Purpose Of Series Input Resistor?

[PepeSilvia]

I noticed some amps have a high value (10s of kOhms) resistor in series with the input. What purpose does that resistor serve?

[Tom Phillips]

If we look at the classic Fender input circuit you are talking about the 68k resistors. They do several things:
1) They sufficiently isolate the two inputs and sum the signals when both inputs are used at the same time.
2) They form a voltage divider attenuator when the #2 input is used by itself. This makes the #2 input ~6dB less sensitive.
3) They act as a "grid stopper" for the input tube. This attenuates high frequencies that could creep into the amp. These are well above the guitar high end. The grid stopper implementation is a compromise since the resistors are so far away from the tube grid pin.

HTH,
Tom

[jrfrond]

If we look at the classic Fender input circuit you are talking about the 68k resistors. They do several things:
1) They sufficiently isolate the two inputs and sum the signals when both inputs are used at the same time.
2) They form a voltage divider attenuator when the #2 input is used by itself. This makes the #2 input ~6dB less sensitive.
3) They act as a "grid stopper" for the input tube. This attenuates high frequencies that could creep into the amp. These are well above the guitar high end. The grid stopper implementation is a compromise since the resistors are so far away from the tube grid pin.

HTH,
Tom

#3 is the most relevant, since it's hard to find even a SINGLE input amp without this series resistor.

[R.G.]

The responses are correct. I personally believe that the mixer and attenuator uses came first, and later the RF and interference blocking was noticed. These resistors are almost universally put out in the jacks themselves, which is the worst place to put them for grid stopper action (stoppers go as close to the grid socket lug as you can get them!) but combined with cable capacitance, they do form a crude RF attenuator. But I don't actually know what went on in the head of the originators of this technique.

The series grid resistors do get in the way. It's not common knowledge, but a tube grid clips more softly the lower the impedance it's driven from. Drive a 12AX7 grid through a 1M resistor and it can clip like a razor edge. Drive it from 10K and it clips much more softly because the added ability to eat grid conduction electrons on the part of the driving source lets the grid go ahead and enhance conduction to the plate for a bit into positive-grid territory. The clipping gets softer. This is the basis of Class AB2 power amp drives; it works on input tubes too. Putting a booster on the front end of an amp with a quite small series grid resistor can do some neat things for front end clipping. The boosters you use to overdrive the front end get softer and more juicy as the series resistance is reduced.