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Ampeg SVT Classic

Discussion in 'Amps and Cabs [BG]' started by Jon Lucas, Nov 3, 2019.

  1. Jon Lucas

    Jon Lucas

    Jun 2, 2010
    Hey y’all, my classic started making a nasty noise but only on a few lower notes (F# G G# specifically) and one octave up on the same notes but nowhere else. Very strange. Have any of you had a similar issue or know what could be going on? A local amp tech said I need a whole new set of power tubes but I’m not convinced to say the least.
  2. Raf Seibert

    Raf Seibert

    Dec 16, 2013
    That your noise occurs at specific notes and their octaves suggests to me that it might be more likely a cab or physical, problem than an electronic one.
  3. I totally agree. Notes are frequencies, some frequencies cause vibrations others don’t... try moving the amp, putting the head elsewhere then on top of the cab, try a different cab...

    From what I’ve experienced, bad power tubes cause volume variations, general noise and more muffled tone, but on every notes.
    Aqualung60 likes this.
  4. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Yeah what they said. Your problem seems like resonance.

    I played a stage last weekend that had a resonant frequency equal to that of a low D. Every time I played one it shook the whole stage like a mild earthquake.
  5. raybone


    Jul 14, 2006
    A.C Jersey
    Yes so annoying. I have an old bassman 100 that sometimes does that on C and C# up and down the neck, i agree that its probably tube vibrations from those frequecies. I just pick the head up off the cab and move it over a few inches either forwards or back, usually gets rid of it.
    SirMjac28 likes this.
  6. Wasnex


    Dec 25, 2011
    I bought a Marshall 2203 awhile back and it came with some GE6550s. The tubes were working, but a couple of them were totally thrashed. Here's how I found out: I have a variac with a built in ammeter and when I went to check the bias on the amp, I noticed it was set really cold. So reset the bias a little hotter; this is done with the chassis out of the head case, upside down resting on the transformers.

    After I put an amp back together I check the current on my variac to make sure everything is good. Unfortunately, I noticed the current was way off from where I had just set it. Turns out the internal structure of two of the 6550s are mechanically compromised. So when you move the amp around the guts shift inside the glass envelope causing the bias to shift drastically. If you were to shake the tubes, they would make all sorts of rattly noise.

    So if this amp was placed on top of a speaker cabinet, one might assume that the tubes would be susceptible to vibrations coming from the cab. Generally cabs do not vibrate evenly at all frequencies. Some cabs have a few frequencies where the panels vibrate like crazy. So if the tubes are mechanically compromised, and the cab vibrates at a few specific frequencies, it's reasonable to assume the symptoms could be due to worn out tubes. I am not saying this is the problem, but it definitely could be.

    Tubes can do all sorts of weird, random stuff. For example, I had a 12AX7 in the OD circuit of my Mesa Titan that had a tendency to motorboat/oscillate at a low frequency after the amp warmed up. The weird part is if I played my bass, the tube would track an octave down from the notes I played. In other words, the tube was creating a down octave effect. I rolled another tube into the amp and the problems went away.
  7. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    Before Going all new power tubes
    I would try new pre tubes
    and also possibly the 2 pre tubes on the power amp section.

    It probably is resonance of some notes.
    the tubes may work and be fine.
    eventually mica spacers or plates can vibrate inside the tube
    certain notes will rattle the loose plates

    likewise if you gig the amp all the time
    check for tubes that are not seated well
    and i dont like moving tube amps when they are hot.
    constant temperature changes and vibration will kill the internals of tubes

    for the most part its unavoidable

    After hearing many tubes go to crickle crakle and all kinds of early deaths
    the most reliable tube I have found so far is Sovtec 12ax7 LPS and matched phase tube Sovtec 12AU7
    or Tung Sol 12AU7W / 6189

    sometimes you can bang on the amp and if you hear rattles or crackles its loose plate tubes.
    or tap hard across the face plate and it might rattle more in the general area where the bad tube is in the pre
    or tap hard on the back of the chasis if the 2 tubes in the power amp are rattle tattles.
    doesnt always work but they are usually pretty bad if tapping the chassis/faceplate works
    and you dont have to disassembly the amp.

    otherwise with the amp open, good luck SVT is royal pain to open up
    you can tap tubes with a pencil eraser end and microphonic pings or rattles will be heard
    Wasnex likes this.
  8. Wasnex


    Dec 25, 2011

    You may not need to replace all of the preamp tubes. Do the tap test and roll a spare 12AX7 through the 12AX7 slots.

    The amp has a pair of 12AU7s. I suggest a bit off caution of these are replaced, because they control the bias of the output tubes. I would replace them in pairs, and before pulling the old 12AU7s out, set the bias for minimum current in the output tubes. Then when you put the new tubes in, follow the normal bias procedure in the owner's manual. The reason is because new driver tubes will tend to be stronger. Stronger tubes will pull the bias down, which will increase current in the output tubes.

    You can also do the tap test on the output tubes. If the internal structures of the tubes are not sound they should rattle and the tap test will produce a pretty obvious noise in the output of the amp. Also if the tubes are as bad as the ones I described in post #6, the bias will shift whenever the amp is moved.
    SirMjac28 likes this.
  9. Lowbrow

    Lowbrow Supporting Member

    Apr 22, 2008
    Pittsburgh PA!
    I don't know if the tech you mentioned has seen your amp in person.
    If not, before you do ANYTHING try switching on the amp, plugging in, and turning each knob to see if there is any crackle.
    A dirty pot can respond to a given frequency vibration - and can sound like blown speakers or any of a number of electronic ills.
    Generally you can just flick the knob back and forth several times to alleviate.
    I once almost trashed an entire PA system due to this often overlooked and easily solved bugaboo.
    Wasnex, SirMjac28 and Aqualung60 like this.
  10. iiipopes

    iiipopes Supporting Member

    May 4, 2009
    Wow! You are lucky something didn't short internally across the plates and send B+ upstream! I have had good luck with the Marshall 2203 that I used to own re-biasing the output for Electro-Harmonix "Big Bottle" 6CA7 power tubes. A 6CA7 is essentially the plate of a 6550, the guts of a 6L6GC, and the bias of an EL34. It has the same big clean tone as a 6550 but without the harshness, is not as "soft" as a 6L6GC while retaining some top end sparkle, although it does lack a little of the midrange complexity of an EL34.
    Last edited: Nov 4, 2019
    Wasnex likes this.
  11. One day my rig started buzzing and then I noticed my wife left a set of metal banjo finger picks on top of my amp case :woot:
    Lowbrow and EatS1stBassist like this.
  12. Mantis Tobaggan

    Mantis Tobaggan Supporting Member

    Sep 9, 2015
    Tampa, FL
    I am guessing it is a tube issue. Have you tried lightly tapping them?
  13. Wasnex


    Dec 25, 2011
    The amp actually sounded fine and I had played it occasionally for a couple of years with no tube related problems. I had even done some trouble shooting on it to find an intermittent 60 hz hum. I found the feedback wire had somehow become pinched under a standoff. I didn't have my bias probes at that time, so I didn't check the output tubes.

    Once I realized how bad those two GE 6550s were, they went in the trash. The other two were good, so I went through my stash of misfit tubes and found a pair of JJ KT88s that biased within a few milliamps of the GEs. It sounds good and gets stupid loud before the output section breaks into OD. Eventually I will probably throw a matched set of 6550s in it.
    iiipopes likes this.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    First rule out that it isn’t the bass, try another one.

    Then rule out that it isn’t something in the room resonating, try the amp in another room.

    Then rule out the cabinet resonating. Don’t place the head on the cab.

    The last thing to check is the amp.
    Wasnex likes this.
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