That "missing" input resistor.

[jjman]

Not sure about the other models but the Express does not show an in-line resistor on the input to the 1st grid. It looks like many/most amps have one. Is it for blocking RF? EMI? Something else? Would it be better to keep the grounded 1meg resistor on the grid side or the input side of such an in-line resistor? I’m seeing it done both ways including on input #1 vs. #2 of BF/SF Fenders.

I play in some noisy rooms. One has unbelievable interference in the corner of a basement, with all my amps, yet it’s quiet a few feet away. Sometimes you have no choice of where to place your amp so I’m wondering if this is something I should try adding to my build? Anyone add one with any positive/negative results?

[Andy Le Blanc]

its a grid stop resistor ,it augments the tubes miller effect (capcitance)
rolling off high frequency, and should as close to the socket pin as physically
possible , depends on the layout, usually the 1 meg is at the input jack for
convienence ..... interference can have many causes make sure your in a
good grounded receptical and turn off whatever is causing the buzz

[doctord02]

Yeah, like Andy said it's a grid stopper. If you find you need it for noise/interferance purposes try a 10k or a 33k before slapping the typical 68k in there. I also know some folks use a ferrite donut bead and thread the input wire thru it a couple times instead...

[jjman]

I'm using shielded wire and may add a resistor next time I'm in there. There was one point when I was picking up a Spanish radio station during the build but that happened only once. Some of my guitar cables are pretty old which may have been a factor.

I'm thinking that typical amps used these resistors to isolate input "1" from "2" also. With only one input that wouldn't be needed. I've also read that a small cap across the input jack would eliminate RF.

Thanx for the feedback.

[doctord02]

I'm thinking that typical amps used these resistors to isolate input "1" from "2" also. With only one input that wouldn't be needed.

Actually, look at the schematic for almost any Blackface or later Fender, or any classic Marshall, and you'll see 68K resistors on both channels, and both inputs.

[Andy Le Blanc]

there can be a lot reason for interferance..... if it comes and goes with
location it might not be your amp...... or it could be a ground loop in the
wireing (dressing) of your amp.....

[drz400]

Yeah, like Andy said it's a grid stopper. If you find you need it for noise/interferance purposes try a 10k or a 33k before slapping the typical 68k in there. I also know some folks use a ferrite donut bead and thread the input wire thru it a couple times instead...

I dont know where everyone gets 68K from
On a Marshall the 2 - 68ks are in parallel making it 34K ? At least when using the high gain input

[jjman]

Same for the BF/SF Fenders. 2 parallel 68k's when using only a "#1" input. I used a 33k for my homebrew Champ and will probably use something like that next time I'm in there.