input resistors and guitar volume pot

[smolder]

I was reading from Blencowe's book on my commute and he talks about the 68l coupling resistor prior to V1 as being overkill. He goes on to say that it great contributes to the tone blanket many experience when lowering the volume on the guitar... especially guitars with 500k pots. He suggests that 10-33k is plenty, yet I never see inputs set up like this in schematics. Is he wrong?

[Keyser Soze]

http://www.aikenamps.com/InputRes.htm

[TweeDLX]

My Accomplice has one input, one 33K resistor.

[mbratch]

Originally Posted by Keyser Soze

http://www.aikenamps.com/InputRes.htm

That article is about grid resistors, not input resistors.

The reduction in highs that occurs when lowering volume is often resolved by adding an appropriate cap or RC combination to the volume pot on the guitar. I haven't read about lowering the input resistor in the amp. That's an interesting idea...

[elkym]

My 5f1 champ clone has a 22k input resistor.

[DGDGBD]

Originally Posted by TweeDLX

My Accomplice has one input, one 33K resistor.

I think that's the same as two 68K's in parallel in the common dual input setup. have the same single 33K in my single-input spitfire clone.

[jay42]

The use of 68K for DIY is sometimes a simple mis-read of Fender schematics. Unless it's a clone, I don't wire Hi/Lo jacks. In the typical Fender input, those are in parallel at 34K for the Hi input, and create a 50% voltage divider in the Lo input. That's likely, what MB was getting at.

[DGDGBD]

Originally Posted by mbratch

That article is about grid resistors, not input resistors.

The reduction in highs that occurs when lowering volume is often resolved by adding an appropriate cap or RC combination to the volume pot on the guitar. I haven't read about lowering the input resistor in the amp. That's an interesting idea...

seems to be about both, as the input resistor is also a grid resistor on the first preamp stage.

[Don A]

Some Boogies use a ferrite bead instead of an input resistor.

[mbratch]

Originally Posted by DGDGBD

I think that's the same as two 68K's in parallel in the common dual input setup. have the same single 33K in my single-input spitfire clone.

Ah right. Good catch Dave.

[Keyser Soze]

Originally Posted by mbratch

That article is about grid resistors, not input resistors.

The reduction in highs that occurs when lowering volume is often resolved by adding an appropriate cap or RC combination to the volume pot on the guitar. I haven't read about lowering the input resistor in the amp. That's an interesting idea...

The article is about both, but if you read through the article you'll see that the input resistors typically function as a voltage divider for signal attenuation in a hi/lo input setup. When only one input resistor is present, and it is wired to the input before the pull down resistor it also acts as a small attenuator (on the order of 5% or less.)

When wired very close to the grid pin they become a grid stopper and will act as a very high frequency low-pass filter (on the order of 16 kHz for a 68 k resistor.) Lowering the value of the input resistor wILL tend to raise the cut-off frequency, rejecting less of the top end, but this quickly moves out of the range audible by the human ear much less reproduceable by a guitar speaker. The formula is 1/(2*Pi*R*C) where c is roughly 151 pF for a 12ax7.

So, going down to a 33K resistor pushes the cut off value up above 30 kHz.