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There is usually a 1000 ohm resistor in the cathode of the 6BK4, measure the voltage drop across the resistor. Voltage drop should be around .8-.9 volts corresponding to a current of 800-900 microamperes. If there is much more than this the 6BK4 is probably drawing too much current.
Too much 6BK4 current will drag the HV down. Have you traced the voltages back through the 6BK4 grid/cathode circuit to a place where they are normal? I have seen several cases where resistors changed value in this circuit and caused problems.
It sounds like a thermal problem since the HV goes down as it warms up...try spraying freeze spray on the resistors in the 6Bk4 circuit.
Also just to rule out tubes have you tried a new 6BK4 and HV rectifier?
And, keep in mind that problems in the horizontal output tube/damper circuit can cause low HV as well. Measure the screen and grid (not plate) voltages on the HO tube too.
Last edited by Chad Hauris; 01-27-2004 at 08:08 AM.
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