[Amps] 30L-1 Blows Fuse at 600ma

[JOS Earthlink]

Running a 30L-1 with a Collins 32S- exciter at 80-90w.  The 30L-1 tunes up
fine at low-power (20-30w) but as exciter output is increased and the 30L-1
reaches 600ma plate current it blows it's 8a HV fuse.  Maximum output is
about 500-600w before fuse blows.  Happens with either 811s or 572Bs.  Plate
voltage unloaded is ~2000v and when loaded it drops to ~1800v.  On other
30L-1's in the shack, I can easily drive the amp to plate currents of 1.2a
with no blown fuse.  Is this indicative of a bad transformer drawing too
much current in order to supply the 600ma /1800v to the plates?  Any other
possibility likely?

73 Jerry
K1JOS