It may be a beautiful Bulgin or Velleman circular button switch.
Usually such a switch only controls heater current so that sound gradual fades in or out when switching on or off. Just like switching on a beautiful AKG C12a condenser microphone with the which has the Nuvistor 7586 electron tube deliberately underheated at 5V.
Low battery voltage can always supplied to the electron tube.
Such an arrangement may be used for example on the Pleiades V6 circuit.
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