Before totally giving up on it, you may want to check the HV filter caps and rectifiers in the cathode circuit. I don't recall if these use the old ceramic type, or plastic caps - ones typically rated for a few kV. I've had a few of the ceramic ones go bad here and there over the years. To properly test them, besides measuring the C, you should also juice them up to a fairly high voltage, comparable to where they operate, and make sure they don't break down or leak excessively. Usually the 1500 V peak, in DC-mode, on a 576 or 577 is enough to show problems with them - if they can't handle that, then they won't make it at twice that. For testing them without doubt, they should have the full cathode voltage applied, if you have a convenient HV source.
Ed
