Here is a lucid explanation of why 68k input resistors are used on the inputs of guitar amps. Written by Randy Aiken, one of the smartest amplifier guys around. I got this off of a forum on the Internet. -Regis
Here's a question for all you amp gurus, the resistor from the input jack to the first grid of a tube amp, it's usually 68K. I usually see these in amps with more than one input and have assumed that it's to give a lower level input alternative. On Mesa boogies with one input there is no resistor here, I've assumed this is because with only one input there's no need to provide a lower level alternative. What I'd like to know is, are there other reasons for that resistor? What would the repercussions be to removing this resistor and going straight from the input jack to the grid, (of course keeping the 1Meg grid resistor), other than the one I've mentioned above?
Those resistors are not just put on the control grid for signal level attenuation purposes; rather, they also act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode (which is a sum of the grid-to-cathode capacitance and the Miller capacitance).
This low-pass filter does a couple of things: (1) it helps prevent high frequency parasitic oscillation in the tube itself; (2) it helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output; and (3) it can limit grid current when the tube is driven into the positive grid region.
You will notice on two-input amps, such as Fenders, when you plug into the low-level input, the 1 meg grid resistor is shorted out and the two 68K resistors act as an attenuator to cut the input signal in half. The downside of this is that the input impedance drops from approximately 1MEG down to approximately 136K, which is a rather heavy load for a high impedance guitar pickup.
If the resistor is connected in series with the input jack and before the 1MEG grid resistor, there is a small attenuation of the input signal (0.94 times). If the low-level input attenuation is not necessary, the resistor is best placed after the 1 meg grid resistor and should be soldered directly to the grid pin instead of back at the input jack. This will be the best position for RF and parasitic oscillation attenuation.
Bottom line: leave it on there. Hope this helps, Randall Aiken