Microphonics

Discussion in 'Building the Classics' started by Brad Hope, Oct 19, 2018.

  1. Brad Hope

    Brad Hope Member

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    Anyone ever heard of a microphonic cap? I have a 2204 build that sounds like a valve is going a little too microphonic. Tapped all of the valves and it’s v1, but it does it no matter how many valves I swap in. It’s the only socket that does it. No static or popping, just high microphonics that you can hear through the cab. Valve is a little loose in the socket, so if I can find time tomorrow, I’ll pull the chassis and re-flow the solder (just in case) on that socket, retension the pin holes, and shoot it with deoxit. But I read somewhere that caps can go microphonic??? Coupling or bypass cap for v1 maybe?? Anyone else ever had that?
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    It is completely normal for the coupling cap to be microphonic. And when you tap on it, it makes a loud ka-boing noise.
    Any amp you try just about (depends on design) is going to have microphonic caps (just like your amp).

    This cap which is microphonic is coupling to the grid of the next preamp stage.

    The grid of the preamp tube is extremely sensitive, and it's amplifying the mechanical noise of the cap it's connected to.

    However this does not prevent making an "isolated" anechoic coupling cap....(reverb tanks are isolated too)

    But it is true that sound comes out of the speaker, vibrates the amp and capacitors, then is re-amplified. (because the caps inside are microphonic)
    So the vibration of this cap from the speaker definitely becomes part of the final tone.
     
  3. Brad Hope

    Brad Hope Member

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    What caught my attention was the ringing sound after a sudden stop in playing at loud volumes. Then when I tapped the valve, the others are all solid and dead sounding, this one was loose and loud, but every valve I put in v1 does it.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    That is correct, V1 is always microphonic to some degree.
    Depends on what amp design. If the amp is high gain like 2204, the tube is more microphonic. If the amp is lower gain like a Fender twin, it's less microphonic.
    1.You can select a tube that rings less (or buy a selected tube). Tube labs sell low microphonic tubes that are picked out of large batches.
    2. You can put a tube damper on the tube, which calms it down (it adds weight to the shell so the resonant frequency is lowered)
    3. You can put a suppression cap on that tube (cheap way that most manufacturers use). This creates some negative feedback so the ringing is reduced.

    In a 2204, there are 2 main Marshall factory installed methods.
    A. A 7 pf cap between grid and plate of the first V1A preamp stage. (it can be like 3-10 pf 1000V disk cap which is soldered on to the socket)
    B. The hot shield method: the shield of the input jack wire (pin 2 or 7) - is tied to the plate (pin 1 or 6) not ground.
    Marshall used both methods depending on which year.

    You can read more threads about this:
    http://www.marshallforum.com/threads/shielded-wire-questions-2204-build.91685/page-5#post-1756612
     
    Last edited: Oct 20, 2018
  5. Brad Hope

    Brad Hope Member

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    Yea, I put a 1000pf cap on v1, and I use socket shields with the springs inside to keep pressure on the valves. Probably still take a look around and make sure everything is good before I retention the socket and hit it with deoxit. I never really noticed my 2203 or 2555x being microphonic on v1 like this one, but now I want to check those and see. Got a gig next weekend, pretty big. Apparently last year at this annual party, it was the second most Ubered address in the country that day. Whatever that means... Just want to keep from having an annoying issue during the time we’re playing. It’s not too bad, but it alarmed me that I might have a valve going bad, and I don’t want to drive these things hard if one is going bad. So I started investigating and it turns out the valve isn’t bad, but the socket tension sucks.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    "Yea, I put a 1000pf cap on v1, "

    But where on V1? 1000pf which is connected to (?) what.

    See: the ringing of the tube is related to vibration, layout of the circuit.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    2204ptogrid.png

    2204prbp.png

    p to c.png

    2204hotshield.png
     
    Last edited: Oct 20, 2018
  8. Brad Hope

    Brad Hope Member

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    V1b plate to cathode...last pic
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The bigger the bypass cap, the more highs will be killed.
    1000pf will kill a lot of high frequency.
    You might want to try 100pf or some smaller value.
     
  11. Brad Hope

    Brad Hope Member

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    Yea, I may do that. I can’t temember if it was 100 or 1000 now. I meant third pic, scanned right past the last pic. Either way, I’ll either pull the chassis today or tomorrow and make that change (if needed), and then retension/clean. Today’s schedule filled up when the wife woke up, smh!
     
  12. Brad Hope

    Brad Hope Member

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    Without pulling the chassis, I had to have used a 100pf...I don’t think I’ve ever used or bought 1000pf ceramics. 1000v, but not 1000pf.
     
  13. Brad Hope

    Brad Hope Member

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    Definitely a 101k (.1n or 100pf). But I just realized I put the same size on the preamp vol pot, instead of a 102k (1n or 1000pf). Wonder if I should swap that out or leave it?
     
  14. Brad Hope

    Brad Hope Member

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    Made the swap, 102k on the preamp vol. can’t remember why I did that, but I guess I’m about to find out! Retensioning the socket now and all should be good!
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    2204vconshield.png
     
  16. Brad Hope

    Brad Hope Member

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    Yea, it’s shielded. Everything is correct, just don’t like those microphonics. They’re not bad, like they would be if the valve was going bad, but I guess I’m just trying to eliminate something that’s never really going to completely go away.
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    It never goes away completely but you can make it less than it is w/ the tube damper etc.

    [​IMG]
     
    Last edited: Oct 20, 2018

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