• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Question about cathode voltage

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crj11

crj11

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Joined 2008
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I am designing a tube tester and because of the way that it works, I need an N-Channel MOSFET between the cathode and ground to be able to open the circuit at various times during the test. I'm having a hard time figuring out what drain-source voltage rating the MOSFET needs to be.

The plate voltage will be up to 500V, so I could just put a 500V MOSFET there and call it a day. However, low resistance high voltage MOSFETs are much more expensive. From what I have read, I'm thinking that the drain of the MOSFET connected to the cathode will not see the full plate voltage when the MOSFET is off, so I can use a cheaper MOSFET. I was also thinking that I could put a high value resistor across the MOSFET (100K-1M) to help in limitting the off state drain voltage.

Does anyone have any thoughts or observations about the voltage on the cathode when the MOSFET is off that could help me out?

Thanks,

Chris
 
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