For a triode operating in the linear region, the amplification...

The amplification factor $\mu$ of a triode is defined as the ratio of a small change in plate voltage to a small change in grid voltage, keeping the plate current constant.

$\mu = - \left( \dfrac{\Delta V_p}{\Delta V_g} \right)_{I_p = \text{constant}}$

The value of the amplification factor is primarily determined by the physical construction and geometry of the triode, such as the spacing between the grid wires, the diameter of the grid wires, and the relative distances between the cathode, grid, and anode.

It is practically independent of the operating conditions like plate potential, grid potential, and cathode temperature as long as the triode operates in the linear region.

Thus, it strongly depends on the separations of the grid from the cathode and the anode.