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SVT Tube Preamp Layout

Discussion in 'Amps [BG]' started by boroman, Mar 26, 2012.

  1. boroman

    boroman

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    Guys, 've been searching for an hour here for info about the layout of svt preamp and what tube does what (I remember seeing it here somewehre)

    I mean V1-V5.

    [​IMG]

    V1, V2 = channel 1 ???
    V3, V4 = channel 2 ???
    V5 - summed output of the preamp for both channels?

    Is that right?
  2. coreyfyfe

    coreyfyfe Supporting Member

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  3. beans-on-toast

    beans-on-toast

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    Perhaps this is the thread you were looking for. It was for an SVT-HD amp http://www.talkbass.com/forum/thread639101.html

    On the early model SVT Pre-amp:
    V1 12DW7 channel 1 first gain stage
    V2 12AX7 channel 2 first gain stage into tone stage
    V3 12DW7 channel 1 bass and treble tone stage
    V4 12DW7 channel 1 mids tone stage
    V5 6C4 channel 1 and 2 are mixed together and fed into this tube which matches the impedance into the power amp.

    On the early model SVT Power-amp:
    V1 12DW7 (or 12AX7) phase inverter
    V2 12BH7 driver tube for V7-V9
    V3 12BH7 driver tube for V4-V6
    V4-V6 6550A power tube trio
    V7-V9 6550A power tube trio
  4. bigbottomend

    bigbottomend

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    The DW7's are ch1 , the AX7 ch2, both out via the 6C4.
  5. boroman

    boroman

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    So, if I have 12ax7s instead of dw7s everywhere and I have new 3x 12dw7s, which one will be priority to install?

    phase inverter, v1.... and v3 or v4? (i don't have 4 pcs).
  6. bigbottomend

    bigbottomend

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    v1,3 and 4. Put the AX7 in the phase inverter on the power amp.
  7. coreyfyfe

    coreyfyfe Supporting Member

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    It may have been converted to use 12AX7s at one point, so check that out before you go swapping in the 12DW7s. I know mine was and it was quick to change back. In the 80's there was a 12DW7 shortage when they stopped being produced so MTI put out a technical bulletin with three resistors to change in the preamp and I think one or two in the driver section? I'll try to find the info again but it's definitely been posted on here in a few threads.
  8. beans-on-toast

    beans-on-toast

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    I think this is the mod that you were referring to Corey.

    It applies to the older SVT pre-amp only.


    [​IMG]


    Here is an SVT pre-amp schematic for reference.


    [​IMG]

  9. boroman

    boroman

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    Thanks for the replies. Mine was not converted to 12Ax7. It's very early MTI. I bought it and whole set of tubes BEFORE I got the amp. thought the MTI were "all 12ax7".. But this is the very early model with 12dw7s all over the place (on schematic inside too, and also, the old tubes there were Magnavox 12dw7)

    Why AX7 on phase inverter? And it's better to leave there Penta Labs 12ax7s (short plate, high gain) or JJ ecc803 long plate?
  10. bigbottomend

    bigbottomend

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    Because the diagrams, schematics and other info provided by many people in other threads have already explored this. If you have an early SVT it will run DW7's a later MTI will be AX7.

    I once read that long plate tubes have a smoother tone, that is up to you to try and decide for yourself what sounds better for you.
  11. boroman

    boroman

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    Yes I know, maybe my question was misunderstood. I have 3 12DW7s and one 12Ax7 and problem placing it on the design which has four 12DW7s so it was suggested to set 3xDW7 on the preamp and one AX7 on the PI (so it seems that replacind DW7 withj Ax7 on PI is better than e.g. replacing DW7 with AX7 on the preamp - v1, v3 or v4)

    I think I must try them both, but needed just an advice, maybe someone did this before me and will save me some time ;)
  12. coreyfyfe

    coreyfyfe Supporting Member

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    Sorry I misunderstood your statement, obviously. I wouldn't put a 12AX7 in a slot meant for a 12DW7 unless you know which half of the 12DW7 is being used in the circuit. A DW is half an AX and half an AU so it can replace either in the right circuit. In the PI slot of the SVT both halves are used for different functions and I'm not sure if you can easily swap them without modification. David or someone with more technical knowledge may be able to shed more light on this.
  13. bigbottomend

    bigbottomend

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    Earlier in this thread beans outlines that an early SVT can use either a DW7 or AX7 in slot v1 for the power amp section. Later ones, as discussed are all AX7.

    So either will do.
  14. husky123

    husky123 Supporting Member

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    Disclosures:
    Endorsing Artist: Spector Basses
    Sorry for dredging up this zombie but I just wanted to confirm that the v4 position on my new SVT VR is for the channel 1 midrange? I want just a tad more grit there so I'm thinking of trying a high gain JJ in that spot. Thanks fellas.
  15. beans-on-toast

    beans-on-toast

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    The chart above was for a vintage SVT, the VR is different.

    On the VR, the midrange tube in the pre-amp is V3, a 12AX7. The other two 12AX7's are V1 and V5; the 12AU7's are V2 and V4. There is a silk screen lettering on the board indicating V1 to V5.
  16. husky123

    husky123 Supporting Member

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    Thanks a ton beans.
  17. beans-on-toast

    beans-on-toast

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    Let us know how it works out.

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