Zenith Hybrid Low HV

[Plumbicon]

Could also be excessive leakage current in the multiplier block. You should try disconnect it (unsolder the wire coming from the HOT secondary going to the block input) and mesure if you still have excessive sweep tube cathode surrent and reduced boost voltage.
Is the H.O transformer external high voltage winding hot to touch after a few minutes of operation ? That would be an indication of either a defective multiplier block or HO transformer (partially shorted secondary HV windings ).