Amp hum question

[Redfishbum]

Hi all....I have tried to find any history to this question as I'm positive I it has been asked before.  However, I can't seem to find the right search terms.

I built a amp from scratch.  Sounds great.  However, on occasion, it hums when I plug in my guitar.  Hum goes away if I (1) touch metal on my strat or (2) if I turn my tone knob all the way up.

I did not use an isolation washer on my input.  I think I'm going to install on the inputs but thought I would shoot our a question to the forum to see if there is anything else I need to think about.  Strat is a 2004 American delux.

Thanks in advance.

Bart

[shooter]

it hums when I plug in my guitar

so with nothing plugged in it's quiet?
what about if you plugged in, say music, with a different cable?

[Redfishbum]

Cable good.  Tried different cable.  No hum as long as I'm touching the strings or my palm is on the metal of the bridge.  Forgot to mention that if I turn my tone knob all the way up the hum is hugely minimized.  Also, the hum is not unbearable......I'm just a spaz and I notice it.  I should also mention that the hum seems somewhat variable in noticeable volume.

[eleventeen]

Is your amp in some kind of shielded enclosure? Or upside down with open bottom in a bare chassis; near a computer?


Fluorescent lights? Noisy dimmer? Noisy wall wart sitting nearby?


One certainly plausible cause is if your strings aren't grounded on the guitar. OR, if the ring of the output jack doesn't have a good connection to "circuit" ground inside the guitar. Try another guitar? It sounds to me like your guitar is acting like an antenna. 

[Willabe]

No hum as long as I'm touching the strings or my palm is on the metal of the bridge. 

That's normal. That's the way it works. 

The ground wire for the guitars PUP's and controls is hooked up to the bridge/strings. If that ground wire wasn't hooked up to the bridge/strings then it would always hum.

If your on stage and have to use both hands to do something else then you have to turn the guitars volume all the way down.   

[TIMBO]

Some stuff to look at here http://el34world.com/Forum/index.php?topic=12674.msg118757#msg118757

[Redfishbum]

I have tried a different guitar as well and it also has a hum.  No computer issue for interference.  The chassis is in the back with the tubes pointing down and the inputs on the back top.

The hum does seem to be variable from room to room in my house and this might be influenced by the lights or other things plugged in with close proximity to the amp plug.

Not a big deal, when I'm playing it.

Thanks for the thoughts and suggestions!

Bart

[EKDENTON]

here are a few more things you might check out.

http://proguitarshop.com/andyscorner/taming-60-cycle-hum

If your strat doesn't have the hum cancelling pickups the article says try switching to in between settings on your pickups and see if it helps?  the two single coils turned on at the same time may cancel the hum.