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Old 4th August 2009, 09:38 PM   #1
iko is offline iko  Canada
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Default Question: grid current related

I have a question related to grid current. I'm thinking of using a tube at a very low voltage but the only curve that looks more linear at about 20V is when the grid is biased at +2.5V (see attachment, second curve from the left). What are the issues with biasing the grid positive, besides the obvious fact that there's a grid current? I'm a beginner, and any help is greatly appreciated.
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Old 4th August 2009, 11:14 PM   #2
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You're gonna have just esssssssss-loads of distortion. Any sort of Class *2 operation demands a Lo-Z driver that can supply the grid current demand. You didn't specify the type, however, you can find some low voltage plate characteristics Here. It doesn't look like operating at Vgk positive is necessary.
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Old 4th August 2009, 11:29 PM   #3
SY is offline SY  United States
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You've got two issues- your plan to run the plates at an extraordinarily low voltage and your interpretation of what the ip-vp curves mean.

The first is easy to handle- think three figures.

The second is almost as easy- look up "load lines."
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Old 5th August 2009, 03:02 PM   #4
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In my experience:

Tubes are a lot more linear at low voltages than the curves (and popular opinion, old wives tales, etc.) would lead you to believe. You may not need to drive the grid positive. The only way to tell is to experiment.

Gm gets very low and Rp gets very high at these extermes, so you will need a high-Z load. A CCS works well.

Because of the above issues, don't expect to get any significant power out under these conditions. Voltage amp, yes...

Also look at car radio tubes that are designed to run at low voltages.

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