amp singing

[gpbarnett]

Hello Doc,
Noise on the Bottlehead S.E.X. amplifier is rare, but What has happened on my little amp is curious.  2 nights ago, i cranked up the volume on my SEX amp, and i heard music, but suddenly realized that there was no output connected.  It was by accident that didn't plug in my phones, but the music was playing.  So, I looked under the hood to try to determine was was acting as a speaker in the amp.  Well, I found a capacitor that was overheated a bit it looks like, but it was still working.  I got out the stethescope, and found that the autoformers were acting as speakers on both channels!  I'd never heard that before.  I can hear the performance degradation in the left channel from the bad cap, but I can't find any degradation from the autoformers moving enough to produce sound themselves.  It must cause some distortion in the output, since they are physically moving.  Has anyone else had this issue?  The music was loud enough to hear it clearly without a stethescope in a quiet room. 

Mr. Barnett

[Caucasian Blackplate]

That sounds more like the tubes than the autoformers.  In my experience, EL84's tend to produce this phenomenon more clearly and more loudly than any other tubes I have tried. 

Which cap did you notice was looking worked?

[gpbarnett]

Hi CP,
The cap that is balloned in the 220uF 200V cap in the power supply.  The one going to ground.  I The tubes are quite, but the stethescope led me to the autoformers.  the housing acted as a resonator, and that's why i was able to hear the amp sing.  It is both channels.  I guess there is a tiny bit of play inside the transformers, and they move just enough to produce sound?  the tubes are 6sn7 on the SEX amp.

Gerald.

[JC]

I have to wonder if this still happens when you have a load connected?  Perhaps dummy loads would make it easier to tell.

I don't doubt that the transformers can resonate, I'm just wondering if there is any change due to loaded v. unloaded operation.

[2wo]

It’s pretty normal for the output transformers to sing with the music, at least non Parafeed ones. I think it is called magnastriction. The intense magnetic force of the primary current alternately squeeze and release the lams of the transformer in time to the music…John