vintage ampeg svt midrange problem?

Discussion in 'Amps [BG]' started by DrowningBrian, Dec 19, 2013.


  1. DrowningBrian

    DrowningBrian

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    i love mine but i just got spoiled by an orange ad200b. it seems like the mids on the old svts are insane and boxy sounding, maybe its just mine. i just did a cap job and replaced the power tubes with JJ 6550s. its an MTI era svt. maybe there is something up with the amp, i dont know. no weird sounds. i play either a jazz bass or p bass fueling an ampeg 8x10. i played a blueline svt and it sounded perfect. idk. im just gonna shut up and play it but im just curious. killer rig!

    thanks,
    brian
     
  2. B-string

    B-string Gold Supporting Member

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    Sounds like your amp has a fault. Even though I don't care for the MTI era, sound was not an issue with a properly working one.
     
  3. DrowningBrian

    DrowningBrian

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    ive been thinking that too. there should be no reason why my gk800rb sounds better. my tech says these are built just as well as any other vintage svt. just not a pretty.lol. its almost like the mids are funky, it looks like the rocker switch for the mid was replaced at one point, but it works.
     
  4. DrowningBrian

    DrowningBrian

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    maybe the preamp tube that operates the mids is messed up? all of the tubes light up. i just want this thing in good working order.
     
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  6. B-string

    B-string Gold Supporting Member

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    Tubes lighting up only means the wire (filament) that heats up the tube works. With the symptoms you stated I don't think a simple tube swap will do.
     
  7. DrowningBrian

    DrowningBrian

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    thanks for your help brother. i guess i'll be taking her to the tech next week to get a checkup. i hope its nothing expensive.:(
     
  8. B-string

    B-string Gold Supporting Member

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    Could be parts even cheaper than a tube, or no parts at all just failed solder joints.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

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    The preamp tubes can make a difference so check that first. Scrubbing the tube pins and sockets with deoxit to remove oxidation can help.

    As was mentioned, check the solder joints. I've seen joints were the solder looked good but the wires were loose, they could be pulled out easily. So they need to be checked carefully.

    It is possible that there are components in the preamp that have drifted out of spec. It would be good if your tech checked for that. Especially look at the ones in the tone stage.
     
  10. anderbass

    anderbass

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    DrowningBrian, does channel 2 have the same problem, or just ch. 1?
     
  11. Steve Dallman

    Steve Dallman Supporting Member

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    The MTI SVT started using 12DW7 preamp tubes as did the earlier SVT's. Later they switched to 12AX7's along with the requisite circuit changes. A 12DW7 has one 12AX7 gain stage and one 12AU7 stage. The switch was made due to 12DW7's becoming scarce. (There are current production 12DW7's available today.)

    Is is possible your SVT has the wrong preamp tube in the midrange circuit? The tone and effect of the midrange circuit would be affected by the use of a 12DW7 in a 12AX7 circuit and vice versa.

    Many SVT's designed to use 12DW7's were modified to use 12AX7's and these conversions were fairly common due to scarcity of 12DW7's a few decades ago. Now, years down the line, have any modified amps not been identified as such? I would suspect quite a few.
     
  12. Steve Dallman

    Steve Dallman Supporting Member

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    Since a 12DW7 is a 12AX7 stage and a 12AU7 stage, subbing a 12AX7 would create some problems. The 12AX7 stage is far higher gain than a 12AU7 stage, and is also easier to distort (higher sensitivity.) By contrast, a 12AU7 stage is much lower gain, higher current, and is much harder to distort.

    If a 12DW7 SVT had 12AX7's put into it, there would be tonal changes, and distortion problems, unless the 12AU7 stages were modified to match 12AX7 gain characteristics. If a 12DW7 was used in a 12AX7 SVT, I would surmise the result would not be distortion, but rather lower gain and perhaps lackluster tone in the preamp.

    There are also impedance differences, besides the gain differences.
     
  13. beans-on-toast

    beans-on-toast Supporting Member

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    That's a good point. Make sure that the correct tube is in the proper socket. There were circuit changes that needed to be made to run a 12AX7 or a 12DW7. If the chassis is silk screened and says one type, the amp may have been modded so the tech would need to check the component values that are in the amp.


    Here are the changes from MTI.



    [​IMG]


     
  14. DrowningBrian

    DrowningBrian

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    no, channel 2 normal sounds better for some reason.lol. the clarity of the mids is weird on channel 1 normal. its almost like the mids are over powering the bass and treble.
     
  15. DrowningBrian

    DrowningBrian

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  16. DrowningBrian

    DrowningBrian

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    thanks steve! good info. it originally had the 12ax7 in the preamp but the tech put it back to 70's specs with the 12dw7s and a few resistor changes.
     
  17. Steve Dallman

    Steve Dallman Supporting Member

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    The second channel does not have the midrange circuit, just the first channel. The baxandall bass and treble is a separate circuit from the midrange circuit and is identical in both channels.
     
  18. JimmyM

    JimmyM Supporting Member

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    So they say, but it's not in my 69. The second channel in mine is more like a guitar channel with reduced lows. I can get the same sounds out of each, but they don't behave the same way.
     
  19. B-string

    B-string Gold Supporting Member

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    But his is the MTI era release.
     
  20. JimmyM

    JimmyM Supporting Member

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    Hmmm...never tried the second channel in an MTI. If I ever get one supplied again, I certainly will.
     
  21. timv

    timv

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    I've worked on two different MTI era SVT's that had a wiring problem on the midrange of channel one. The midrange frequency switch wiring.

    Have it checked out closely by a tech.

    Both of these amps seemed to have been built wrong from the factory because the solder joints looked to have been original.

    The difference was huge when this was fixed!
     

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