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Interelectrode capacitance and the birth of the screen grid

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42
Much talk about pentode tubes concerns the plate, cathode and control grid. The lowly screen grid doesn’t seem to get quite as much attention and its function is probably not as widely understood. The capacitance created between the control grid and the plate gave birth to the screen grid which will prevent feedback of the AC signal back to the control grid by way of capacitive action due to the relatively large size of the plate. Remember that any two conductors separated by a nonconductor is a capacitor in its most basic form. To reduce this problem, which ultimately causes a reduction in gain, a screen grid is placed between the control grid and the plate.

How the screen grid reduces interelectrode capacitance is by virtue of the fact that two capacitors in series will have less capacitance than either of the two capacitors. The same formula for computing impedance for two parallel speakers can be used to compute series capacitance. Cool!

What is a good operating voltage for the screen grid? Having the screen grid at a higher potential than the plate is not good for conduction nor is so low that it is more negative than the control grid. Personally I feel that it should be run at the highest voltage possible as long as conduction across the tube is not compromised. A choke filter inserted in the power supply between your main filter and the screen filter should put you in good shape to keep the screen voltage in check. Another option commonly used would be a 1K dropping resistor.
Also it is important to note that a bypass cap is needed to make the screen grid operate to its greatest potential. Like at the plate, the ac component will be inverted to be 180 degrees out of phase with the input signal and will cause a loss in gain by opposing conduction so a cap attached from the screen to ground effectively eliminates this undesired condition.
I’ll save the purpose of screen resistors for another discussion.
-ht
 

Tone_Terrific

Silver Supporting Member
Messages
34,464
Also it is important to note that a bypass cap is needed to make the screen grid operate to its greatest potential. Like at the plate, the ac component will be inverted to be 180 degrees out of phase with the input signal and will cause a loss in gain by opposing conduction so a cap attached from the screen to ground effectively eliminates this undesired condition.
Could you link to a schematic that shows how this is set up and/or an amp that uses it?
 
Messages
42
Could you link to a schematic that shows how this is set up and/or an amp that uses it?
Your screen supply filter cap(s) that form a pi filter with the main filter are in play here so there is no additional cap being used. Using the term "bypass" is done to explain its function in regards to preventing the screen grid from opposing conduction through the tube.
 

VacuumVoodoo

Member
Messages
1,545
Your screen supply filter cap(s) that form a pi filter with the main filter are in play here so there is no additional cap being used. Using the term "bypass" is done to explain its function in regards to preventing the screen grid from opposing conduction through the tube.
This is incorrect. Screen grid resistors isolate the SGs from supply capacitor. You need a very small AC impedance between SG and ground, that is a smoothing capacitor directly on each screen grid --> ground.
Usually 10-22uF works beautifully explanation.
Carry on.
 
Messages
42
This is incorrect. Screen grid resistors isolate the SGs from supply capacitor. You need a very small AC impedance between SG and ground, that is a smoothing capacitor directly on each screen grid --> ground.
Usually 10-22uF works beautifully explanation.
Carry on.
My article made no indication that a screen resistor was in the circuit. In fact at the end of the article I left any talk of the screen resistor for another discussion. The article was meant only to show how the screen grid affects tube performance. With no screen resistor, the screen filter will perform as I have indicated.
 
Last edited:

donnyjaguar

Member
Messages
4,194
Use the standard bypass capacitor calculation for the capacitor off your Sgrid factoring in the "dropping" resistor value. Ie: for 1MΩ drop to Sgrid and 20Hz desired knee, use 68nF.
 






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