Resident SWR bass amp gurus needed

[chicago_mike]

Hey guys,

I have an swr sm900 that I did a couple mods to, and could use a little help getting rid of a little bit of hum.

Mods I did are:

Replaced the 6800uf caps in the power supply with 15000uf.

Replaced the .02 tube blocking caps (wima) with orange drop .033

Replaced the older poweramp boards with the newer Boards. They still use the Sanken 3264's and everything else, the board is just a little sturdier in construction. Older boards were blueish in color and the new ones are the dark green, power 2000 boards.

Changed the single shaft master volume control to a concentric control, works just like it should.

I also put in a 12ax7 China B, as some people say thats actually a real good tube...I really have no clue.

Everything works and it sounds killer. I just get a little hum. NOT 60 cycle hum, its more like 120 or maybe even higher.

I have not yet biased the new power amp modules...if I do, would this possibly eliminate the hum? I do have the bias procedure and will be able to do that this week.

Also, I know when the fan kicks on, the sound of the fan goes into the poweramp as well, how could I get rid of that? Relocate the ground wire? Where would I do that?

Lastly, before I swapped the boards, it would pop when I would turn the amp on, you would hear the pop thru the speakers and swr would say thats fine, normal operation. Cool, but now it doesnt do that. I double checked my wiring and everything is where its supposed to be. Just wondering if anything on the boards changed??

Thanks for any and all help.

Mike

[nigelwright7557]

A 120Hz hum sounds like full wave hum.

I spent quite a long time trying to get rid of hum in my 12ax7 circuit. In the end I used DC for the heater.

The inputs to the 12ax7 are very high impedance so maybe screened cables might help on the input.