Hum Vanishes When I Touch Chassis

[TheNewSteveH]

I was playing my new head today, and I noticed it has a slight high-pitched hum that goes away when I touch the chassis. The 60 Hz noise is very low.

Any clues?

[tubeswell]

Most likely grounding related

[Keyser Soze]

I have no good explanation for the phenomenon.

But if it were happening to me I'd be very concerned at the possibility that my body was proving a better ground source than the actual ground wire. Copper wire generally being a much better conductor than human flesh..

So, might be worthwhile to double check that connection.

Also, check the actual resistance between the input jack ground connector and the chassis. With a sensitive input even a few ohms there can be problematic. Some people recommend a small value capacitor from input ground to the chassis, especially if the grounding scheme places the chassis ground point far from the input.

[TheNewSteveH]

The main ground is very sturdy, but I can look at it.

My jacks are the Switchcraft type without insulating washers, so they are grounded right where they are. I have the washers, but installing them seemed like a giant pain. Maybe I'm paying the price.

What's a good value for the capacitors you're talking about?

[Keyser Soze]

If your jacks are not isolated then they are already at chassis potential, both AC and DC, so a cap is unnecessary.

But, if it weren't then a good value would be something like 0.01 uf.

[hackworth1]

If you havent already got them there a 3/8" star washer is good to put behind the input jack(s) for added bite and better grounding. Just one on each jack on the inside of the chassis of course.

[TheNewSteveH]

I stuck some shielding under the chassis today. I learned that upholstery staples go through thin aluminum just fine.

I have not heard the 120 Hz hum since putting the sheet down, but I haven't tested it much. I tried it with and without the guitar plugged in.

I do get a little low, rumbly 60 Hz, but it's only when the volume is at over 2 o'clock. There is also a certain amount of what sounds like hiss.