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12ax7 heater problem

Started by Ice-9, February 12, 2009, 10:47:56 AM

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Paul Marossy

#20
February 12, 2009, 05:57:21 PM
Quote from: Renegadrian on February 12, 2009, 05:49:46 PM
He has a 4049 based voltage multipler!

Oh, I must have missed that little point.  :icon_redface:

Ice-9

#21
February 12, 2009, 06:44:02 PM Last Edit: February 12, 2009, 06:57:23 PM by Ice-9
Quote from: Paul Marossy on February 12, 2009, 05:47:30 PM
Quote from: R.G. on February 12, 2009, 12:14:49 PM
Let's play ohm's law.

Quick, what voltage drops across a 390 ohm resistor when you pull 150ma through it?

V = 0.15* 390 = 58.5V

Mother Nature is whispering in your ear "You ain't gettin' 150ma through that resistor, guy. Not with those voltages."

OK, dumb question time. How can you have 58.5V when your supply voltage is only 18V?!

This refers to the 390r resistor, ohms law , v=I*R   V=150ma for heaters * 390 ohm for the resistor  58.5v that the resistor trying to drop, and thats not possible from 18v so the resistor needs to a lot smaller value.

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