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EQUIPMENT FORUMS >> SE84UFO >> Bias switch setting
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Message started by thethanimal on 08/10/20 at 01:54:34

Title: Bias switch setting
Post by thethanimal on 08/10/20 at 01:54:34

On my new SE84UFO running 99 dB full range speakers I get a decent amount of hum if I flip the bias switch back towards the input tube. I also hear a big jump in treble energy. Is this expected on high efficiency full range drivers?

-Than

Title: Re: Bias switch setting
Post by Geno on 08/10/20 at 03:44:56

Than, I experimented with the bias setting a good bit when I first got my amp. I finally decided I liked it in the position closest to the input tube. That is where mine has stayed. My full rangers are 99dB also. I never have gotten a hum, but this setting is definitely a little louder than pointed to the front. You may run that by Steve, as you should not have any hum.

Title: Re: Bias switch setting
Post by Geno on 08/10/20 at 03:50:35

Also, you might try a different input tube just to make sure you donít have a faulty one.

Title: Re: Bias switch setting
Post by CAJames on 08/10/20 at 15:06:39

Sounds like the input tube to me, esp. if it is 60 cycle hum. It is possible the hum will go away after several hours and the tube breaks in.

Title: Re: Bias switch setting
Post by thethanimal on 08/11/20 at 01:18:08

Well the hum and treble brightness are gone. (Actually a little hum if I get my ear within a foot of the speaker but even then itís marginally above ambient room noise.) I did go from bare wire connections to banana plugs and removed the nearby receiver and the speaker wire running from it to my KEFs. So maybe some environmental EMI was removed or the tube/amp broke in a little more, or some of both. Either way it seems like Iíll need more time on the amp before settling on the bias switch position, and maybe I should revisit the Hi/Lo switch as well.

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