Hum alleviated by adding go-nowhere lead?

[professormudd]

Refer to this layout: https://ampgarage.com/forum/download/fi ... &mode=view

I noticed when the PAB is engaged, there is a hum, similar to a grounding hum.

I found that when I turn presence to max, it goes away. I also found that when I held a wire a few inches in length with a stripped end up against the middle lead of the cut control trimmer ( Trimmer labeled "A" in this image: https://ampgarage.com/forum/download/fi ... &mode=view ) the grounding sound goes away. Now, if I attach the other end to ground, it comes back.

Can anyone help me understand why the hum goes away with an empty lead soldered in place?

[R.G.]

There are many ways that hum can get into the audio path. The hum you hear is the composite of all the possible hum input ways added together. And even the "hum" isn't necessarily pure. It's composed of any real power line frequency signals that leak in, possibly phase shifted from different ways they sneaked in, and with 2x, 4x, 6x, ... the power line frequency from ripple on B+, as well as power line frequency and 2x power line frequency "spikes" from rectifier switching and from other equipment sharing the power lines in the walls. Fluorescent lights are one notorious source of the 2x power line spikes.

All of these sound like "hum" if you're not careful and practiced in your listening - a connoisseur of hum as it were.

In your case, one possibility is that the one-ended wire is picking up some hum from the air around it and adding this into the hum mix. If it happens to be the right phase compared to the other added-up hums, it can partly or almost completely cancel the other hums. This is in fact the reasoning behind the "hum-balancing" kind of heater references. A pot strung across the heater voltages with its wiper tied to ground can introduce cancelling hum into the tube heaters that can be tuned to get rid of hum from other places.

[professormudd]

There are many ways that hum can get into the audio path. The hum you hear is the composite of all the possible hum input ways added together. And even the "hum" isn't necessarily pure. It's composed of any real power line frequency signals that leak in, possibly phase shifted from different ways they sneaked in, and with 2x, 4x, 6x, ... the power line frequency from ripple on B+, as well as power line frequency and 2x power line frequency "spikes" from rectifier switching and from other equipment sharing the power lines in the walls. Fluorescent lights are one notorious source of the 2x power line spikes.

All of these sound like "hum" if you're not careful and practiced in your listening - a connoisseur of hum as it were.

In your case, one possibility is that the one-ended wire is picking up some hum from the air around it and adding this into the hum mix. If it happens to be the right phase compared to the other added-up hums, it can partly or almost completely cancel the other hums. This is in fact the reasoning behind the "hum-balancing" kind of heater references. A pot strung across the heater voltages with its wiper tied to ground can introduce cancelling hum into the tube heaters that can be tuned to get rid of hum from other places.

Thank you for the detailed explanation. Interesting stuff. So it seems to really isolate the source of hum to an internal vs external influence you have to test the amp in at least 2 entirely different environments. Presuming it is an internal cause (which I am pretty sure it was/is), the correct way to address it is by adjusting wire paths to avoid interaction between signal and B+. Is this correct?

For the time being, Is there any harm in leaving the open lead as if I ensure it is insulated and not at risk of making a connection?

Side thought: I wonder, in lieu of shielded wire, would it work to run signal wire through a small piece copper tubing that is grounded to the chassis, like conduit?

[Bev Wood]

Hi folks - how is this for a hum cure? I was playing electric violin in a pub with a dodgy earth - but I noticed that every time I touched the jack plug body, the hum vanished. No time to work out why -and could not do anything whilst playing, of course. Fortunately, I had some spare cable, so I stripped some wire and wrapped it round the jack. I then stripped a few inches off the other end, splayed out the strands, placed them in my shoe-and put my sweaty feat in said shoe. No hum during the gig. This is my first send into the Amp Garage, so please excuse me!

[professormudd]

Hi folks - how is this for a hum cure? I was playing electric violin in a pub with a dodgy earth - but I noticed that every time I touched the jack plug body, the hum vanished. No time to work out why -and could not do anything whilst playing, of course. Fortunately, I had some spare cable, so I stripped some wire and wrapped it round the jack. I then stripped a few inches off the other end, splayed out the strands, placed them in my shoe-and put my sweaty feat in said shoe. No hum during the gig. This is my first send into the Amp Garage, so please excuse me!

Sweaty feet don't fix many things, but apparently there is at least one thing they can fix.