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Silicon · 7th Nov 2018, 6:29 pm

Hello Mike G4WYZ,

When you were suggesting an alternative MOSFET you wrote - 'Which are rated at 800 volts at 2.5A, this explains why they run cool, requiring no heat sink.'

They will only run cool if they are passing a small current or dropping a small voltage.
If you test an EF86 at 4mA at 250V they will run cool.

If someone tries to test a power valve at low voltage and high current, the MOSFET in the anode regulated supply will get hot. It will need a heatsink.

7th Nov 2018, 6:29 pm	#1509
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,161	Re: The "Sussex" Homebrew Valve Tester. Hello Mike G4WYZ, When you were suggesting an alternative MOSFET you wrote - 'Which are rated at 800 volts at 2.5A, this explains why they run cool, requiring no heat sink.' They will only run cool if they are passing a small current or dropping a small voltage. If you test an EF86 at 4mA at 250V they will run cool. If someone tries to test a power valve at low voltage and high current, the MOSFET in the anode regulated supply will get hot. It will need a heatsink.