1. The Gear Page is run by musicians for musicians. We listen, we learn and if we misstep we are not afraid to do the right thing. We proposed some changes to the Emporiums. Based on feedback from members, we have decided to not go ahead with those changes. However, it has also highlighted that we need some community input into what is working and what is not working for members here. Primarily focused on the Emporiums, we'd like input on your thoughts about TGP and how things work in the Emporiums for you and how you'd improve them. The discussion thread on the is here!

    Dismiss Notice

When all pots go scratchy (not due to bad pots!)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ericb, Oct 9, 2006.

  1. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Hi guys I've had a few amps where I developed major pot scratchiness ... all of a sudden, i.e. things working fine 1 day , and the next the majority of pots on the front panel were intermittent,scratchy, too interactive, etc.. It's happened on a trainwrecky clone, a 90's Orange and a Marshall 6101 over the coarse of my travels.

    This is NOT due to bad pots(i.e deoxit/cramolin,etc have no bearing on it) , and NOT due to bad tubes or bad tube sockets.. Is this what happens when a certain resister goes, or a grounding issue?

    It's probably a very basic question but it's now happened to an amp that worked fine this past week and I"m bummed.

    Thanks for any advice/help


    Eric
     
  2. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    A leaky coupling capacitor can put a DC voltage on the front panel pots, causing them to sound scratchy. A preamp tube with too much grid current can also make pots sound scratchy. I suppose a bad ground could also do it. The quick test is to put a meter across the pot in question and see if there is any DC voltage on it.

    Randall Aiken
     
  3. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT

    THANKS SO MUCH for your answer.. It'll be a day or two between work and kids where I can get back at it, but I'll check exactly what you suggested and take some reaadings 1st. THANK YOU again

    Eric
     
  4. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Quote:
    Originally Posted by reaiken
    A leaky coupling capacitor can put a DC voltage on the front panel pots, causing them to sound scratchy. A preamp tube with too much grid current can also make pots sound scratchy. I suppose a bad ground could also do it. The quick test is to put a meter across the pot in question and see if there is any DC voltage on it.

    Randall Aiken




    THANKS SO MUCH for your answer.. It'll be a day or two between work and kids where I can get back at it, but I'll check exactly what you suggested and take some reaadings 1st. THANK YOU again

    Eric

    HI There, I am most definitely NOT an amp tech ,but I play one on the internet. ha... I've messed up almost as many projects as I've fixed things, but I've found that is the only way to learn. ANYWAY , there was a bunch of DC Voltage on the Treble pot and Volume pot. I unsoldered a 500pf cap that was going to the treble pot from the board and sure enough it was passing loads of DC voltage.. I played the amp WITHOUT this connected and everything worked perfectly again .. The pots were almost totally disfunctional (unfunctional?) with it connected... I'm going to replace the cap and hopefully I will have my treble pot AND my amp back . THANKS a bunch for your advice... The builder was cool as hell and gave a few tips similar and I read some advice on the hoffmanamps webpage. Great info there , as well as on yours. The internet is such a great place when GENEROUS people like yourselves help . I just wanted to post this so people would hear that yes there certainly are various things that will mess up the pots on your amps , and I could tell it wasn't the pots being shot. If this works, I'll be ready to tackle the Orange or120 also as I think it might be a similar issue.

    Thanks ,
    ERIC
     
  5. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Those silver-mica caps seem to be bad about leaking and causing random noises and DC on tone stacks. It is best to use another type of dielectric or put a large capacitor before the tone stack or in series with the small treble cap to block DC. One of these caps cost me return shipping two ways on a warranty repair on one of my first amps. That's an expensive lesson, so I check them carefully now, or avoid them altogether.

    If you ever run into him, ask Mark Norwine (of Carlson Turbo Pup fame) about 470pF silver mica caps, too. He has a great story about them... :)

    Randall Aiken
     
  6. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Thanks again Randall. If you get a chance to see this, do you notice a difference in sound in using the silver mica caps vs. polyester , vs ceramic etc on these tone stack caps? This amp that I'm dealing with (which is a Trainwreck-type clone.. don't wanna piss people off about clones so didn't mention it!) , but the amp sounds absolutely phenomenal .. Well even without this cap hooked up currently it sounds great so I might just get another kind of cap to replace this one! Anyway, if you read this , and wouldn't mind taking a second to give your views on the tonal differences, I'd love to hear them! (so would the other readers I'd imagine. )
    Thanks so much again

    Eric
     
  7. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    All caps can sound different, but usually the differences are subtle, and it depends on the application. The problem is that small values in the below 1000pF range are not generally available in anything except silver mica or ceramic. You may find a polystyrene cap, but they usually have very thin leads that are not roadworthy.

    Ceramic types can vary widely, depending on the type, from good (COG/NPO) to bad (X7R/Z5U), but most exhibit high microphonic tendencies.

    Silver micas are very good, but can exhibit the DC leakage problems and seem to be prone to hygroscopic problems or soldering damage if the lead coatings get cracked, and they occasionally pass DC for no good reason as you have seen.

    It would be nice to find a good high-voltage polyester or polypropylene film capacitor in the below 1000pF range with nice thick leads, but I don't know of any.

    Randall Aiken
     
  8. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Once again , thanks for taking the time to provide all of that info! This is a 500 picofarad silver mica, so sounds like there aren't a huge amount of options. Yep this one was soldered perfectly and looked brand new and shiny with no visible cracks or breaks at all.. Heading to try to find a replacement this afternoon....
    Eric
     
  9. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Just figured I'd let anyone know who's reading this, I couldn't find a 500pf/500volt cap today , as all we have are Radio Shacks here , but had a 1000pf (.001uf) 500volter, so wired that in , just to doublecheck that this was the source of all my problems, and YEP Sure enough , that was it.. 1 bad $1.00 part caused the whole amp to malfunction. Unbelievable but true.. PLUS there was no 'visible sign' of this, as the amp is wired beautifully and there were no burn marks etc like with a bad resister etc. Anyway, now I just have to get the right value and I'm back in business.. I'm going to have to take apart the Orange or120 I have also , as that amp has all kinds of bad noise on the pots and seems like it might be the same kind of thing. (That won't be as easy though to work on I'm sure !)

    Eric
     
  10. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    I can't verify it, but I think the failure mechanism might be related to the soldering technique. It would probably help to hold the leads with a pair of needle-nose pliers or other type of heat sink right at the body of the cap to avoid heating it through the leads when soldering.

    If you only have 1000pF caps, you can put two in series to get 500pF, with extra DC blocking protection, to boot. It's cheap insurance.

    Randall Aiken
     
  11. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    Now why didn't I think of that! :jo Oh yea , you're the professional! I'll wire them in series today. Double protection is always a good thing. Thanks once again for taking the time to help a random guy on the internet. I'd do the same for you someday if I can:AOK

    Eric
     
  12. novosibir

    novosibir Member

    Messages:
    50
    Joined:
    Oct 9, 2006
    Location:
    Nuernberg, Germany
    I'm using 470p/630V axial polyester caps from ERO/Roederstein in my British Purist as the treble cap, seen on the photo located where the violet cable goes off:

    http://www.larry-amplification.de/britishpurist/brit.pur103.jpg

    and an axial 470p/630V polypropylene cap from BC/Philips in my DINO amp as the treble cap, seen on this photo located where again the violet cable goes off:

    http://www.larry-amplification.de/dino939/dino043-01.jpg

    Here in Germany I get them at Farnell, so I think that on your side of the pond you should get them at Newark.

    The ERO is a nice and warm sounding cap with not too much pronouncation of the highs, the BC is more sounding straight into your face.

    Larry
     
  13. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Is that the blue one? That looks like it has some nice thick leads. I'll have to look into those. Most of the yellow plastic-wrapped one have leads that are too thin for my tastes.

    Edit: It looks like those are RoHS-compliant, too!

    Edit #2: And it looks like they are made of unobtainium here in the US... :(

    Thanks!

    Randall Aiken
     
  14. Jackie Treehorn

    Jackie Treehorn Member

    Messages:
    1,871
    Joined:
    Jan 20, 2004
    Location:
    New Orleans, LA
    Eric, go polystyrene. I switched to polystyrene from silver mica in my express type amp a few months ago and it's a significant improvement in the top end. Triode electronics has high voltage polystyrene 470 pf caps. I bought around 10 and found one that measured appr 481, which seems close enough!
     
  15. pfrischmann

    pfrischmann Member

    Messages:
    3,002
    Joined:
    Jan 6, 2002
    Location:
    In the ATL.
    Larry,
    That workmanship is beautiful! Do you have more pics or a website?
     
  16. ericb

    ericb Silver Supporting Member

    Messages:
    8,413
    Joined:
    Jan 4, 2002
    Location:
    VT
    THANK YOU .. you guys are awesome

    Eric
     
  17. VacuumVoodoo

    VacuumVoodoo Member

    Messages:
    1,567
    Joined:
    Jan 9, 2005
    Location:
    Gothenburg, Sweden
    The yellow caps are ERO made by Vishay type MKT1813 and are ROHS compliant. Really good coupling caps. DATASHEET-MKT1813
    The blue caps are type KP460...464, also made by Vishay, originally designed for filtering networks in telecom applications. Good thing is they are available in 1% tolerance, 47pF and up, E24 series as standard but you can also get E48 or E96. Perfect for frequency response defining applications. ROHS compliant. DATASHEET-KP460
    Vishay owns most of old european component brands.

    My caps of choice.
     
  18. reaiken

    reaiken Member

    Messages:
    1,805
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    I searched Mouser, Allied, and Newark for the BFC2464 and BFC2463 series and only found a couple values. I haven't searched for the yellow ones yet (I really liked the looks of the blue ones!). I may have to special order them.

    RA
     
  19. novosibir

    novosibir Member

    Messages:
    50
    Joined:
    Oct 9, 2006
    Location:
    Nuernberg, Germany
    @ Randall

    Yes, as VacuumVoodoo already said, the blue ones are the BC/Philips (now Vishay), but in my catalogue they aren't marked as RoHS compliant.

    Further I've seen now, that they are no longer available in 630V, but still in 400V, what also would be plenty for a treble cap.

    I get the caps from Farnell www.farnellinone.de the European section of Newark through the parts #106-792

    The values, available in 400V are 150p; 220p; 330p; 470p; 680p and 1,000p - further in 630V are available: 47p; 68p and 100p

    If you have a problem to get this parts, simply drop me an email and tell me the values and numbers you need - no problem, to ship the needed amounts over to you.

    Larry
     
  20. novosibir

    novosibir Member

    Messages:
    50
    Joined:
    Oct 9, 2006
    Location:
    Nuernberg, Germany
    Right now seen, that the not RoHS compliant part is no longer available - the now RoHS compliant replacement is the # 1215580

    Larry
     

Share This Page