I Thought It Was a Ground Loop

[chinrest]

I have a fresh JTM50 build, and I've been trying to get rid of that last little bit of 120Hz buzz. I did all the divide and conquer troubleshooting moves (tube swap, different ground schemes, component check, etc.), but no luck. I was sure it was a ground loop (bright channel volume and tone effects the buzz), but not one of my ground scheme changes made a dent in this buzz. I was about to give up and stick it in the cabinet and live with a little buzz, when I got me to thinking... What else beside ground loops can cause the buzz?

I made a few observations, and then a weird discovery: Sheilding the chassis helps a little. The PT is noisy and makes a 120Hz (I think) electrical noise when I switch out of standby.

Here's the kicker: the buzz is the same with no input, or a guitar input BUT - I plugged a pedal board to the input, and the buzz disappeared!!! Huh? The pedal board terminates with a buffered delay.

I'm befuddled. My pea brain says that the impedance may be lower, and then the brain shuts off. Any ideas?

[Amp Kat]

Make sure all your input grounds and input jack are wired correctly. Check the pots to make sure they are all star grounded and reading shorts to one ground point and that the ground point is terminated all together at a master star point. Make sure the 3rd grounding wire is isolated by itself and check the bias ground point as it can cause buzz also.

[Enzo]

If it5 stops buzzing when you plug something into the input jack, my first reaction is that the input jack is not grounding itself off when empty.

Turn the amp on, nothing plugged into the input, and turn the controls up to hear the hum you have. Now bring your fingers right near the input jack. Does that increase the hum? If so, then your grounding contact in the jack is eithe dirty or misformed, or it is not wired correctly.

[chinrest]

Make sure all your input grounds and input jack are wired correctly. Check the pots to make sure they are all star grounded and reading shorts to one ground point and that the ground point is terminated all together at a master star point. Make sure the 3rd grounding wire is isolated by itself and check the bias ground point as it can cause buzz also.

I've honestly tried a number of different proven ground schemes, with no improvement. That's what makes me think it's not a ground loop.

If it5 stops buzzing when you plug something into the input jack, my first reaction is that the input jack is not grounding itself off when empty.

Turn the amp on, nothing plugged into the input, and turn the controls up to hear the hum you have. Now bring your fingers right near the input jack. Does that increase the hum? If so, then your grounding contact in the jack is eithe dirty or misformed, or it is not wired correctly.

The buzz is still present with V1 pulled, so it's not the input jacks, right? The fact that the buzz disappears only when a buffered pedal board is plugged in is freaking me out. A clue? A red herring?

[Enzo]

SO if you just plug a guitar into the input, and dial it to zero, the hum remains?

[chinrest]

SO if you just plug a guitar into the input, and dial it to zero, the hum remains?

That is correct. The hum is there with a guitar plugged straight in. Am I nutty to think that the low impedance output of the pedal, or the wall-wart power supply offer some clue here? I'm a hopeless red herring hunter.

[Enzo]

Even with the volume on the guitar at zero?

Pulling V1 does not kill the buzz? OK, leave out V1 and it still buzzes, and then plug your pedal board in. Does that still kill the buzz?

[chinrest]

Even with the volume on the guitar at zero?

Pulling V1 does not kill the buzz? OK, leave out V1 and it still buzzes, and then plug your pedal board in. Does that still kill the buzz?

That is some wild-ass troubleshooting. It sounds absurd, and I know it's completely absurd, but I told you—if you flip a red herring in the air, I'll go for it like Sea World. I'm not ashamed to admit that I tried your suggestion. A week had passed since my last troubleshoot, and I had fresh ears, and perhaps my perception wasn't as skewed as before. What actually happened was the noise of the pedal board masked the hum.

So... at full amp volume, with the volume pedal at zero, the pedalboard hiss is louder than the hum. Great. I still have a mildly annoying hum. Why do I always expect the 120Hz buzz to be something other than a ground problem? I feel like I've tried everything except a true, one-point star. Back to the troubleshooting gallery....

[chinrest]

Isn't it funny (no, not really) how sometimes stepping away for a little while often helps solve the problem? Today I did two simple things: 1. Ground the volume pot side of the mixing resistors--hum remains; 2. Ground the other side of the mixing resistors--hum gone. It was lead dress. I shortened the v2 grid wire about 1/2" and flew it in the air off the chassis. Finito.

There must be a psychological term for this. I know it's either ground issues or lead dress, but I really want to believe it's something else. "There's a slight hum, perhaps I should replace the PT." Next time I have a problem with hum, the first thing I'll do is buy a new guitar.)