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Old 13th September 2005, 06:24 PM   #1
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Question what would happen if...

Hey all,
Help me think this thing through. If you have a pentode, and you simply disconnected the G2 and G3, then would it
A) act as a triode
B) collect lots and lots of electrons on G2 and burst into flames
c) none of the above
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Old 13th September 2005, 08:01 PM   #2
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Answer C, if C2 and C3 are floating without reference to ground or other parts of the circuit...lots of distortion (secondary emission) as the plate electrons are knocked from the plate by other higher energy electons and strike the control grid .

dr._sleep
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Old 13th September 2005, 11:06 PM   #3
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I get it. The grids are still in the way, so they are going to muck things up, inlike a real triode.
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Old 13th September 2005, 11:32 PM   #4
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I had G2 become disconnected - open circuit on a KT88 Ultralinear Amp.

What happens is that the current in the tube drops from 55mA to 0.2mA.

You need voltage on that G2 for the tube to operate.

Cheers,
Ian
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Old 14th September 2005, 02:21 AM   #5
Enzo is online now Enzo  United States
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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.
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Old 14th September 2005, 02:33 AM   #6
cerrem is offline cerrem  United States
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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
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Old 14th September 2005, 03:06 AM   #7
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HA! If everone explained these things in terms of chicks...it would be much easier to understand.
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Old 14th September 2005, 03:10 AM   #8
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That was beautiful Cerrem!

*applauds*
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Old 14th September 2005, 03:34 AM   #9
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Aren't "What ifs" fun! Blowing up tubes in one's mind is far less messy (and expensive) then blowing them up in real life.
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