[G6Tanuki]

Anything more than 315-320V on the anodes of a 6V6 is asking for trouble in the long term; as noted upthread adding series resistors either in the links from the power-transformer to the rectifier anodes, or in the connection from the mains-transformer to earth, will help tame things.

[In practice it doesn't matter whether you use resistors in each rectifier anode-feed, or one resistor of the same value but twice the wattage between the power-transformer centre-tap and earth - electrically they give the same result, so which you use is entirely a matter of physical convenience and thermal dissipation]

I'd also check the valveholders; if there is a poor or intermittent contact between the anode-pin of the valve and the anode windings of the output transformer this can rapidly kill a valove: if the anode connection is lost, all the electrons being given off by the cathode then get collected-up by the screen-grid - which is not designed to handle the current and can end up being melted by the energy of this unscheduled electrode-bombardment.