Success of the valve resulted from the use of the slip-coated cathode which enabled a high mutual conductance to be achieved without serious grid emission.
The mutual conductance of such tubes is highly variable with grid bias.
Have them tested for mutual conductance, gas and shorts before you install them.
If the quality or mutual conductance remains high, the tube being tested has 90-95% of its useful life remaining.
As a result, mutual conductance of the circuit and its gain can be improved.