Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Andre Oliveros on October 25, 2010, 01:36:31 pm

Title: Fender bassman 100 hum!
Post by: Andre Oliveros on October 25, 2010, 01:36:31 pm

hey guys im fixing a fender bassman 100, already did the caps, and new tubes, but the amp has a loud hum, i was thinking that maybe the AC cord its giving this, what do you guys think? i have not replace the original cord for a ground one could be these the problem? the hum is noticiable as soon as you turn on the amp, no matter if it has a guitar plug or not the hum appears. any ideas?

Thanks

Dre.

Title: Re: Fender bassman 100 hum!
Post by: LooseChange on October 25, 2010, 01:47:24 pm

Add a grounded power cord for sure.

Title: Re: Fender bassman 100 hum!
Post by: phsyconoodler on October 25, 2010, 03:00:26 pm

Yes to the grounded cord.But,do a simple test while it still has the two wire cord.Simply reverse the plug in the wall socket and see if it hums less.No? Then you have another issue causing the hum.Look the amp over and test all the grounds.Also make sure it has either the heater center tap grounded or a pair of GOOD 100 ohm resistors from the pilot light contacts to ground.If one of those is bad it will hum quite a bit.
  Then of course the usual tubes,solder joints etc.

Title: Re: Fender bassman 100 hum!
Post by: Andre Oliveros on October 25, 2010, 05:06:13 pm

Thanks ill check that,

Title: Re: Fender bassman 100 hum!
Post by: worth on October 25, 2010, 11:14:48 pm

It's a bad hum balance pot.. replace it , or use a
"virtual center tap",(100 ohm resistors from the pilot light to ground ).