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RE: Tube/solid state DIY problem

The LND150 cascode is a CCS in series with the 220K resistor. It sets the voltage at 121.3V at the gate of the BFS862. This implies a current of ~0.55mA. This current source can be tested with a couple of 9V batteries and a 10R resistor. Connect + battery to the B+ connection and connect the 10R resistor to the R1/P1 node. The other end of the resistor connects to battery -. Measure voltage drop across the resistor. Should be about 5.5mV. If that is working, it's likely the BFS862 is toast. The DN2540 is a 400V part. If you don't have a jig to test MOSFETs just replace the DN2540. They are super cheap. I use a lot of CCSs and shunt reg. and keep spare MOSFETs and transistors on hand.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln


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