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Mircophonic Tube Diagnosis - SVT

Discussion in 'Amps [BG]' started by Old Fart, Mar 1, 2014.


  1. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    MTI SVT. When I tap on the preamp tubes (the ones in the top part of the chassis), all of them make a sound. Are they ALL microphonic?

    The tapping was done with a LONG, lightweight wooden skewer stick.

    This amp has a significant hum. It also has a crackling sound like sizzling that comes and goes, but is not obnoxious....yet.

    Any ideas? Anybody know a good amp tech in the Seattle area?
     
  2. Sartori

    Sartori

    Joined:
    Apr 9, 2009
    Location:
    San Francisco Bay Area, CA
    Long wooden skewer sticks are my preferred method of testing for microphonic preamp tubes as well.

    And yeah, it's certainly possible for them all to be microphonic. A while back, four of my amp's six power tubes were microphonic, with the last ones on their way out, too.
     
  3. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    Dang. I hope not. All the tubes in this amp are vintage GE tubes. The kind with different numbers (for example, the 12DW7 tubes are actually labeled "188-5" or something).
     
  4. rodl2005

    rodl2005

    Joined:
    Jun 1, 2005
    Location:
    Tasmania, Australia
    Often if one is microphonic, it'll transfer it's 'sound' thru the others. Are you sure one isn't 'louder' than the others?
     
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  6. T-Bird

    T-Bird

    Joined:
    Apr 29, 2007
    Location:
    Finland (Northern Europe)
    Hi.

    Most likely not as much as to be a problem.

    The sound that a microphonic tube induces into the amplification signal chain is pretty easy to spot.

    It sounds like a chime (as would be expected), while a healthy (=less microphonic) tube produces just a click or a thump.
    The further up the chain You go, the less the effect/amplitude of the interference.
    Obviously.

    Being an electromechanical device in nature, a vacuum tube will almost without an exception make some sound when tapped with a stick.
    So one could say that all vacuum (or gas filled for that matter) tubes are microphonic to a degree.
    The Nuvistor design was developed to overcome some of the problems, but then came the transistors and small signal tube technology was brought into a screeching halt.

    Sounds like a need for a cap job and/or an oxidation problem somewhere.

    Good idea about the tech.

    Regards
    Sam
     
  7. Sartori

    Sartori

    Joined:
    Apr 9, 2009
    Location:
    San Francisco Bay Area, CA
    What is this Nuvistor design you speak of?
     
  8. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps, EMG Pickups
    12dw7's are still semi-reasonable NOS. Whatever you do, don't get JJ's. They sound good enough but they're not very well made. I've had good luck with most NOS 12dw7's, one of my favorites being Amperex. I'm liking JJ 12ax7's in my Heritage SVT and V4B but I've had two of the 12dw7's go belly up within the first year.
     
  9. T-Bird

    T-Bird

    Joined:
    Apr 29, 2007
    Location:
    Finland (Northern Europe)
    Hi.

    A quote from the article in the link below:

    "The Nuvistor is often regarded as a last desperate effort by valve manufacturers to stem the flow of 'transistorisation' which was becoming a torrent by this time."

    http://www.r-type.org/articles/art-150.htm

    There are plenty of Nuvistor examples in (DIY) audio, but the absence of aftermarket production makes Nuvistors less than an ideal choice.

    Regards
    Sam
     
  10. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    This SVT had its caps replaced recently, and the tech cleaned everything up. I think he did a thorough job. He knew of, and replaced the electrolytic cap that is in the preamp section for example, and he also knew about the preamp jack with switch contacts. The amp sounds way better now with the new caps. I think he did a good job, but he did not fix the crackling, and the hum appeared when I got it home from the recap job. This amp tech does not have a tube tester. So.....I am shopping for a different tech. In the Seattle area.
     
  11. JOE EDO

    JOE EDO Supporting Member

    Joined:
    Aug 28, 2007
    Location:
    Central New York
  12. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps, EMG Pickups
    If you're in Seattle, you should call Ampeg and see if they can recommend someone. I don't know if Hodgy still does repairs since he moved out of the Heritage Shop into customer service, but he might.
     
  13. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    Thanks, Jimmy.

    I'm hoping a knowledgeable person will chime in here. I've got the back of this SVT off, and I wonder if I can swap some tubes around and see if the hum or the crackling will go away. I also have a VT-22 here, which has a 12DW7 and a 12AX7 I could 'borrow.'

    I guess it's just a matter of borrowing those tubes and swapping stuff around. With the amp off, I presume. Should I worry about turning the amp off and on too quickly? I'd rather do this with some coaching.
     
  14. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
    Location:
    Apopka, FL
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    I would at least let the tubes warn up for 3-4 minutes when swapping, just to make sure no ugliness manifests itself once the tubes are warm. And of course, you might want them to cool down before you touch them.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    The 6C4 tube is know for going microphonic. If you are going to change one, try that one with a known good one. Since you have a new set of electrolytics installed, it could very well be a tube that is causing the hummmmm.
     
  16. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    By the way Jimmy, each time I put this head away for a few weeks or months then drag it out, it's an OMG experience. I've decided to gig this thing, despite the weight. It's just too good to leave at home. I will need a wheeled flight case, but it'll be worth it. Tone MONSTER, I say. And I can prove it. ;)
     
  17. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    A spare 6C4 I do not have, Beans. But....I'm not sure microphonism is the problem. I've done a little more reading, and I don't think I have a microphonic tube. They make a 'tink' noise when tapped, but they all sound about the same and the 'tink' is short-lived.

    However, I do think the humming and/or the cracking could be coming from a tube and I am hoping to swap 12DW7 and 12AX7 tubes around, to see what happens. Any comments, cautions, observations, suggestions, etc?
     
  18. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
    Location:
    Apopka, FL
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    Endorsing: Ampeg Amps, EMG Pickups
    MTI SVT's are VERY underrated by people who never played them. They sound like SVT's to me and they usually go cheaper than other vintage SVT's.
     
  19. Old Fart

    Old Fart

    Joined:
    Mar 11, 2011
    This one might as well be a Magnavox SVT. Early 1980. All vintage GE tubes. American transformers (we think). Tone that I simply cannot explain here. Rich, harmonic, defined, fat, growly, articulated....these are some of the words that come to mind.
     
  20. JimmyM

    JimmyM Supporting Member

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    Apr 11, 2005
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    Apopka, FL
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    Endorsing: Ampeg Amps, EMG Pickups
    I hear you...it's why I went back to tube amps in 2008 after thinking I couldn't do one-nighters all the time with an SVT. Turned out I really can't do one-nighters all the time with an SVT ;) but I still bust it out sometimes anyway.
     
  21. headband

    headband Supporting Member

    Joined:
    Oct 18, 2013
    Location:
    Indiana
    The approach I take for determining the amplifier stage with issues is to pull all preamp tubes and see if the noise is present when you tap on the chassis or the output tubes. Then insert the driver tube, and see if it is the culprit, and so on for each tube from those closest (in the circuit) to the output tubes all the way to the first stage tube. You can isolate the problem in this way. There are many circuit components that can cause this kind of problem - capacitors are notorious for this as well. They can be checked with the chassis removed and tapped with a chop stick - but this is best left to someone that is very familiar with high voltage circuit repair - voltages in these circuits can be lethal.
     

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