, thankfully.

[suncrush]

Thank you!  I figured that was the case, but wanted to check before I plugged it all in.

How does one keep from blowing fuses?  Does the resistance rise nearly instantly?  The power supply I have is a 750 mA supply, and I'm concerned it won't fire up.

[Brisance]

probably the supply will handle the initial transient. You could start them from a reservoir cap if in doubt(in fact if supply has enough reservoir caps to begin with, won't be a problem most likely)

[suncrush]

You guys are the best.  Thanks!