#124 Noise. Need help diagnosing

[lgehrig4]

I completed this build a few months back and have been testing/tweaking since. Last week I finally got it into a head cab and then noticed that it was noisier than before. It was never dead quite before but I'm pretty sure the clean channel was.

Is seems as though the noise is coming from the clean channel and then just gets more intense when you switch to the OD. This buzz I affected by both the preamp volume and overdrive drive knobs and the buzz actually has a different character from each. I can't explain but you can see this in the videos.

Other things to note are that when a guitar is plugged in and the guitar volume is off, the noise is louder than when you starting turning up the guitar volume. Also, the noise is on the FET channel too but just sounds hissier. Does this sound like a preamp tube issue?

https://youtube.com/shorts/Dd0StfnOsYg?feature=share

https://youtube.com/shorts/wd6oomkIDds?feature=share

[neskor]

it sounds like 120Hz
check this topic:
https://ampgarage.com/forum/viewtopic.php?f=6&t=38716

[martin manning]

I agree, predominantly 120 Hz. Looks like when you turn the OD or Master down the buzz goes away, which says the source is before the OD drive pot. Since it is present in clean mode, the source is before the OD input. Put a patch cord across the loop jacks and see if that makes any difference.

[lgehrig4]

I read the other thread but in that case the buzz was a happening from the start. This just started in my case, after few months after completion.

Martin, its not the OD master, but rather the drive that greatly controls the volume of the buzz but you can clearly hear that it is present on the clean channel as well.

I tried the patch across he loop jacks and it was still the same.

Some more clarity on my original post. The volume (far left) mostly changes the character of the buzz, but does not add or remove it. Almost like a buzz tone control or buzz wah wah effect. The second video demonstrates this.

I have been testing/playing the amp in the same spot as you see in the videos. This literally started after I installed the chassis in the cab last week. I'll test the preamps later.

Update

- Clean channel with the volume all the way off = still hear buzz
- OD channel really makes this louder with the drive knob
- Tried new V1 = no change

[fred.violleau]

Did you happen to move some cables when you got your amp into the cab? It sounds like a heavy 120Hz buzz.

These could be tricky to find but this one is very obvious and probably one of your heaters or AC run is too close/ parallel to a signal path?

Fred.

[lgehrig4]

Did you happen to move some cables when you got your amp into the cab? It sounds like a heavy 120Hz buzz.

These could be tricky to find but this one is very obvious and probably one of your heaters or AC run is too close/ parallel to a signal path?

Fred.

No, nothing was touched inside the chassis. I have it out now. Changed V1 and V2 and I've been tapping around with a chopstick. Here is the chassis. There's nowhere to move the heaters to be further away from the board. I thought of this though and foldered rubber workout grips over the preamp heaters to see if the noise would at least change.

final1.jpg

final2.jpg

[Lothy]

I had the same problem. With my build it was the heater balance resistors not grounded at the same spot. It looks like that is the case with yours too:

Screenshot 2026-04-11 084249.png

Because you use 2 different sockets for the balance resistors, you might have used the same heater line twice. You can avoid this, by running the resistors from the same socket.
Make sure both heater lines have a balance resistor, twist them together at the other end, and the ground goes to exactly the same spot.

Cheers
Gerhard

[lgehrig4]

I had the same problem. With my build it was the heater balance resistors not grounded at the same spot. It looks like that is the case with yours too:
Screenshot 2026-04-11 084249.png
Because you use 2 different sockets for the balance resistors, you might have used the same heater line twice. You can avoid this, by running the resistors from the same socket.
Make sure both heater lines have a balance resistor, twist them together at the other end, and the ground goes to exactly the same spot.

Cheers
Gerhard

Thank you! I will try that

edit: I just traced the wire in the photo and indeed that is the same heater wire getting both resistors

[Lothy]

I had the same problem. With my build it was the heater balance resistors not grounded at the same spot. It looks like that is the case with yours too:
Screenshot 2026-04-11 084249.png
Because you use 2 different sockets for the balance resistors, you might have used the same heater line twice. You can avoid this, by running the resistors from the same socket.
Make sure both heater lines have a balance resistor, twist them together at the other end, and the ground goes to exactly the same spot.

Cheers
Gerhard

Thank you! I will try that

edit: I just traced the wire in the photo and indeed that is the same heater wire getting both resistors

There you have it! Swap one resistor to the other lug should do it.

[lgehrig4]

It's tight in that area. Could I accomplish this by instead soldering the 2 resistors directly to these heater lugs and the center ground in this photo?

IMG_5504.jpg

[Richard1001]

It's tight in that area. Could I accomplish this by instead soldering the 2 resistors directly to these heater lugs and the center ground in this photo?

IMG_5504.jpg

Yes, but first remove the resistors you already put in.
Even better would be a 200 or 250 ohm pot, center to ground and both outer lugs connected to the heater wires.
This way you can set the balance for the lowest hum level.

[lgehrig4]

It's tight in that area. Could I accomplish this by instead soldering the 2 resistors directly to these heater lugs and the center ground in this photo?

IMG_5504.jpg

Yes, but first remove the resistors you already put in.
Even better would be a 200 or 250 ohm pot, center to ground and both outer lugs connected to the heater wires.
This way you can set the balance for the lowest hum level.

Thank you for confirming and for the suggestion!