SWR SM 900 hum..need help.

[basspro557]

Have a 90's pre fender Swr Sm 900 thats humming when the master is turned up at around 3 oclock..When i turn the master down it hums less..Its this normal???

[JimmyM]

sounds pretty normal to me. the louder you turn something up, the louder you turn up its noise floor, too.

[basspro557]

Hm..I had a Gk800 and the master was set up to max and it never hummed.

[JimmyM]

well maybe your tube is going out or something. that's usually the first thing to check. beyond that, it's tech time.

[murphy]

3 o'clock would be very loud.....perhaps it's picking up loop noise from the wiring in your house? Try a different location?

[kpc]

Had that head. It's typical.....I sold it.......

[basspro557]

I hear ya...I will replace the tube and get back to you all...Thanks

[basspro557]

Does anyone know which brand tube came stock on the pre.fender models??

[basspro557]

Come on guys someone out there has to know...

[ThrashMaster]

who cares what brand it was, anything currently available isnt gonna be the same anyway, even if it was the same brand

that said, most amps of almost any brand from that era came with Sovteks, or Svetlanas if they were tryin to get all fancy

[murphy]

I think SWR had Groove Tubes originally

[JimmyM]

i had an sm-500 that came with a groove tubes tube. but any decent 12ax7 will work.

[T-Bird]

Hi.

My SM400 had a GT tube back in the day as well, but since I wasn't the original owner (obviously, since those babies cost two months pay + 6 month wait), and GT was the only "quality" tube brand sold around here at that time, it doesn't really prove anything.

At least on the older preamps, Steve designed the circuit to be really insensitive to tube rolling for various reasons, and I for one can't tell the difference between different tubes on my ST220. It was a long time ago, but that was how the SM400 behaved as well.

I have no experience about the SM900, but if the output sections are anything like on the ST220 and SM400, "exessive" humming usually accompanied with "exessive" heat, often indicates that it's best to re-bias the output section. Especially if difficult loads are used.

Regards
Sam

[basspro557]

Thanks guys,,By the way Thrashmaster NOT all 12ax7 tubes sound the same.Russian tubes sound thin while Chinese tubes are warm and fat.