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    Macrophonic Tube ?

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    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Macrophonic Tube ?

    Post by arledgsc on Mon Jul 08, 2013 11:01 am

    Not to be confused with microphonic or mechanical vibrations pickup by the tubes and amplified out the speakers.  I actually think I witnessed a tube mechanically vibrating to the sound of the music and modulating the air directly.  

    Back story is I recapped my amp and the particular caps are notorious for long break in cycle.  So this gave me an opportunity run in Shuguang KT88-Ts and Psvane 12AU7s also known for extended break in.   If I am force to endure bad sound for a while I may as well kill two birds with one stone so to speak.  Sound was indeed very strange for the first days and was all over the place but I was tracking The Police - Walking on the Moon when Sting was singing "walking on the mooooonnnn..." at low volume but I could also hear a raspy sound directly from the amp and not from the inside.  I repeated the song phrase and the sound did not go away.  I couldn't isolate which tube (or component) and forgot about it.  And have not heard it since the very early retubing.  I've heard of singing transformers but never a vacuum tube.  Anyone else have a similar experience?

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Macrophonic Tube ?

    Post by GP49 on Mon Jul 08, 2013 4:23 pm

    Yes, especially in power tubes where there is a heavy current flow, the internal parts of tubes can and do vibrate in concert with the current flowing through them. We just don't hear it most of the time because the loudspeakers are in the same room as the amplifier. But those of us who have run tests on amplifiers using dummy loads know this!

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Macrophonic Tube ?

    Post by kaner on Mon Jul 08, 2013 11:01 pm

    Happens pretty easily in guitar amps where the tubes are located right behind the speakers. I built a small practice amp that had that same problem any time I hit an A note or chord. I changed out the power tube to a NOS RCA and it cleared up. Try swapping out power tubes and it will probably disappear.

    kaner

    Posts : 88
    Join date : 2011-09-20

    Re: Macrophonic Tube ?

    Post by kaner on Mon Jul 08, 2013 11:10 pm

    I just read your post again. I was referring to mechanical vibrations that work their way into the sound. Are you saying the tube created its own vibrations?

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Macrophonic Tube ?

    Post by sKiZo on Tue Jul 09, 2013 12:13 am

    Put cones on them rascals and see how loud they get!

    I suppose anything's possible ... sound is vibration, after all. Can't imagine it does the tubes any good though. Might want to look into some damping rings?

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Macrophonic Tube ?

    Post by arledgsc on Tue Jul 09, 2013 9:26 am

    Thanks for information.  Yes, it is the other way around from microphonic.  The tube apparently making noise and not through the speakers.  But GP49's response makes sense as he tests amps on a resistive load.  High voltage, thin plates, and enough current can cause small mechanical vibrations that are picked up by the ear.  Haven't heard it since the first day of the retube job so probably quickly settled out.  But was very strange indeed and very quiet but horrible like a speaker made of glass!

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