Blackface Proreverb Microphonic!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by HammyD, Mar 23, 2006.


  1. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    Hi Guys,

    I have a 67 Proreverb which was gone over from tubes to caps and new speakers just a few years ago. It hasn't seen much use due to a change in jobs but I got it out last week and it still sounds wonderful.

    But it appeared that one of the 12AT7's was microphonic so I bought a couple of new GT12AT7's and put them in v3 & v6. It has GT's throughout with an ECC83 in the v2 slot.

    But it is slightly microphonic. You can tap on all of the preamp tubes and get a deffernt new noise that with continued light tapping will cause the noise to go away. You can even tap on the volume knob with it turned down and hear the noise.

    Now its not alot of noise but it is still there....

    I even thought it could be the crappy ac here so I brought out an old UPS we used for a video editing suite. Its quite large and has new batteries. Still the problem persist!

    The tech is a great guy but he's on the road and I am not sure what to do next. Its not a major distraction but my fear is that something may be loos and cause a major problem if not addressed.

    Any thoughts or ideas?

    Thanks,

    James

    ps: Here's a jpg of the amp.

    [​IMG]
     
  2. brad347

    brad347 Member

    Messages:
    4,795
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    wouldn't be surprised if all the groove tubes were microphonic. I don't have a lot of confidence in them.

    THAT SAID, often the best sounding tubes are slightly microphonic. Most would agree in an A/B test that a slightly microphonic tube sounds more "alive" than a completely dead tube. My favorite tube is an old Amperex (labelled silvertone) made in Holland that's microphonic if you tap on it but it doesn't cause any feeding back or other problems in the amp and it sounds simply great.

    Are you sure the tapping is a microphonic sound? I wonder if there's some sort of grounding issue... does it have a 3-prong plug fitted?
     
  3. Boogie92801

    Boogie92801 Member

    Messages:
    2,134
    Joined:
    Aug 24, 2004
    My BFPro is outfitted (by the previous owner) with GT's throughout. It has decent lower and mid volume tone but cranked it went squishy. Before I changes the ground to a three prong it was very buzzy and bit scarey. Right after that cranked it to try it out... the room we were in was way too "shinny" and it was loud and hence compressed the room into MUD... untill we turned down to about three... but I think it could benifit from some decent tubes. Tell me a source for some decent 6L6's to run in it. I think I have about four nice RCA and mullard 12 AX7's, but I would like to hear some ideas on power and rectifier tubes for it. As of late I have been buying tubes for my EL 84 amp and have no clue on what 6L6's I should look for.
     
  4. brad347

    brad347 Member

    Messages:
    4,795
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    NOS 6L6s are getting damned expensive. Seek out some quality new production (jj or the like) is what i would do, or if you have lots of disposable income you can't beat black plate (or grey plate) RCAs.
     
  5. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    Thanks for the tips guys!

    I don't play at high volumes and the noise is just a very slight, well sorta like the sound when you hold a seashell to your ear. Is this just normal for a tube guitar amp of its age?

    BTY I used an old UPS with new batteries, a very large one used to a video editing system and a lot of the noise went away so I am think some of it may be due to line noise.
     
  6. brad347

    brad347 Member

    Messages:
    4,795
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    whoa.... if its a hissss sound then i'd check the caps. When have the electrolytic caps been changed last?

    It's possible for even an old amp to be dead quiet.
     
  7. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    It had the electrolytic changed when I had the other upgrades done. It sounded great when I first got it back. I wonder if its a cold solder or a loose wire. The tech has been exceptional and everyone who heard the amp was blown away by the sound and they are a lot more experienced then I am!

    Thanks for the info Brad!!! This was the type of information I was looking for!! (Something that would save me form blowing up the amp or electrocuting myself!)
     
  8. Shea

    Shea Member

    Messages:
    540
    Joined:
    Sep 7, 2004
    If your only problem is that seashell sound, then I don't think it's microphonics. That's just a sound that old carbon comp resistors make. You could either have your tech replace them, or just learn to live with it and enjoy it as part of the vintage vibe. But when it gets too loud to bear, or when it turns into an annoying crackling and popping noise, it's time for them to go.

    Now, if you really have a microphonic problem, see if the leads from the input jacks to V1 and/or V2 are solid-core wire. That's what was in my 68 or 69 Twin Reverb, and it was so microphonic that it would start feeding back on itself and make this hellaciously loud howl. Replacing those leads with flexibile shielded cable helped fix it, but I'm pretty sure it was the flexibility and not the shielding that did it. The problem with those solid-core input leads was that they would pick up vibrations from the combo and transmit them to V1 and V2, which would amplify them.

    Seems like there was something else I did to fix the microphonic problem in that amp, but I can't remember it to save my life. Ceramic caps have a reputation for being microphonic, so maybe you could either replace them with silver micas or glue them down to the circuit card with some silicon sealant. I don't think I did that in my amp, however.

    Shea
     
  9. Shea

    Shea Member

    Messages:
    540
    Joined:
    Sep 7, 2004
    PS - is the Beacon still around? (Can't remember if that was in Spartanburg or Greenburg).

    Shea
     
  10. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    Thanks Shea! It did not make the sound when I first got it back from him and he said it was dead quiet. it began a few weeks later but I assumed it was from the noisy AC at the house. It is very low but present at all times even at "idle." Since my goal is to play jazz again and the venues will be very small and eventually record I will need to address the problem.

    Yes, the Beacon is still here! If you're in town, drop me a line and we'll go by and experience the ambiance!LOL
     
  11. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    Hi Guys,

    I ddid some tube swappign and found an offending ecc83 in the v-2 slot and much of the surf noise went away. No real micophonics when tapping the tubes. But when I touch the V2 tube or touch the volume control or the input jack on the front there is the sound I think of as a coming from a ground loup! You know, a little buzz....

    When plugged in the guitar sounds find except for the ground loop sound until I touch the strings or touch the metal cover on the cable......

    Any thought? Ideas?

    BTY Shea Thanks for the e-mail and the pics!!! Very cool!
     
  12. slider313

    slider313 Silver Supporting Member

    Messages:
    7,230
    Joined:
    Jul 28, 2005
    Location:
    NC
    If you have a microphonic 12ax7 in V1, it can cause V2 to be more microphonic. Try a real quiet tube in V1 and see if it helps.
     
  13. HammyD

    HammyD Supporting Member

    Messages:
    4,660
    Joined:
    Dec 1, 2002
    Location:
    South Carolina
    Great info! It did help swapping tubes...but still off to the tech for a checkup! Thanks.
     

Share This Page