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The late model CTC 38's use precision resistors in the 6BK4 circuit rather than a pot, to prevent people from fooling with the HV control. Have you measured the voltage drop across the 1000 ohm resistor in the cathode circuit? (note, 400 volts on both ends of the resistor). Should be about 0.9 volts (900 ma cathode current in the 6BK4). Sometimes the resistors in the circuit will go off value resulting in too much 6BK4 current, low HV, and a poor picture.
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