[Charlie]

You can't bring your Horzontal circuits up on a Variac because your horz osc won't reach the correct operating frequency. As a result, the osc won't provide the horz output tube enough drive... then that would make the output tube overheat and ruin the tube. Removing the output tube keeps you from putting the strain on the tube and HV circuits that follow.

Once you've ran the chassis for a while with no apparent problems, then try firing it up with those tubes in place. If you've replace all your caps, chances are you'll be alright. The only thing to keep a lookout for is any HV leakage at the flyback and surrounding area.

Good luck!