A Question of Ohms ?

Discussion in 'Amps and Cabs [BG]' started by Tom Neves, Aug 3, 2000.

  1. I have just invested in a 70s SVT and I am in Bass heaven. All the things you guys have said about this wonderful amp is totally true. I notice there is no Ohms switch.
    The back panel only has a 4 ohm input for speaker and a 4 ohm ext. speaker jack.
    I have 2 Ampeg cabinets,(see profile).
    Will running 2 cabs in any way cause a problem with the amp?
    Am I correct in my assumption that one cab is a 4 ohm load and two running together would be 2 ohms?
    Please......simple english for a simple guy [​IMG]
  2. I don't have an SVT, but according to the schematic, the output transformer has a 4 ohm tap and a 2 ohm tap. So you could run a total of (2) 4-ohm cabs from it by using the ext. speaker jack and the main speaker jack. The ext. speaker jack switches the impedance tap from 4 to 2 automatically when a plug is inserted in it.

    Somebody correct me if I'm wrong on this. I'm only going by schematics.

  3. jcadmus


    Apr 2, 2000
    I'm not an Ampeg expert by any means, but I believe the SVTs were originally designed to handle up to two four-ohm cabs (for a total impedance of two ohms). Any of the Ampeg aficianados listening in?
  4. So are you guys saying it's O.K. to run the two 8 ohms cabinets either one or both at the same time?
  5. Player


    Dec 27, 1999
    USA Cincinnati, OH
    If I remember correctly the 70s SVT went down to 2ohms (like the V-4b which has a selector switch for impedence). This is always a minimum rating, meaning it won't hurt to higher impedence. Higher impedence More resistance) just means less volume.


  6. danhei


    Jan 21, 2000
    <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Player:
    If I remember correctly the 70s SVT went down to 2ohms (like the V-4b which has a selector switch for impedence). This is always a minimum rating, meaning it won't hurt to higher impedence. Higher impedence More resistance) just means less volume.

    This is NOT correct. Tube amps have impedence switching schemes because tubes want to see a certain impedance. That's why there are switches or jacks that do the switching automatically. Now, depending on the amp, you can mismatch impedances to some degree but I wouldn't really recommend it. Since you have 2 8 ohm cabs for a total load of 4 ohms, this is what you need to do: run both cabs parallel into the 4 ohm jack. You could make yourself a special cable or box or maybe one of your cabs has an output on it.

    Solid state amps can handle any impedance that's not less than their rated minimum impedance (typically 2 or 4 ohms). But tube amps have impedance selection schemes for a reason! Don't mismatch impedances if at all possible. You really should run both cabs off the 4 ohm jack, not into both jacks which, to my understanding, is where the amp wants to see a 2 ohm load (2 4 ohm cabs).

    I can tell you how to make a box/cable to do this if you're interested.

    Psycho is the SVT guru so maybe he'll have a different/better understanding but I think I'm correct.


  7. <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by danhei:
    This is NOT correct. Tube amps have impedence switching schemes because tubes want to see a certain impedance. . . . .


    I agree wholeheartedly.

    "Preach it,Brother! Testify!" [​IMG]

    This allows tube amps to get full power into different ohm cabinets, if they are designed for that impedance. (I mean the SVT above can do full power into a 2 or 4 ohm cab, and shouldn't be run with just an 8 ohm cab, unless Phsyco says it's alright from his experience.)

  8. Psycho,
    Hi guys, just got back from a trip.
    Now I am really confused !
    I have been running both my SVT and V4b with a single 8 ohm cab.
    The V4b has the ohm switch on the back and is set at 8 ohms.
    From what I understand, the SVT will automatically switch impedance depending on how many 8 ohm cabs I use.
    Am I O.K. with what I'm doing on the V4b?
    If I use 2 cabs with the V4b should I move the switch to 4 ohms?
    Sorry to drag this out, I have looked at some of the other links on ohms and still need clarification. I guess I am looking for a simple answer, being I'm a simple guy.
    Bass Players.............What a crazy bunch!!!
  9. timv

    timv Supporting Member

    Jun 7, 2000
    Chandler, AZ
    The svt is made to either run at 4 ohms or 2 ohms not above or below. The 4 0hm jack is connected to the four ohm output transformer tap and if you connect a cord to the extension cab it makes both jacks connected to the 2 ohm output transformer tap. So, for two 8 ohm cabs you want to run a speaker cord from the main output jack to one cabinet, another cord out of that cab to the other cab. One 4 ohm cabinet would connect to just the main output jack. 2 4 ohm cabinets could be hooked a couple different ways. Either, one cabinet connected to the extension jack and the other cab hooked to the first cabinet or, one cabinet hooked to each of the output jacks. I hope that makes sense. If not, i'll try to answer any other questions about the svt. I have a 77'. It is very nice.
  10. timv

    timv Supporting Member

    Jun 7, 2000
    Chandler, AZ
    Tom: The V4 is fine running at 8 ohms w/ the switch set at 8 ohms but, the svt isn't made to run at 8 ohms at all so you aren't getting the most power out of your head. I think as long as you don't play really loud and push the amp hard you are probably ok running at 8 ohms but someone else may have something different to say. But, if you play pretty loud i wouldn't run it into an 8 ohm load and it probably is a good idea not to anyway, just incase. Youo don't want to do damage to an svt. Good luck finding another one if you hurt it bad enough. It took me 2 months to find mine. With the 8 ohm load, the output tubes aren't seeing the proper load they want to see so the currents and voltages are wrong for the tubes and the output transformer. I hope this helps. Tim
  11. Another question?
    Can I change my existing Ampeg 4-10 8 ohm cabinet to 4 ohms?
    Ditto on my Ampeg 2-12 cab?
    Bass players....what a crazy bunch!!!
  12. reel big bassist

    reel big bassist

    Mar 27, 2000
    I have an SVT-300,it's bassicly the power section of an svt.
    I was wondering, is thier a way
    you could modify the svt, to accept 8,4,and 2 ohm loads?
    I have a Genz-Benz 410T-XB(8ohms), and a home made 4x10(8ohms).
    The homemade 4x10 doesn't have a lot of power handling, so I'd like to run just the Genz benz off the svt, but I'd lose a lot of power, and it would eat the tubes at high volume levels. Could this modification be done, and if so would it affect the tone?

    Greg P

  13. RobbieNuke


    Jan 22, 2008
    Dennis Krager (a technician for Ampeg in the the 70's) wrote a memo that stated since a lot of SVT owners were only buying one cabinet (which was only rated for 240 watts), Ampeg intentionally rewired the impedance switching jack; so the 2 Ohm tap on the 'speaker' jack and the 4 Ohm tap was on the 'extension speaker' jack were switched... this would reduce the output if only one cabinet were used. And this would also reduce the claims for blown speakers under any warranty.

    I use only one cabinet (a later version 810E rated for 600 watts) with my late 70's SVT and put a 'dummy' plug (open) in the 'extension speaker' jack which allows me the correct 4 Ohm tap matching my transformer to my 4 Ohm cabinet for a full 330 WRMS.
  14. oinkbanana


    Oct 20, 2011
    if you're running your svt with a dummy plug, aren't you just tricking your amp into running on the 2 ohm tap? why would you run it at 2ohms with a 4ohm cab?
  15. RobbieNuke


    Jan 22, 2008
    The 'dummy plug' switches it to the 4 Ohm tap. Since it's open there is no additional load so the amp only sees the 4 Ohm cabinet. However, since I don't have an Ohm meter that measures that low of a resistance, I can't be certain. Only going by what I read in the memo. When I insert the plug there seems to be a bit of gain, not unlike changing the impedance selector switch on other amps.

    I haven't been inside the amp in awhile, but the next time I am, I will try to compare the wiring with the schematic. I may modify the amp with just two parallel jacks and install an impedance selector switch (like the MTI/SLM production units).
  16. oinkbanana


    Oct 20, 2011
    so if you plug just one cab, without a dummy plug, it runs the amp at 2ohms?
    seems backwards.