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Old 7th January 2011, 07:12 PM   #1
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Default Very basic grid question????

Please look at this data sheet: [URL="http://www.tkhifi.com/datablade/6C45P.pdf"]

The grid is pins 2 and 8, the diagram says pin 2 goes to one side of the grid and pin 8 goes to the other. Are there really "sides" to the grid?

I see lots of schematics that have "grid stoppers" connected to pins 2,8.

What doses 2,8 mean?

If I short pins 2 to 8 and "stop" with a 330 ohm resistor from the short, don't I short out the grid?
Or do I add two resistors, one from pin 2, and another from pin 8?
Or do I have one resister from either pin 2 or pin 8, depending on how my physical layout lends itself to the resistor leads?

I hope this isn't too basic of a question, but I'm at a loss. About a year ago, I cobbled together a 6C45 spud and I shorted pins 2 and 8 and "stopped" with a 220 Ohm resistor. Now when I revisit the schematic, I'm at a loss why the grid isn't shorted out. I no longer have the bread boarded amp to cut the short to see if there is a difference.

I'm never too proud to show my ignorance!

Ed
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Old 7th January 2011, 07:36 PM   #2
DF96 is offline DF96  England
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Yes, most valve grids have two sides, corresponding to two support rods. In most cases the external grid pin is attached to just one of these - this is sufficient as the whole grid is metal anyway. Some high frequency valves, intended for grounded grid operation at VHF/UHF, have two or more grid connections externally in order to reduce the inductance of the grid connection - this improves gain and stability.

When such a valve is used for audio it is usually necessary to add a grid stopper in order to deliberately reduce its RF gain and so avoid instability. Many people use both grid pins for audio and then a grid stopper is needed for each pin. I don't know why they do this, as I think a single connection and single stopper is sufficient.

Connecting pin2 to 8 externally is, at audio frequencies, merely paralleling an existing internal connection so does no harm or good. You can use either pin for the grid. 220 ohms seems a bit too small for a stopper - I would use at least 1K. There was some nonsense on here a few months ago about small value stoppers being better, and that seems to have sent people off down the wrong path. If you use both pins then you need a stopper on both pins.

I wonder whether your confusion is caused by the fact that the standard way of drawing a valve grid makes it look like a set of separate bits of metal when in fact it is all connected together. If you use a meter you will find a dead short between pins 2 and 8.
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