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One thing you can check without actually measuring the HV is the regulator current (6BK4.) You measure the voltage drop across the 1000 ohm cathode resistor. (Caution! 400 volts on both sides of the resistor!) Normal current draw is about 800-1000 microamps, which is about 0.8-1 volt dropped if I remember correctly. You are overloading the system if you are drawing say 1500 microamps, you are dragging down the HV with too much 6Bk4 current. On the other hand if the current is say 300 microamps your HV is probably too high and the tube may be weak if you can't adjust the HV control to make the 6Bk4 draw more current. Now this is not a total substitute for the real thing, using the HV probe to measure the anode voltage, but it can help you check out the 6Bk4 stage.
Using these tests has helped me find bad 6Bk4 tubes as well as off-tolerance resistors in the 6BK4 circuit.
Last edited by Chad Hauris; 11-14-2003 at 11:50 PM.
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