Current limit of EL84 grid? - diyAudio
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Old 21st February 2003, 10:30 PM   #1
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Default Current limit of EL84 grid?

Hi All,

What would be considered too much current on the grid of a 6BQ5?

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Old 21st February 2003, 10:41 PM   #2
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Default GRID CURRENT.

Hi,

Quote:
What would be considered too much current on the grid of a 6BQ5?
On wich one?

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Old 21st February 2003, 10:44 PM   #3
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The signal grid.

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Old 21st February 2003, 10:49 PM   #4
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Default 6BQ5

Hi,

So what is your concern ?

G1, the control grid is hardly taking any current at all in normal operation.
Look at a datasheet and it will give you typical operation points.

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Old 21st February 2003, 10:52 PM   #5
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I guess my concern is what the result will be on the EL84 if I run my 5687 in the gain stage at 18 - 20mA other than sounding better. I didn't think it would be a problem. I looked at the data sheet and could find nothing about maximum current on the signal grid. Only a spec for the shield. Thanks Frank.

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Old 21st February 2003, 11:03 PM   #6
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Default 6BQ5

Hi,

Gavin,naturally you wont find anything on the control grids maximum dissipation (Watts,current) since it won't be taking any of significance.

I think your concern is one of : will I send my powertube into clipping with the 5687?
To which my answer is: normally not unless you send the 5687 into clipping already.

Look at the function of the grid as if it was the flowcontrol element of a mechanical valve and you may understand its' function better.

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Old 22nd February 2003, 02:10 AM   #7
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Gavin,

Your question really is: does the current of the driver flow 'through' the grid of the next stage? to which the answer is "no".
The current of a voltage amplifier flows within its own plate circuit - without the current, there would be no rise and fall of voltage across the plate resistance (the "load"), and hence no amplification. The current is DC. In an RC coupled stage the coupling capacitor blocks DC - so the "grid circuit" is an open one.
(there is some current demanded by the input capacitance, and to disacharge and charge the coupling cap - but it's very small, and that's another topic.)

So, basically you can drive the most fragile tube with 100mA of driver current if you like. No damage will occur.
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Old 22nd February 2003, 04:04 AM   #8
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Thanks Frank and Joel. I feel better now.

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Old 22nd February 2003, 04:07 AM   #9
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The control grid will not draw any current (well, some, but very very small amounts) when it is negatively biased.

now if G1 was positive in comparison to the cathode, then you'd pull some current worth mentioning.. (positive grid is denoted by a subscript '2' after the class, for A, AB, and B. I dont know about class C for sure.. but class C is a RF thing anyways..)

certain tubes need to be positivley biased if you want to get anything out of them, its just the nature of their curves.. (811 comes to mind)
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Old 22nd February 2003, 04:16 PM   #10
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Actually class C isn't an RF thing... if you want an efficient PA system, class C PP is the way to go. And there's plenty of class A and B RF stages. It's just that you can get much better (performance, efficiency, etc.) out of class C.

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