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Vintage SVT ohms ?

Discussion in 'Amps [BG]' started by kringle77, Oct 22, 2009.


  1. kringle77

    kringle77 Supporting Member

    Joined:
    Jul 30, 2004
    Location:
    Massena NY
    My 70's svt has to see a max of 4 ohms but can push two 4 ohm loads. Can I chain to 4 ohm cabs together and them into one of the 4 ohm speaker outputs or do I have to pluge one cab into each output.

    I know that some amps outputs are just wired in parallel but, I wasn't sure if the old SVT's outputs are tapped off an output transformer and therefore, must be used seperately. Thanks.
     
  2. projectMalamute

    projectMalamute

    Joined:
    Jan 9, 2008
    Location:
    Westfield, MA, USA
    Pretty sure the transformer is tapped for two or four ohm loads, plugging in to the extension speaker jack switches to the two ohm tap.
     
  3. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps
    You have to daisy chain two 8 ohm cabs into one 4 ohm speaker out on the SVT for it to work right. The head will see two 8 ohm cabs as two 8 ohm loads and not a single 4 ohm load unless you daisy chain.
     
  4. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    You have to connect one 4 ohm cabinet to the main speaker out and the other 4 ohm cabinet to the external speaker out.

    The main speaker out on the vintage SVT is intended for a 4 ohm cabinet. Same goes for the ext speaker output.

    The output transformer has 4 and 2 ohm taps. When you connect a cabinet to the main speaker out, you are connecting to the 4 ohm tap on the transformer. When you connect a second cabinet to the external speaker out, the two cabinets are connected in parallel internally and switched to the 2 ohm tap.

    To connect a 2 ohm cabinet, plug it into the ext speaker out AND connect a 4-pin plug to the main speaker out that is wired to short pin-2 to pin-3.

    As Jimmy said, if you have two 8 ohm cabinets, you have to (parallel) daisy chain them together to give you 4 ohms and then connect them to the main speaker out.
     
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  6. JimmyM

    JimmyM Supporting Member

    Joined:
    Apr 11, 2005
    Location:
    Apopka, FL
    Disclosures:
    Endorsing: Ampeg Amps
    Let me also add that's for the old ones. The newer ones have ironed that little kink out, but you still need at least a 4 ohm load.
     
  7. kringle77

    kringle77 Supporting Member

    Joined:
    Jul 30, 2004
    Location:
    Massena NY
    Pins 2 and 3? Stereo jack?
     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    On the earliest SVT's, the main speaker out had an attached cable with a four pin XLR type connector. This is where the short pin-2 to pin-3 comes into play.

    If your vintage amp has a 1/4" jack on the main speaker out, it would be best to have someone take a look at how the two output jacks are wired on your amp before attempting to connect a 2 ohm cabinet. Sometimes modifications have been made.

    Normally, the main 1/4" speaker out jack is shorted to ground when nothing is plugged in, that is, no speakers are connected. This helps to protect the amp if it is turned on with no speakers connected. Not a good thing to do with a tube amp, but a short is better than an open circuit. If you were to plug a cab into the ext out with nothing connected to the main speaker out, all you would have is a short to ground. It wouldn't work and could harm the amp.

    If you plug a 2 ohm cabinet into the main speaker out 1/4" jack AND insert a standard 1/4" plug (with nothing connected to it) into the ext speaker out, you will switch the main speaker out jack to the 2 ohm transformer tap.

    As I said above, it would be a good idea to have your output jack wiring checked out before attempting this.
     
  9. xoir

    xoir

    Joined:
    Oct 25, 2007
    Hi folks,

    I would like to have some advice of what could be happening to my amp, this is the situation:

    I’m using and old SVT 73 (recapped/biased/retubed) with two cabinets: SVT-215E at 4ohms and SVT810E at 4ohms, the problems occurs only very few times when I’m using the two cabinets connected to the main and external outputs, the amp distorts whatever is connected to the main speaker output, but not the signal hooked in the external speaker output, if I connect only one cab to the main speaker 215 or 810 at 4ohms the amp sounds and seems to work good, but If a connect the second speaker in the external output (on the fly) the main speaker signal sometimes distort and sometimes seems that is not sending the full output signal to the main output, and sometimes seems that the output volume of the external speaker is reduced while the main distort, if I disconnect the external speaker (on the fly) the signal is recovered in the main speaker output and sounds good.

    Any ideas?

    Thanks!
     
  10. xoir

    xoir

    Joined:
    Oct 25, 2007
    Ok... now is doing it every time... :(

    I think that I'm having an ohm mismatch issue in the OT.. I tried troubleshoot with an IMT-1 in the main speaker output without luck...

    [​IMG]
     
  11. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    The first thing that I would check is the ext spkr jack, as shown at the right in the image. Check for any shorts or loose wires wired to the jack. It has switch contacts that oxidize and can reduce you signal. Clean them well with Deoxit if you have some. Keep any cleaning solutions away from the insulators, just use it on the metal contacts. Also check that those screws on the jack are tight.

    That's a neat impedance matching box. Got a shot of the inside?


    [​IMG]

     
  12. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    DO NOT plug and unplug "on the fly" either! You must be in standby at least, NEVER in full operate.
     
  13. xoir

    xoir

    Joined:
    Oct 25, 2007
    Ok, the issue is related to the speaker output jack in the amp, ground probably?

    In order to remove the distortion or loose of volume in the main speaker the ext-speaker output needs to be semi-plugged, If plug the cable all the way in the amp jack I get the issue, if I semi unplug the cable I got my full rich sound without any issue on both cabs.

    This is how is connected the ext-speaker cable, I did not notice that one of my old cables is curved but definitely was damaged from the cabinet side, not the amp side, should I open my speaker cabinets to check if anything is loose? I’m totally sure that did not damage anything in the amp side jack. why I get the issue fixed with the plug half-way?

    Here is how it work :/

    [​IMG]

    Old Plug

    [​IMG]

    Beans,

    The transformer is sealed, same size as the OT from B15 :D

    [​IMG]

    [​IMG]
     
  14. xoir

    xoir

    Joined:
    Oct 25, 2007
    I plug and unplug on the fly leaving always one speaker active in the amp :D
     
  15. CL400Peavey

    CL400Peavey Supporting Member

    Joined:
    Nov 7, 2011
    Location:
    Grand Rapids Michigan
    I still wouldnt do that. The amp has a impedance selector switch yes? Very bad idea to be switching and unplugging these things on the fly. Just asking to cook your output transformer.
     
  16. xoir

    xoir

    Joined:
    Oct 25, 2007
    Already try with other two cabs, and it's the same problem, so the issue is in the amp :/
     
  17. CL400Peavey

    CL400Peavey Supporting Member

    Joined:
    Nov 7, 2011
    Location:
    Grand Rapids Michigan
    I suspect you may have damaged your output transformer from unplugging cabs "On the fly".
     
  18. xoir

    xoir

    Joined:
    Oct 25, 2007
    The issue is fixed with the plug half-way inside jack, so it may be related to what beans-on-toast told, but I suspect from a ground issue.
     
  19. B-string

    B-string Gold Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    With the plug partly pulled out the impedance switching is not active and you are running a 2 ohm load on the 4 ohm OT tap! It could still be the contacts as you may have burned them plugging with the amp live. Or you damaged the 2 ohm tap on the OT.
    I repeat DO NOT PLUG or UNPLUG WITH THE AMP OPERATING. Unless you like having problems?
     
  20. xoir

    xoir

    Joined:
    Oct 25, 2007
    I never run the amp "live" without speakers :) I mean ever, the issue began a couple of weeks ago, I unplug the external speaker "live" only today I'm sure about it, I could not find information that plugging the external speaker output live could harm the 2 ohm tap in the OT, besides I'm aware that is not recommended, I don't think at this point, that the OT is damaged because running it at four ohms or with one cabinet the amp works right.
     
  21. CL400Peavey

    CL400Peavey Supporting Member

    Joined:
    Nov 7, 2011
    Location:
    Grand Rapids Michigan
    Well you must have it all figured out... Why the questions here then?
     

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