Grounding shield of input connection

[jlboxlc]

If you connect the buss to ground at the power supply, you will need to use an isolated input jack (run the shield connection over to the ground point of the cathode components of the first stage). Then you will need to add a 0.01uF capacitor (ceramic disk caps are good for this) from the shield terminal of the input jack directly to the chassis with the shortest possible wires. This will keep radio frequencies from getting into the amp.

This is from Aiken's site. In this situation, is the shield(drain) being connected to the shield terminal of the input jack, then to the ground point of the first stage cathode components, with the cap being added from input shield terminal to chassis, or is the shield(drain) being connected only at the first stage ground point, and the cap is connecting the input jack shield terminal to chassis ?

[jlboxlc]

???

[reaiken]

You have to have a DC connection from the shield to the amp, so yes, you have to connect the shield at both the jack and the cathode ground point of the first stage, but don't connect the shield to the chassis at the input jack if you have it connected elsewhere, like back at the power supply. You then connect the capacitor from the shield lug on the jack directly to the chassis with as short a wire as possible.

If you connect both the shield to ground and the power supply to ground, you will usually end up with a lot more hum.

The best way to ground the amp is to ground it right at the input jack and not at the power supply. This way you get minimal hum, best RF rejection, and don't have to use the extra capacitor or isolated input jacks. By the way, this is the grounding practice recommended in the old Mullard manual.

Either way, you always should use the safety ground (third prong) on a short wire right at the power entry point.

RA

[EFK]

I've noticed a lot of old amps from the 1950s run all the grounds "backwards" compared the way most today do it, in that these old guys ran them the way Mr. Aiken describes, all back to the input jacks.