Grid is Referenced To Ground through resistor? WHAT?

[Eddiegnz1]

in The schematic below

https://www.dropbox.com/s/kjj7xz205q5412h/PreAmpAnodeLoad.JPG

The text that goes with it says;

"The Grid represents no load to the incoming signal. The grid is referenced to ground through the Grid resistor. The Grid resistor represents the input impedance"

I don't know what any of that means but I'm very interested in learning what it means that "the grid is referenced to ground through the Grid resistor".

Can anyone explain that to the level of extreme beginner?

Kind thanks,
Eddie

 

[SY]

If the grid draws no current (and it shouldn't), the voltage drop across the resistor is zero by Ohm's Law. So the grid is at DC ground.

 

[Eddiegnz1]

I'm absolutely certain that someone here understands exactly what you mean, SY. ;-)

but I would definitely benefit from a slightly more expanded explanation if you know what I mean.

I understand that the audio signal changes the voltage of the grid which in turn causes changes to the current that flows from the cathode to the anode. But I still don't know what it means that "the grid is referenced to ground through the Grid resistor".

But I do kindly thank you for offering an explanation.

 

[TheTubeAmper]

If you've ever worked with digital circuits and especially mosfets, than you should be familiar with the terms "pulled up" or "pulled down" where you use a resistor on the gate to sort of bias it to positive or "pull up" or you can bias it toward ground or "pull it down" to ground.

Hopefully you know what I mean by those terms, if not I'm not sure how much more I can explain it other than it just sort of biases it, instead of leaving the pin floating, it biases it toward ground.

 

[Hearinspace]

I'll try,
If you look at the basic operation of a triode, the rate at which current flows through it is controlled by the grid - which you understand. The grid acheives its control of current flow by means of its voltage relative to the cathode. (Voltage is always a relationship between two points)

In the cathode biased triode in your schematic, there is a 2K2 resistor between ground and the cathode, through which all the tube's current flows. Because all the tube's current flows through that resistor it will raise the cathode above ground by the amount of the voltage dropped across that resistor. (The actual amount is governed by Ohm's Law) So now, if we connect (reference) the grid to ground, it will share ground's relationship to the cathode, ie. by the amount of voltage dropped across the cathode resistor.

OK so far?

But now however, whatever signal source is used to supply the grid with a driving (audio) signal will be shorted directly to ground. Not good unless you want to mute the circuit, so we put a resistor between grid and ground to keep ac signal at the grid but allowing the grid to maintain its voltage relationship to ground - and thus to the cathode - at DC. . . . . and this works as long as there is no dc current through the grid resistor. If grid current does flow then that will alter the picture, so the cathode biased triode is usually set up so as to prevent current from wanting to go up through the grid resistor. (Don't worry about it, yet)

If this still doesn't make sense I'd suggest you do a search on tube biasing. There's a ton of stuff out there , including

NEETS

Inside the Vacuum Tube

Understanding the Tube Amp: Biasing the Triode

 

[DF96]

If the grid draws absolutely no current and the coupling capacitor is perfect with no leakage then the resistor is not needed, except when first switching on. Real life is different. The grid will draw a little current and the capacitor will be ever so slightly leaky so we need a resistor to set the DC bias on the grid. As the currents are very small the resistor can be rather large and still do its job.

 

[fpitas]

"Referenced to ground" in this case implies a DC reference, or bias point. As others have said, the grid may not be precisely at DC ground. I'm not sure what you're asking, either.

 

[Merlinb]

iCan anyone explain that to the level of extreme beginner?

Ever walked across a carpeted floor, then touched a grounded object and received a shock?
Your body gets charged, raising it to a high voltage relative to ground. When you touch the grounded object, your body discharges back to zero volts.

Same thing happens with the grid. It too can collect charge and float up to some unknown voltage (not what we want!). Adding the grid leak resistor is like keeping your hand always on a grounded object- you stay firmly at zero volts. So does the grid.

 

[12E1]

Ever walked across a carpeted floor, then touched a grounded object and received a shock?
Your body gets charged, raising it to a high voltage relative to ground. When you touch the grounded object, your body discharges back to zero volts.

Same thing happens with the grid. It too can collect charge and float up to some unknown voltage (not what we want!). Adding the grid leak resistor is like keeping your hand always on a grounded object- you stay firmly at zero volts. So does the grid.

More precisely... the grid stays (almost firmly) at zero volts apart from the fact that the input signal is coupled to the grid via the input coupling capacitor. So the grid sees the rapidly varying voltage of the input signal but with the average voltage being zero volts rather than any other average voltage that may happen to be present on the input connector.

(not to take anything away from MerlinB's post - just trying to clarify for the OP)

 

[jjman]

For an “extreme beginner” I would say that the important takeaway from that statement is that the signal flows from the previous section to ground thru the grid resistor. None of the current of the “input” signal flows into the tube thru the grid, although the signal’s VOLTAGE is present on the grid. All of the current goes directly to ground thru the grid resistor but the grid “sees” this happening. The grid is monitoring the signal but not taking any of it. The grid is just a wire floating inside the tube and connected to nothing inside. It is a magic wand that “tells” the electron to move faster or slower.

There are some exceptions and variables but it’s best to 1st learn this concept before learning the exceptions and such.