Guitar Gear Forum

Just built my first champ project, and I’m excited about how good it sounds. Unfortunately, there seems to be a loud 60hz hum, that starts about 15 seconds after the amp is turned on. I’ve tried two different power tubes with no change in the sound, however the hum does not happen when the power tube is out of the amp. The volume control does not affect it. What could it be? Here’s a photo of my build.

It's most likely heater hum. The twist on those filament leads looks rather loose. Take a wooden chopstick (or similarly non-conductive item) and move those wires around while the amp is on and see if you can make the hum louder or softer.

Thanks, I checked that already and it seems fine. Poked around with a chopstick and couldn’t hear anything through the speaker. Should I redo the filiments? I didn’t realize they were that loose.

First, try chopsticking those preamp signal wires a bit and see what that changes. Then try to get the filaments as far away from signal wires as you can reasonably manage. If they have to cross paths with anything else, do it at right angles.

I also suspect the rectifier filament may be creating an issue with all of that extra length.

jgreenwd wrote:First, try chopsticking those preamp signal wires a bit and see what that changes. Then try to get the filaments as far away from signal wires as you can reasonably manage. If they have to cross paths with anything else, do it at right angles.

I also suspect the rectifier filament may be creating an issue with all of that extra length.

No change with the preamp wires after chopsticking. I have some wire I can redo the filiments with. Will do that and report back

Hopefully this will clearly show what I'm talking about. You want the filament wires to sort of hug the outside wall of the chassis and reach in to the tubes. Twist em up as tight as you can reasonably manage. I usually chuck up one end of the pair in a drill and mount the other end in a vise.

I would start by twisting up 4 that will go to the 6V6 about 2-3 inches in length. Connect those to the socket pins and bring them to the wall. Check continuity with a meter, then run one pair to the 12AX7 and the other pair to the PT (and on to the lamp). You'll waste some wire, but it'll help keep things tidy.

For what it's worth, single-ended amps are noisy. You don't have the common mode rejection that is inherent to push-pull amps. So, you may never get it as quiet as you would like.

Redid the filiments, using some NOS cloth wire I had laying around. Hum might be slightly reduced but it is still loud. What else could be the culprit? My safety ground is separate from the red/yellow and green/yellow PT taps. The preamp and power amp are grounded as well.


I know single ended amps are noisy but this is much louder than my vibro champ

Is the hum louder than a guitar signal running through it? Something could be grounding out on the backside of the board.

You could try removing the filament center tap and putting in an artificial center tap or a humdinger instead. If the filament windings aren’t balanced, this will cause hum, necessitating either of these solutions.

Maybe move the wires running to the speaker jack and see what effect that has.

The likely culprit is a high current AC line in close proximity to a signal wire. Distance between the two is the solution to that.

This is the tedious and frustrating part of DIYing an amp. It never really gets any less frustrating.

Not sure if this will help...
- Try unbolting the input jack from the chassis to see if the hum disappears... if so, you will need a plastic sleeved, isolated jack.
I did this on an old Roland practice amp and the plastic jack did the trick.

Thanks for the replies everyone! It was a pain, but I found the culprit. The ground connecting the left two filter caps had come loose under the board. Redid the solder joint and boom! Quiet as it should be. Thanks!

Glad that was...