Input Network - Shorting Jack

[Sir Cuitous]

I humble myself yet again to the esteemed members here to ask this question revealing my true inexperience.

If I have a single input with a 1M resistor to ground. Do I need a shorting jack to prevent the grid from seeing the open circuit when an instrument is not plugged in? Can I use a non-shorting jack along with the 1M resistor when I have just a single input?

[tubeswell]

You can use a non-shorting jack but there will be more probability of noise when there is nothing plugged in. The shorting jack diverts the input signal path to ground when there is no plug in there so as to cut out any unwanted 'antenna' effect.

[Sir Cuitous]

Is the 1M resistor to ground on a single input circuit not enough to cut out noise?

[Chuck H]

The 1M R to ground provides the ground reference for the first triodes grid as part of the bias. It is in the circuit even when your guitar is plugged in. It will keep the input stable, but not quiet. With a non shorting jack the input will be as if you plugged in a cable with no guitar at the other end. A shorting jack makes it as if your guitars volume pot were turned to 0 when nothing is plugged into it.

It will "work" and nothing bad will happen to the amp, but a shorting jack is ideal. It makes no difference at all in the amps performance. When a cable is plugged into the input, whether the jack is shorting or not becomes mute.

Since you probably don't plan to use the amp without plugging something into it, I say use the jack you have until you can get a shorting jack.

Chuck

[Sir Cuitous]

I just replaced it with a Switchcraft shorting jack. Thanks to all for the clarification. I have some other questions about input networks which I will post in a few days.