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SVT 6C4 Microphonic Issue

Discussion in 'Amps and Cabs [BG]' started by Jazz_bass2664, Apr 29, 2015.

  1. So I just got my 1974 SVT in the mail a few days ago. WOW! What a beast of an amp. After a thorough cleaning and checking of parts, I decided to power it up. It sounded great but was making some occasional whooshing sounds and tapping on the preamp chassis made a pinging noise.

    Today I dug deeper into it and ran the amp without the preamp through the power amp in and everything sounds as it should. In the preamp, I found that the 6C4 driver tube is extremely microphonic and most likely needs to be replaced.

    What would you guys recommend getting for a replacement? Ive heard that most all of these tubes have microphonic qualities but are some brands are a bit better than others? Also should I buy a few of them NOS and then see which one works best?

  2. Buy many of them (NOS), sort through to find the least noisy. That is the real weak spot the original. If you find one that is real quiet it usually means the tube is bad ;).

    You might try some tube dampners if you can find one for the 7 pin.

    www.thetubestore.com - Duende Criatura Tube Rings
    Last edited: Apr 29, 2015
  3. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I quit worrying about in mine. Doesn't seem to have much of a negative effect.
  4. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    I have about 10 of them and just swapped around until I found a decent one. I also use a cinch on mine to help damp noise. I think it's the medium height one here.
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Look for a 6100/6C4WA. It is a ruggedized version intended for applications where mechanical shock and vibrations are encountered.

    Like Corey said, buy a bunch and hope for the best. At least they are relatively inexpensive.

    Some people rewire their amps to accept a 12AU7 instead of the 6C4. Makes life easier.
  6. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    AES has NOS 6100/6C4WA for $4. I think the last time I ordered some I got three or four and they were splits between JAN or GE labeled.
    beans-on-toast likes this.
  7. Well I just ordered 4 RCA tubes so hopefully Ill find a good one in there. 4$ a piece isn't bad at all so I don't mind ordering a few more.
    beans-on-toast likes this.
  8. Mark Reccord

    Mark Reccord Supporting Member

    I'd probably do this myself if microphonic 6C4s were causing me a lot of problems. As it is, I bought a stack of 6C4s about 10 years ago and cycled through them to find the quietest ones. I stuck the quietest one in and it's still there.
    beans-on-toast likes this.

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