Gibson G 30

[gbono]

I've run into a hum issue (60Hz) on this amp. After fixing the blown output section and replacing the leaky electrolytics this amp is working again. I upgraded the line cable to a grounded type and when the ground is attached to the chassis I get a very noticeable increase in 60 Hz at the output.

The ripple on the voltage rails is .025V but I get almost .3V on the output terminals with the chassis grounded. Ground loop?

[Jazz P Bass]

Did you ditch C12 at the on/off switch.
That's the 'death cap'.

[gbono]

Death cap doesn't have any effect on the noise issue - though it appears to help suppress the switch arc/pop. I usually replace these old ceramic "death caps" with proper Y rated film caps with UL/CSA approval.

[gbono]

See attached - I'm thinking of adding the following circuit to eliminate the noise issue. I can use fiber shoulder washers on the input jacks - anyone try this?

[52 Bill]

Have you checked the outlet wiring? Is there a voltage differential between the ground and neutral legs? When you lift the ground wire from the chassis is there a voltage differential between the chassis and the ground pin?

[gbono]

There is a voltage between neutral and ground at the outlet of .120 V and there is 105V between the ground pin and chassis with ground wire lifted. I used an old HP handheld DMM to make the measurements.

[soundguruman]

What you are saying is that the chassis is hot.

[gbono]

Yes when I get a chance I'll check the power transformer - I'm assuming that would be the likely reason for the 105V on the chassis?

[soundguruman]

What you are saying is that the chassis is hot.

somehow there is voltage on the chassis...

[Jazz P Bass]

Yeah, maybe try to isolate the PT to see if that is the issue.
If it is, it's bad.

[eschertron]

There is a voltage between neutral and ground at the outlet of .120 V and there is 105V between the ground pin and chassis with ground wire lifted. I used an old HP handheld DMM to make the measurements.

...So the neutral is NOT at ground potential. Is the HOT at ground? I've lived in more than one domicile where the outlets were all backwards, but that's a different problem.

Any connection between L1 or L2 and the chassis should be severed. In a 3-wire arrangement, only the ground wire should make contact with the chassis. If you have done that, then yes, it's possible that the power xformer is not isolated properly. However, my money is on something in the original wiring between the power cord entry and the xformer. Lift or ditch C12, If you haven't replaced it.

[gbono]

L1 a L2 are isolated from the chassis - I tested the PT and no winding is connected to the chassis.

At the outlet L1 to ground measures 120V. Line neutral and ground are all wired correct at outlet per circuit tester. I did find a piece of test gear with chassis grounded on the same circuit.

Lifted C12.

Amps plays well but I would like to reduce the hum. I installed the hum suppressor circuit with little change in noise level.

I'm now seeing 10-11V between chassis and safety ground.