[Technomaniac]

In such a situation I would use a Variac (Variable Voltage Transformer) and meter the HT while slowly bringing up the mains voltage from about half normal. You may find that the voltage when loaded doesn't go above the specified level. You would want to increase the input voltage slowly, to give the heaters time to get the valves drawing current. A light dimmer that uses a triac (rather than a thyristor) can be used as a poor man's variac in most cases. Be careful with those Jones Plugs, the cases can become live if not earthed. Better not to touch the metal cases.