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Noisy DI on SVT CL?

Discussion in 'Amps and Cabs [BG]' started by poorbassist15, May 7, 2019.

  1. So after remedying some minor issues with this amp(namely replacing filter caps) I've been able to take my SVT for a test drive this week at a few larger gigs, only one of which had provisions for a DI, and the DI was very noisy. Now the amp isn't exactly the quietest beast on the planet to begin with, the fan is incredibly noisy, there's still 60 hz hum in it(as is the case with the Magnavox SVT I've been lucky to borrow), although not really anything that I wouldn't hear from any other tube amp, and it really doesn't obstruct the actual live sound with the amp.

    The problem is is that the DI is obnoxiously noisy to the point I had to box it to FOH, I didn't try to check the polarity switch as I had a tube fail within 4 songs(figures) but I was wondering if the switch on the amp itself would have an effect on it. I'd really like to DI it when soundguys won't mic it because it sounds gooood. Does anyone else have this issue?

    I should also say I'm not the world's greatest technician by any means, but I can read a schematic and use a probe and have already serviced the amp as well as other tube amps so I'm well aware of the precautions in this case.
  2. Wasnex


    Dec 25, 2011

    I looked at couple of different schematics. Some show just a transformer tapped off the preamp and some show a transformer that can be tapped Pre or Post followed by a couple of Op Amps. Obviously the first step is figuring out which schematic is the correct one for your amp.

    If your amp has a Pre/Post switch, the tap for the Post DI is electrically the same as the preamp out. If your amp doesn't have a Pre/Post switch...it's Post only.

    I would set the Master Volume fully CCW and see if the DI is still noisy with the DI in Post. I believe V2B is the only tube stage between the Master Volume and the DI. So if you pull V2 and the noise remains, look into the DI circuitry. If the DI goes quiet when you turn the Master Volume down or pull the tube, then look into the preamp.

    I couldn't find a schematic of the preamp that shows how the Pre DI signal is derived. The available schematic shows NC for the relevant pin on J3 (pin 11).

    Good luck.
  3. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The pre/post switch is an option on the 2 pro (and VR) but not the CL, the power amp schematics floating around the net are labeled for both CL and 2 pro but the pre/post and level are not on the CL.

    If I'm remembering right there is an old tech bulletin for a ground loop noise in the preamp, if you can find it you should check to see if your amp is covered under this bulletin since you mention a consistent 60Hz noise.

    For the DI I would suggest trying a ground lift adapter on the XLR (the CL also doesn’t have the pin 1 lift option) to see if the noise goes away.
    Last edited: May 7, 2019
    Wasnex likes this.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Wasnex likes this.
  5. My cl is indeed older us, this is interesting, I'll follow this lead.
    beans-on-toast likes this.
  6. I checked IC1 and it appears to either have been a revision or had been remedied at some point. What I did find, however, were several questionable pots, including the volume that had been hit so hard it's loose and crunchy. I wonder if this has been the source of my woes all along. I can't believe I didn't notice that before. I tried to rearrange the cables going to the preamp section, trying to give myself some space between them and the HV(I assume I can't find any schematics for the preamp on this thing, somehow.) and the filament wires(a twisted pair, singular) maybe that will help but I have practice tomorrow so we shall see.

    I wonder if the sovtek that tested good in V2 is also causing problems, I had a tested good Chinese in V1 that quit giving the goods earlier. Who knows, I've got a 75% GE 7025 I might stick in there to say the only things not NOS are the driver and power tubes.
    beans-on-toast likes this.
  7. I'll try that, I feel like that would significantly diminish it. Odd that the 2 pro has all these things remedied and the CL doesn't.
  8. Wasnex


    Dec 25, 2011

    See if this is the preamp schematic you need http://www.tangible-technology.com/schematics/Ampeg/SVT/Classic/Ampeg SVT CL-3.pdf

    Looks like the wrong DI schematic though, since it has pre/post.

    Here's the root directory Index of /schematics/Ampeg/SVT/Classic/

    If you turn down the Master Volume, the only gain stage you should hear is V2B. (Edit: I believe it's actually V2A but its the second gain stage on the schematic for V2 which uses pins 1,2,3)
    Last edited: May 7, 2019
  9. Wasnex


    Dec 25, 2011
    If your using a Sovtek 12AX7LPS in V2 it could possibly be the tube. The schematic shows 100V on the cathode of V2, pin 3. From what I have read, the New Sensor tubes with spiral wound filaments don't tolerate high cathode voltage very well. Tube might test okay be have a lot of hum and be prone to cathode to heater shorts.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Tighten the pot nuts, then reflow the solder connections.

    A common problem with the older amps is cracks in the solder joints of the power tubes. This causes crackling sounds. The solution is to reflow the solder joints. Remove the power tube to do this.

    They fixed this issue in later revisions by installing two metal hex standoffs, one on each tab of the power tube socket. One side is against the tube sock, the other is on the PCB. This reinforces the socket, reducing the force on the tube socket when the tube is inserted. If your amp has these standoffs, you should be fine. If not, reflow.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Removing the tube board is shown in TIB0003-1. THey describe the power tube issue in CSB0013.

    The SVT_CL_ServiceBulletin was recommended but not urgent. A lot of amps are operating as they were originally built. They build them as per the bulletin now. The 220 ohm resistors can help with tube life. It is important to use the correct resistor, one with the correct wattage and type, they are designed to blow like a fuse in the even of an issue. See the NFR25 file for the screen resistor: 1/2W, 220 ohms.

    Attached Files:

    Last edited: May 7, 2019
  12. callofcthulhu


    Oct 16, 2012
    Thanks, this is a useful reference.
    beans-on-toast likes this.
  13. I believe mine already has the standoffs(you'd think I remember I had it apart not one week ago), and I really can't remember so I'll have to look so it may have the resistors installed as well. That's a good idea to do anyway, I'll do that too.
    beans-on-toast likes this.
  14. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

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