Looking through the data sheet for the 5751 I noticed the maximum rating for 'Grid Resistance' was 0.5MΩ. Does this refer to the largest value of grid leak resistor one can use this this tube? If not, can any one tell me what it refers to? What happens if you exceed it? 500kΩ seems like a woefully small value for setting the input impedance of a gain stage. Relatedly, can any one tell me what the upper limit on a grid leak resistor is and the logic involved in arriving at this value? I have seen them as high as 5.6MΩ. Isn't higher = better, generally? What do you lose by increasing this value?