Murphy Never Sleeps - Champ Hum

[DocG]

So I got this homebrew 5F1 Champ clone, made with old iron and tubes, on fleaBay. No cab or speaker. I connected it to the speaker in another amp and powered it up. Sounded great, no noise.

I bought a Weber 10" speaker and found a hi-fi speaker cabinet by the side of the road. Really! Then I pulled the tubes out of the amp while I figured out how to mount the chassis and the new speaker in the cabinet. Because I am woodworkily challenged, this process took a long time. Say a month. Maybe this is significant, I dunno.

Today I finally finished stretching the grill cloth and redrilling the speaker mounting holes, and cutting up the old cabinet back for ventilation and open backitude. Plugged the speaker into the amp, plugged the power cord into the wall, turned it on....

....and damned if it hadn't acquired a loud hum. Turning the volume control had no effect on the hum. Plugging in a guitar had no effect on the hum. The guitar can be heard through the amp, and the volume control affects its volume.

All the caps and resistors in the amp are modern. A month ago the amp was impressively humless. Why would a month off, or removing and replacing tubes, cause hum to happen?

Any hints?

[Sidney Vicious]

I thought that kinda stuff only happened to me . . . .

[DocG]

Well, there's good as well as bad luck. The cab from the roadside is solid 3/4" mahogany!

[Sidney Vicious]

Sweet!!!

I love roadside stuff.

And before too long one of the resident experts will chime in with advice on fixing that hum.

Free stuff + free advice =

[emu!]

Are you using the same outlet in the same building in the same room as before? Did you acquire any neon signs since last time you used the amp? Try it in another building or at least another room...then report back.

Oh, and is it the same guitar and same cable as before?

[muchxs]

"My amp hums" checklist:

Power supply grounding issues:

Does your amp have a grounded AC plug?

If yes, is it plugged into a grounded outlet?

If yes, is the "grounded" outlet really grounded? The third conductor should terminate at a solid ground on a water pipe down in the basement.

If you think you might have a ground problem plug into a "ground fault" outlet. NEC mandates that a ground fault outlet be installed in kitchens and bathrooms. Unplug the toaster or the hair drier, if your amp trips the little button on the ground fault outlet you don't have a "real ground.


O.k., it's a homebrew. Is the wiring tidy? Are all the solder joints good? Has some internal connection that was connected a month ago been compromised of disconnected?


Tubes. There are only three of them, the 5Y3 will either be good or it won't. I'd swap out the 12AX7. While I was right there I'd make sure none of the connections at the tube socket are touching each other. Pins 4 and 5 should connect to each other, pin 9 is your other fillament connection. Maks sure nothing else is touching at the socket, it's gonna hummm...

Are the inputs wired correctly with 68k grid blockers and a 1 meg grid leak? Did the builder use decent switching jacks at the input or did they save a buck and improvise?


Pet peeve: Many builders pay no atttention to lead dress, if it looks sloppy it probably is sloppy. There's middle ground here, some guys spend hours wiring things up with miles and miles of wiring hidden under the circuit board. Less wire is generally better.

[PeteG45]

Did you try re-seating the tubes? You might want to try a little contact cleaner.

[DocG]

Thanks for the suggestions!

The Champ is such a simple circuit that hum had to be a power supply capacitor that wasn't capaciting. There's only three of them, so I figured I'd just replace them all. I made up an order at justradios.com (nice folks, recommended), and just before I clicked the "Complete Order" button I decided to verify the actual part against the schematic.

Took off the back of the cabinet. I couldn't read the value on the big electrolytic, so I poked it with a Sharpie (our motto: Scared Of Shocks). It fell on the floor! I think the builder made the mechanical connection and then forgot to solder it in.

Now to discharge the caps, resolder as necessary, and recheck for hum. I bet it's gone.

[Later - Yep, that was it!]