What proves a tube is microphonic?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jchilly, Sep 26, 2005.

  1. jchilly

    jchilly Member

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    When I tap the pre amp tubes with a pencil no noise....play a muted chord I hear the ping? Input jack is way micro...but none of the tubes????What gives....................
     
  2. jchilly

    jchilly Member

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    no help to the long time HC'er newbie GP?
     
  3. Boogs

    Boogs Member

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    Replace it with another tube... problem goes away = bad tube.


    :)
     
  4. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    All tubes are microphonic to some degree. When that degree gets too great (e.g., causes feedback), it becomes a problem. As with pickups, tube microphonics can add an airiness to the tone and as such is somewhat desirable.

    As a new guy, you might want to know that it's generally bad form to bump after only a few hours. You might not get an answer as quickly as you might want/expect but folks here are pretty darn good at responding to such queries. Finally, it might have gotten answered more quickly had it been posted in the Amp Technical Info section instead of the Amps & Cabs section.

    I've moved it to the tech section.
     
  5. Reeek

    Reeek Member

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    Boogs pretty much nailed it. It really pays to have spare preamp tubes and power tubes around. The reason is that under "good" conditions such as low volume in a combo where high virbration is not an issue, you may not detect microphonics but you may when the volume is boosted the amps can go crazy on you. I've even found situations where a tube was more prone to be microphonic depening on which socket I had it in. Microphonics are also often exacerbated by certain frequencies.

    Good luck!
     
  6. jchilly

    jchilly Member

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    Thanks guys and input both on the tubes and my forum posting! I have changed out tubes and it is still there....

    HIlls
     
  7. StompBoxBlues

    StompBoxBlues Member

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    Just last night I was browsing through a new "tube guitar amp" book. Lot's of knowledgeable folk there.

    On microphonics, one expert mentioned (from memory):

    First, turn the amp on, do NOT plug anything into the amp (no guitar, or cord at all) and turn the volume (maybe gain too) up all the way.

    Next go from tube to tube, with a chopstick lightly, or back of fingernail and lightly tap the tubes one by one.

    You will hear the tap, BUT you are trying to listen for a "piiiiiinnnng" that they described as "like when you ping a fine crystal glass" that sort of hangs on for a long time.

    Also, (this is me now...no expert) first rule of troubleshooting is get rid of variables...rough troubleshooting...that is why the expert suggested the no plug-in and gain, get rid of any possible problems from guitar...

    Anyway, in a similar thing, if you have preamp tubes going to the reverb for example, turn the reverb off...if it goes away it is that one, if not leave it off and continue.

    If you have an effects loop, with tubes driving it, don't plug into that.
     
  8. StompBoxBlues

    StompBoxBlues Member

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    One other thing. I'm guessing you know the sound of microphonic, but is there a chance you could be getting fooled by some other vibration that sounds similar?

    If you can get someone to hold or touch various parts of the cabinet when you play and see if it stops?

    Also, what kind of amp is this we are talking about, what and how many tubes, and how old are they?
     
  9. jchilly

    jchilly Member

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    I have done all these suggestions other than taking a toothbrush and playing with all the PTP wiring. This is an old Sovtek MIG 60 modified my Scott Splawn of Splawn amps. 3 ECC83's brand new, JJ 34 tubes brand new. Does it only when played and not when tapping the tubes. I have tried with 3 cabs, out of headshell in headshell in another room no matter what it keeps doing it!
     
  10. StompBoxBlues

    StompBoxBlues Member

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    Sounds like you've covered all the bases. Have you sent any email to Splawn asking for suggestions?

    The only thing I see you haven't done, is to change out all tubes one by one. Do you have the old set?

    Even if you don't, if you can afford it, get a backup set and do it one by one?
     
  11. Reeek

    Reeek Member

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    So it's a head?

    Hmmmm

    I owned a MiG-60 and it was very reliable and certainly not hard on tubes being a head and all with plenty of air circulation. Dead quiet when operating and a very mean machine indeed.

    I'd call Splawn.

    The ONLY thing I hated about it was the very heavy transformers that didn't balance well when picking it up by the handle. It always wanted to list to one side ;)
     

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