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Old 3rd January 2011, 04:06 PM   #1
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Default Minimum Gain Stage Rg Value?

I can find a lot if info about the Maximum Rg value (data sheet, equations, etc.) but nothing on Minimum.

I know in general you want max Rg so you get as much gain as possible. But what if you want to use Rg to help attenuate gain?

I see a lot of schematics that use a pot and keep the specified Rg in place. The problem I have with this is if the pot is all the way up Rg=0 and that doesn't seem right (no scientific support, just seems wrong). I would prefer to design an attenuation circuit that insures some min. Rg value.

Is there a way to determine what minimum Rg can be? I would settle for a rule of thumb.

T
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Old 3rd January 2011, 05:16 PM   #2
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Hi,

I'm assuming you are referring to the arrangement where the incoming signal goes through a potmeter track to ground, and the wiper is connected to the next tube's grid.

When using such a volume pot it is best to retain Rg so the grid doesn't go open circuit when the wiper loses contact with the pot (which is a typical failure mode due to dirt on the track, or mechanical vibration).

If the pot is turned all the way down then yes the grid circuit resistance is zero but then again muting is usually exactly what is desired when turning a pot all the way down.

Note however that the load impedance formed by the pot + the Rg, as seen from the previous amplification stage, does not change very much when the pot is turned. So the effective gain of that stage is not affected very much.

HTH
Kenneth
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Old 3rd January 2011, 07:28 PM   #3
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Quote:
Originally Posted by Troncones View Post
I can find a lot if info about the Maximum Rg value (data sheet, equations, etc.) but nothing on Minimum.
The minimum is Rg= 0: dead short between grid and AC ground. Of course, you won't get much in the way of usable signal. Making Rg negative makes an oscillator, and is usually avoided.

Quote:
I know in general you want max Rg so you get as much gain as possible. But what if you want to use Rg to help attenuate gain?
You definitely do not want to do that. Reducing Rg will force the driven stage to operate into a more nearly vertical loadline, and that will mean less headroom and more distortion.

Quote:
I see a lot of schematics that use a pot and keep the specified Rg in place. The problem I have with this is if the pot is all the way up Rg=0 and that doesn't seem right (no scientific support, just seems wrong). I would prefer to design an attenuation circuit that insures some min. Rg value.
In this case, what they're doing is using Rg as a guard resistor so that the grid doesn't go completely open circuit if the volume pot fails open (as they have a nasty habit of doing).

Quote:
Is there a way to determine what minimum Rg can be? I would settle for a rule of thumb.
There really is none at all. In some RF designs, Rg can be made as small as 50R or 75R in order to match the characteristic Zi of the coax used to make the connection. You're more often concerned with making Rg as large as possible. In those cases where you do need a Lo-Z input, select Rg to match as required.
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Old 3rd January 2011, 08:03 PM   #4
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The grid resistor generally serves as the input impedance for a tube stage and, as such, its value is determined by the best compromise for an appropriate input resistance. The grid resistor for following stages is in parallel with the plate resistor of the proceeding stage making its choice significant. To help understand its importance, compute the composite resistance of the plate resistor and the grid resistor and use that resistance for your load line when determining operating points for the stage. You will see that lowering the grid resistor for the following stage makes a big difference in the load line making it more vertical and generally increasing second harmonic distortion. So for practical reasons the grid resistor is made as large as possible to reduce the load on the following stage.

Many tube manuals will give a minimum grid resistance for power tubes. This is where the compromise is really apparent. The driver stage wants to see an "easy" load, i.e., a large grid resistor while the power tube needs a low resistance between grid and ground so that any grid current will have a easy path to ground. When the electrons that collect on the grid have a high resistance path to ground, the grid starts to collect them making the grid more negative and decreasing current through the tube. This will obviously create distortion as the tube no longer follows the grid signal.

So grid resistors, just like marriages, are a function of compromises.
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Old 3rd January 2011, 08:19 PM   #5
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i must have the perfect value for a grid resistor then
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Old 3rd January 2011, 08:47 PM   #6
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Thanks Everybody,

If I understand correctly, the Rg for the input stage can be changed and the composite value (Rg and Tube) can be used as part of input impedance and an attenuation circuit. In addition the Grid should always have a reference to 0 with some resistance.

Regarding Rg for the driven (output stage), I should finish reading the chapter on Gain and Coupling in the Army "Theory and Application......" book (for starters).

T
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