I haven't analyzed it carefully but it does look to me like Hero and TCM have mentioned. It appears that the output power is higher until Q3 is turned on. Maybe Q3 changes the oscillation frequency. Time constant of R6, R6', and C11 works out in the neighborhood of a second. This may be to either strike the arc, heat the filaments, or heat the gas in the tubes, or all of the above.
I also agree that it does not need to be "fixed".
Note: If anyone plans to build this thing, the 'ground' symbol on the diagram is just a common point and is not Earth.