Ampeg SVP-CL : 4 tubes and their funtion?

Discussion in 'Amps and Cabs [BG]' started by MuzikMan, Aug 29, 2005.

  1. Does anybody know what part of the circuit each of the 4 tubes are in? I'm a total noob with tubes but I'm resarching the subject to get a better understanding of what to expect tonally if I changed tubes.

    All I know right now is it has 2 12ax7 and 2 12au7 tubes. I would like to find out if I could concentrate on 1 tube for tone or do they all affect tone equally. For example, if I had one Telefunken and three JJ/teslas where would you put the Telefunken?

    I would also like to hear your experience with tubes in this preamp. I hope someone can help :D
  2. you could always think of it more as :

    One 12AX7
    One 12AU7
    One 12DW7

    because with one of the 12ax7's and 12au7's your only using half of each tube, which is the equivilant of the 12DW7 IIRC

    i may be wrong, but it seems the first one in the circuit usually colours the sound most, so that'd be the one nearest the side with the inputs, i think
  3. Ok that's how they are emulating the classic SVT with the 12dw7's right? I wonder if I can dig up an old SVT schematic and apply it to my preamp (as far as the tubes go).

    Where's Ampeg insider (Jerrold) when you need him?
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Right here, noting that the answer was pretty much given.....

    Yes, the 12AU7 and 12AX7 are used for the appropriate halves of the original 12DW7 tube.

    Much better than using the 12AX7 for everything, since all the original values can be used....leading to performance as per original.
  5. Do you have any info on the specific placement of each tube in the circuit?
  6. tadawson


    Aug 24, 2005
    Lewisville, TX
    I forgot to bring the schematic in to work with me, but the Ampeg Schematics show that there is one tube as a preamp in both channel one and channel 2 (12DW7 originally), a second in each channel for tone (12DW7 as well, IIRC) and a third in channel one for the midrange tone contro. There is then a 6C??? as a summer of both channels, and off to the power amp section. In the power amp, (my memory fades a bit more here . . ) there is an input amp/phase inverter, and then two driver tubes, one for each side of the power amp, and then the six output tubes.

    Both the 12DW7 and 12AX7 are dual triodes. The 12DW7 was unique in that they were an unmatches pair - the second had bigger plates, and more power capability (3 watts or so vs. 1.2 for the first) and the 12AX7 is a matched set. The Ampeg published mod for anything but the SVT was to pull out the 12DW7 and put in the 12AX7 - no other changes, since they pin out identically. In the SVT, there were three resistors that change, to alter the biasing on one of the tubes, but I don't recall which one off the top of my head. If anyone wants to know specifics, as well as the tube numbers to function, let me know and I will try to get online while I have my docs with me and can get you that information.

    And if anyone knows of a "rough" SVT head that might be a candidate for a rebuild, let me know. I have an urge to rebuild one of these from the dead (tubes being so simple and all . . . ) and no desire to pay top shelf for the new SVT-CL or a "cherry" original . . .

    - Tim
  7. Ok I saw the schematic for the SVT while browswing the net. It definately isn't the same as my pre, but may still give me an idea of what's up. My amp is only one channel so I'm guessing that one side of the first 12ax7 and one side of the first 12au7 would represent the 12dw7 in the gain position of a single channel of an SVT. The other side of those same tubes may combine to represent another 12dw7 in the tone section.

    If I am on the right track there would be a possibility for 2 more 12dw7 clones, or maybe 1 12dw7, one unused side of a 12ax7, and one 12au7 left to feed a power amp. Who knows! :D I think I'm just going to get some JAN Phillips 12ax7wa tubees and some JAN 12au7 tubes. It seems like a good place to start in my quest for tube knowledge.
  8. tadawson


    Aug 24, 2005
    Lewisville, TX
    Sorry, missed that you just had the pre. I have the schematic for that as well (at least for the 80's unit, and the reissue should be the same). I will try to look tonight. My guess would be that there woult NOT be a split between the 12AU7 and 12AX7 to replace the 12DW7 - all the Ampeg mods publishes are 1 to 1, 12DW7 to 12AX7, and I can't think of any reason that they would do this one differently, unless, of course, that the reissue is NOT the same circuit as the original . . . .

    - Tim
  9. This is mostly speculation on my part but I have read somewhere that a 12dw7 is essentially the same as 1/2 of a 12ax7 and 1/2 of a 12au7. I have also read that the SVP-CL is "based" on the preamp circuit of the classic SVT head.

    FWIW the SVP-CL is not the preamp section of the SVT-CL that is currently in production. (What's up with that?)
  10. I wonder why they didnt just use 12DW7's, they werent availible in the 80's and early 90's IIRC, but are readily availible now
    KaseOfBass likes this.
  11. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The tube near inputs (12AX7) is the one to use the premium low-noise tube.... it is the input preamp.

    As far as the SVP CL versus SVT CL, I think its confusing too. Sounds like they should be the same, but they are not.

    "Based on", because we don't use the 12DW7, so it can't be "identical to"............ and the board isn't laid out exactly the same as, so that isn't "identical to" either......

    Go play one...... and don't sweat the definitions.
  12. they wanted a decent preamp, and yeah, as i said at the top of the thread, it is the same as the 2 halves
  13. Thank you! That answers the primary goal of my post. As far as "based on" goes, I wasn't trying to imply any negativity in that statement. I simply was not aware of what that specific term meant.

    I do own the SVP-CL and I like it very much. I'm just not able to leave well enough alone so I gotta play with my tubes (almost as fun as twisting my knobs!) :D
  14. tadawson


    Aug 24, 2005
    Lewisville, TX
    Sounds like you got pretty much what you needed to know already. Apparently I had a brain fade, and found that the page in my schematics that I thought was the SVP was another variant of the preamp deck for the SVT, so sorry if I got your hopes up. . . . .

    - Tim
  15. All is good :D
  16. Coco'


    Apr 24, 2001
    Milano, Italy
    I'm the owner of a great svp-cl. I had to change two tubes cause they were microphonics. What other tube brands do you suggest?
    Have you changed yours? What did you use?
  17. I recently installed a 1960 GE longplate in the first 12ax7 slot and 2 RCA cleartops (1960's?) in the 12au7 slots. The second 12ax7 is the original GT (sovtek) 12ax7WA.

    After 2 gigs I have concluded that the bass is tighter, the mids are clearer, and the highs lost a harsh quality that it had before. All around the sound is more focused and together.

    I wouldn't say that the tone (flat) has changed much and I don't feel like I have a new amp now (didn't want one anyway) but the changes were well worth the $40 I spent on tubes.
  18. Sparks


    Dec 15, 2004
    KaseOfBass likes this.
  19. Phew! 8 yrs! A zombie thread record??:eek:
  20. Blues Bass 2

    Blues Bass 2 Supporting Member

    Oct 3, 2001
    Davenport Iowa
    Zombie thread or not I'm glad it got brought back . I had some tube trouble with my SVP-CL pre and this would have helped . At least it gives me an idea what each tube is for .