What are the affects of the grid resistor on the first gain stage

[hesamadman]

Ive seen the grid resistor vary from 33k on an ac30 to 68k on a 2204 but what does this value affect?

[sluckey]

On amps that have two or more input jacks feeding a channel, the resistors serve to isolate two instruments so the volume control of one instrument does not affect the other instrument. The resistors also mix the two inputs together just as the mixing resistors seen at the output of some two channel preamps just prior to the phase inverter.

The resistor also works with the Miller capacitance of the tube's grid to reduce or reject RF interference.

The value is not critical but usually lies between 10K and 100K.

[hesamadman]

Oh ok. Thanks for that explanation. When i was doing some things on my breadboard I changed this value to see if there was much difference. Never noticed anything.

[HotBluePlates]

... When i was doing some things on my breadboard I changed this value to see if there was much difference. Never noticed anything.

Try very high, like 220kΩ-500kΩ. You will hear some degree of treble roll-off. Smaller value = roll-off happens at a higher frequency. Small emough = roll-off in super-sonic or RF range.