The triode is fixed-biased by the grid-charge current that flows through the 1meg resistor. To explain this: The hot cathode always emitts some electrons that have enough energy to fly up to the grid. The grid resistor closes the current path between grid and cathode so that the grid discharges across the resistor and following Ohm´s law builts up a negative voltage between grid and cathode = ground.
This way of biasing works for many small signal tubes like 6DJ8, ECC88, 6922 but also for 12AT7, 12AX7 etc.. It works for pentodes as well, like DAF96, 1T4. It won’t work for power tubes like 2A3 or so because the bias voltage is in the range of -0.3..-0.8VDC only.