Question on the grounding wires

[cddc]

I heard some faint noise when the music was not playing today (not sure if it is from the tube/grounding/or other interferences), so I checked the grounding of the Crack first.

Surprisingly I found no black grounding wires is connected to the grounding tab (safety ground). The ground bus starts from 20 -> 14 -> 12 -> grounding terminals on the headphone jack -> grounding terminals on the pot -> solder tabs on the RCA jacks.

But there is no direct wire connection from the RCA solder tabs (or anywhere on the ground bus) to the grounding tab (safety ground).

The only possible grounding connection from the ground bus to the chassis is through the RCA jacks. But there are 2 white plastic washers separating the RCA jack from the metal chassis. So the only possible grounding connection is through the physical contact between the RCA cylinders (the portion between the 2 washers) and the RCA holes on the chassis. However, I think that connection might be unstable, because 1.) there is no solid solder connection there; 2.) the physical contact can become loose when temperature changes;  3.) the surface of RCA cylinders & chassis holes can get oxidized over time.

So I am wondering if it's okay to solder a grounding wire from the RCA solder tabs (or 14U/12U) to the grounding tab (safety ground) so that the ground bus is firmly grounded? Thanks.

[mcandmar]

From memory the front terminal strip by the 12au7 acts as a star ground to the chassis.  You don't want more than one connection or you end up with a ground loop.

[Laudanum]

Yes.  From 3L (I think) back through the volume pot to the RCA jacks.

[Deke609]

Ditto.  Both the safety ground tab and T3 are bolted directly (and therefore electrically connected) to the huge aluminum chassis, and so for practical purposes can be thought of as the same point.  But you might want to check that the nut and bolt at T3 are tight.

cheers, Derek

[cddc]

Ditto.  Both the safety ground tab and T3 are bolted directly (and therefore electrically connected) to the huge aluminum chassis, and so for practical purposes can be thought of as the same point.  But you might want to check that the nut and bolt at T3 are tight.

cheers, Derek

Thanks.

Yes, the safety ground tab and T3 are both connected to the chassis/ground, but they only ground themselves and are not grounding the 'ground bus' I mentioned above.

Only 3L grounds the 'ground bus' to the chassis ('ground bus' -> 3L -> chassis -> safety ground tab -> ground of the IEC power entry socket).

[cddc]

Yes.  From 3L (I think) back through the volume pot to the RCA jacks.

Yes, you got hawk eyes. There is a solid grounding connection through 3L (I missed that bolt   ).

The 'ground bus' is grounded to the chassis through 3L (and partially thru the RCA cylinders).

Thanks a lot!

[Paul Birkeland]

(and partially thru the RCA cylinders).

There should be absolutely no contact between the RCA jacks and the chassis plates.  The insulating washers are there to insulate!

[Deke609]

Yes, the safety ground tab and T3 are both connected to the chassis/ground, but they only ground themselves and are not grounding the 'ground bus' I mentioned above.

Only 3L grounds the 'ground bus' to the chassis ('ground bus' -> 3L -> chassis -> safety ground tab -> ground of the IEC power entry socket).

We're saying the same thing! By T3 I meant terminal 3U/L.

cheers, Derek

[cddc]

We're saying the same thing! By T3 I meant terminal 3U/L.

cheers, Derek

LOL...my bad.

I thought you are referring to the PT3 transformer. But you are referring to 3U/L.

[cddc]

There should be absolutely no contact between the RCA jacks and the chassis plates.  The insulating washers are there to insulate!

Oh...really? Even we put insulating washers there, there will still be contact between the RCA cylinders and chassis holes unless we also put some insulation on the RCA cylinders. But why there should be no contact there, shouldn't the RCA cables be grounded, I didn't get it? 

[Paul Birkeland]

Oh...really? Even we put insulating washers there, there will still be contact between the RCA cylinders and chassis holes

No, one plastic washer on each jack has a shoulder to keep it from touching the chassis plate.

But why there should be no contact there, shouldn't the RCA cables be grounded, I didn't get it? 

If you have a connection to the chassis there and the black wire ultimately ending up at 3U, you have two completely different ground paths and you take the chance that you'll end up with ground loop hum. 

[cddc]

Great, thank you very much for explaining the grounding, PB!

Now I got it why there should be no contact there.

Glad to hear there is a shoulder on the washer to insulate the RCA cylinder. Attention to detail...LOL