Bassman Hum Problem

[weirdotis]

Hi guys... I have a 67 Bassman (AB165) that is experiencing a lot of 60 cycle hum. I replaced all the filter caps today and it is still doing it. I am at a loss as to why this is happening... could it be the diodes? Thanks in advance.

[selloutrr]

http://www.geofex.com/ampdbug/hum.htm

[Gaz]

Did you make sure the 2 100R resistors that form the heater's artificial center tap are grounded? if those somehow came loose, you'd get a loud hum.

[solderstain]

You didn't say anything about this, but I'll bring it up anyway, because it something I (and many others here) deal with: What's the wiring in your house like?

In the past year, I moved to a house with older wiring, and the room that all my amps are in has 'dirty' power. Amps that sounded nice and quiet in my former house buzz in this house when plugged into a couple of the circuits in the house. Drove me nuts when I was developing an amp that I'm going to be taking to market soon. I knew I'd done everything 'right', but in my amp room, it buzzes anyway. I ended up taking it over to a friend's house and plugging it in there. Nice and quiet.

Do you have other amps to try in the same outlet? Do they buzz too? You may have done everything right but your wall power may be a problem.

Is it safe to assume that you've replaced the two-prong power cable with a modern and safer three-prong cable?

Another thing I've run into on older Fenders with the death switch is a bad ground cap. Try disconnecting that thing and see if that helps. It's actually a good idea to disconnect that altogether. It's unnecessary, especially after changing out the power cable.

[Gibsonman63]

The Bassman AB165 has the bias balance circuit and not a bias voltage adjustment (Blackface). If your amp has not been modified to bias voltage adjustment, adjust the bias balance adjustment for the least amount of hum.