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in a pentode vacuum tube how does voltage and current affect screen operation? Answered

i know that the 2 grid (screen) affects the current on the plate, but how does that work? does more voltage on the 2 grid make more current be on the plate ? also how does the amount of current on the screen affect all of this. Finally, is the grid current limented? Or will i have to connect a resistor?

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gmoonBest Answer (author)2010-04-22

Don't get hung up on the operation of the screen. YES, it can be used to throttle current, but that's not it's primary function.

Keep the screen voltage below it's spec. maximum. Always keep it below the plate voltage. A series resistor to limit the screen current is a good idea, too.

The problem here is you're not up-to-speed on a couple of important concepts-- one is bias, the other is the operation of the main (control) grid.

The main grid is the one that has the most effect on current. It's the main "valve." The tube needs to be correctly biased so the grid is negatively charged in respect to the cathode. That grid voltage should be just negative enough that a voltage change on the grid will alter the current flow through the valve.

Expecting the screen to function as a variable current regulator is a bit like throwing an anchor out the window of your car instead of pushing the brake pedal...

I mentioned how to measure current flow in a pentode previously...

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electricfan (author)gmoon2010-04-22

so what your saying is that 1 grid is like a corse ajustment and the 2 grid is the fine tuning right

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gmoon (author)electricfan2010-04-23

Not exactly.

To use the car analogy, the main grid is like the accelerator, and the screen is the more esoteric adjustments like ignition timing, fuel / air mixture, etc.

Yes, you can control the engine somewhat with those other adjustments (and ruin the engine if they are wrong), but they're not really a "control mechanism."

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electricfan (author)gmoon2010-04-22

by the way why can't you take a multimeter and measure the current taken by the plate? can't you just measure that instead of measuring cathode consomtion?

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gmoon (author)electricfan2010-04-23

Measuring the plate might mess with the operation somewhat.

Plus you'll get the total current, since the cathode (and the suppressor grid, which is tied to the cathode) is the "source" of all the electrons, which "jump" off there...

I suppose you could just hardwire a cheap VOM between the ground and the cathode, if it's specs can handle it. But using a 1 ohm between cathode / ground is the way many ammeters work anyway--measure the voltage drop across a small resistance (like 1 ohm.)

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electricfan (author)gmoon2010-04-22

thanks again gmoon. i will be sure to thank you and steve when i publish a ,ible on my vttc

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steveastrouk (author)2010-04-22

You'd REALLY benefit from going to talk to Radio Amateurs (Hams) Some of them will be absolutely delighted to pass on what they know about valve design to a new generation.

It really is time for book learning. I've spent an inordinate amount of time in the last week learning enough to advise you.

Steve

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electricfan (author)steveastrouk2010-04-22

thanks for your help, now its time for ebay!

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