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can you run a vacuum tube under its rated filament current? Answered

I have a 811a tube and a 3 amp filli. transformer. I know that the 811a runs on 4 amps, but can't I use it with no harmful effects? I know it will run not as well, but will it kill the tube?

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Your transformer won't be able to cope, unless you can turn the voltage to the tube down as well.

what do you mean, will the transformer go into saturation, and what do mean by turning down the voltage?

The transformer will supply 3Amps, it will saturate and be unhappy trying to supply 4A. The only way to match the load to the transformer it to add some load to the filament, so that it only pulls 3A, or tap the transformer so it puts out 3/4 of the voltage.

Your tube won't work right, and you won't damage it under-running it, but your tubes' filament resistance determines the current that it will demand from the supply, not your transformers ability to supply it.

Steve.

Thanks, I've found 2 power packs, one 12v 300milliampa and one 4.8 volt phone charger(330mili). i plan to use a home made step down transformer to bring the voltage down to 6.3 and for the phone charger, i have the phone and the battery gives 6.3 volts so i will rig the charger circuit to the battery to give 6.3 volts. then the amps will add to about 3.95 (.05 won't make a difference right?) by the way i'm using it to make a vttc. also as a back up i have 2 compactatrons (6jm6) if the current is too little. wish me luck

Paul.

Sounds messy, but it just might work....
Good luck, as you asked ;-)
Steve

i love vacuum tubes, i love there glow, but i hate filiment transformers, there too heavy, or expensive, or most likely both!

Switched Mode Power Supply - very small for the power rating.