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Author Topic: Question on grid current  (Read 1045 times)
WX2S
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Posts: 702




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« on: February 09, 2013, 07:49:46 AM »

Hi, all,

It's been a long time since I've played around with tubes. Could someone tell me what is happening physically when I have excessive grid current? Am I driving past the limit of bias so that the grid becomes positive with respect to the cathode?

Thanks and 73,
-WX2S.

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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
G3RZP
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Posts: 4464




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« Reply #1 on: February 09, 2013, 08:12:03 AM »

Yes. You've driven the grid positive - provided no drive, no grid current. If a tube is gassy, you can get grid current in normal operation, and some power tubes will give a small amount of grid current with no bias because of electron velocity being enough for them to reach the grid. Which is why you often see a maximum value of grid to cathode resistance.

Depending on the tube and the intended class of operation, grid current may be normal. OTOH, grid current could be a tube killer - depends on the tube.
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K8AXW
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« Reply #2 on: February 09, 2013, 08:33:33 AM »

Tube grids are a delicate element in the tube that doesn't tolerate excessive grid current very long, as the plate would. 

This is one of the reasons for monitoring grid current in an amplifier and why some amplifiers go so far as to have a high grid current trip circuit.

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WX2S
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Posts: 702




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« Reply #3 on: February 09, 2013, 09:00:57 AM »

Thanks, gentlemen. I was testing a medical pulled tube I noticed excessive grid current and I'm glad that I probably didn't damage the tube or the amplifier. My amp has overcurrent protection, and it didn't trip. It just went into the red for a second or two.

So the problem is probably not the tube, but the operator. :-)

73,
-WX2S.
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73, - Steve WX2S.
I subscribe to the DX Code of Conduct. http://dx-code.org/
KASSY
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« Reply #4 on: February 09, 2013, 11:41:24 AM »

I've seen it go both ways.  It was explained to me like this.  In an instant-on tube, the grid is in the way between cathode and plate, and can't help but be hit by electrons.  So there's always some grid current.  But if the grid gets overheated, then it starts emanating its own electrons, and grid current can go the other way.  That's bad because you get distortion.

But the warm-up tubes somehow aim the cathode electrons in beams that shoot in between grid wires, so there should be next to zero grid current if you're operating it correctly.

The electrons are supposed to be drawn to the highly positive plate, but if the amp is underloaded, then the plate voltage swing can drop very low, and now the grid appears more positive than the plate, leading to high electron collection and grid current.

I think that's how I remember it.  I notice on the amp I'm using that if I load it more heavily it makes less grid current.  AL82, pair of instant-on 3-500s.

- k
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G3RZP
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« Reply #5 on: February 10, 2013, 12:47:31 AM »

The main matter is the class of operation. Class A1, AB1 and B1 (B1 is pretty rarely found) have no grid current. Class A2 (very, very rare), Class AB2, Class B2 and Class C run grid current.

Grounded grid RF power amplifiers are generally Class AB2 and run with grid current. The amount varies with the loading.
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