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Old 20th Aug 2015, 9:58 am   #1
ms660
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Default Control Grid Resistor?

I've just been looking at the data sheet in the link below for the 6V6, it gives what I assume to be two maximum grid resistor values, 100k for fixed bias and 500k for cathode bias, does anyone know why such a large difference? I've noticed similar on some other valve specs.

http://frank.pocnet.net/sheets/191/6/6V6.pdf

Lawrence.
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Old 20th Aug 2015, 10:10 am   #2
G8HQP Dave
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Default Re: Control Grid Resistor?

Cathode resistor bias is partly self-adjusting, as an increase in cathode current causes the grid to go more negative (with respect to the cathode). This means that a little bit of grid current does less harm than for fixed bias.
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Old 20th Aug 2015, 10:49 am   #3
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Default Re: Control Grid Resistor?

What's confusing me is the quote below from the navymars site on valves, where it says grid current will flow back from the grid to the cathode via the grid resistor, I'm reading that rightly or wrongly that the lower the value of the grid resistor the more grid current can flow which means according to the 6V6 valve spec more grid current can be allowed to flow using fixed bias using a 100k grid resistor as opposed to using cathode bias using a 500k resistor.

I'm still not fully with this.

"We found that the positive charge on the grid
caused more plate current to flow. However, when the grid becomes positive, it begins to act like a small
plate. It draws a few electrons from the space charge. These electrons flow from the cathode across the
gap to the positive grid, and back through the external grid circuit to the cathode. This flow is known as
grid current.
In some tube applications, grid current is desired. In others it is relatively harmless, while in
some, grid current causes problems and must be eliminated"

Lawrence.
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Old 20th Aug 2015, 12:40 pm   #4
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Default Re: Control Grid Resistor?

Panic over (I think)...Found a couple of articles which seem to sort it out, this one in the heading Grid Current:

http://www.john-a-harper.com/tubes201/

And this one in section 6.1:

http://sound.westhost.com/valves/design.html

Lawrence.
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Old 21st Aug 2015, 5:50 am   #5
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Default Re: Control Grid Resistor?

Interesting what was said about the problem of driving OP valves with low grid stopper R value's in a linear fashion etc. Found the transistor driver for OP valves especially so.

Noticed you've been hitting the books Lawrence, or the online equivelent. What are you thinking of making or is this research?

Andy.
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Old 21st Aug 2015, 8:54 am   #6
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Default Re: Control Grid Resistor?

Just swotting up..

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