In a pentode power amp - I work with guitar amps - if the tube is not conducting, the first place I look is to see if the screen grid resistor is open. No screen voltage, no signal through tube.
If you let the signal grid float, you better hope the thing is cathode biased, otherwise your plates will start to melt. Unless the screen is also floating.
If you let the signal grid float, you better hope the thing is cathode biased, otherwise your plates will start to melt. Unless the screen is also floating.
Pentodes are designed so that the SCREEN grid voltage does all the hard WORK ..But the PLATE gets all the glory......
The SCREEN voltage attracts all the babes (electrons).....but they walk right on by him and run right into the Plate with open arms
Without the SCREEN Voltage the Plate would get diddley squat...
Forces that "attract" electrons are related to Voltage Potential = Distance squared...... In simple first order terms...
Examination of pentode curves will show this.....
The flat section of the curves known as the saturation region has very little change in current with respect to change in plate voltage.... But change the screen voltage for that same curve and you will see a massive change in current...this shows there is a transconductance affect in driving the screens of a pentode...
In the real world ...a pentode is far from perfect and the screen will draw current durring operation.... ideally you should have 0 screen current, but this is a small current that occurs from the electrons that reflect from the plate based on the kinetic energy of the electrons and what the suppressor grid is able to handle....
YOu will sometimes see the screen grid glow when the tube is pushed ....
Chris
The SCREEN voltage attracts all the babes (electrons).....but they walk right on by him and run right into the Plate with open arms
Without the SCREEN Voltage the Plate would get diddley squat...
Forces that "attract" electrons are related to Voltage Potential = Distance squared...... In simple first order terms...
Examination of pentode curves will show this.....
The flat section of the curves known as the saturation region has very little change in current with respect to change in plate voltage.... But change the screen voltage for that same curve and you will see a massive change in current...this shows there is a transconductance affect in driving the screens of a pentode...
In the real world ...a pentode is far from perfect and the screen will draw current durring operation.... ideally you should have 0 screen current, but this is a small current that occurs from the electrons that reflect from the plate based on the kinetic energy of the electrons and what the suppressor grid is able to handle....
YOu will sometimes see the screen grid glow when the tube is pushed ....
Chris
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