Effect of input resistor on tube amp

[funkgang49]

I was hoping someone could explain the effect of the input resistor value (usually 1 Meg) - raising or lowering would have on tube amp performance?

[roberto]

The effect is due mainly on the previous part, so the guitar.
And it depends on the pickup you use, but lower values mean less signal to the amp.

With piezos, if you lower this value you cut lows too.

[pdf64]

It basically sets the input impedance of the amplifier, and so to be pedantic, it will impact on the signal source, eg guitar pickup, rather than how the amp itself performs.
See http://buildyourguitar.com/resources/lemme/ especially Fig 14, to get a good understanding of the pickup circuit (which that resistor is a part of) and how the various parameters affect the response.
2M is the limiting value (when using cathode bias) http://www.mif.pg.gda.pl/homepages/fran ... /ECC83.pdf
Pete

[Structo]

It's called a grid leak resistor and sets the input impedance.