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 guys
around. I got this off of a forum on the internet. -Regis
Why 68K Input Resistors
plexibreath wrote:
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?
JKB
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 1MEG 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