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ampeg svt mti era head preamp tube question...

Discussion in 'Amps and Cabs [BG]' started by DrowningBrian, Feb 25, 2013.


  1. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    i have an 80's mti ampeg svt. the preamp seems too hot. i play a passive p bass. is it because they subbed the 12dw7s for the 12ax7s? is the mod expensive to change 12ax7s to 12dw7s? i've heard its only a few resisters. ive played 70's svt's and they just sound better, must be the 12dw7s. OR maybe its time for a cap job.lol.
     
  2. You mean 'HOT" as in temperature or too much gain?
     
  3. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    too much gain. ive played 70's svt's and they have a better gradual volume.
     
  4. gastric

    gastric Professional product tester for hire

    Jun 8, 2009
    Raleigh, NC
    Source Audio BETA tester
    Tubes are easy to reach on these why not pop the back off and replace V1 with something else? Also pop the front off and check the power bias it's pretty easy. PM for help if you need any.
     
  5. Change V1 to a 12AY7 or 12AU7 will drop the gain down.
    12DW7 is one half of a 12AX7 and one half 12AU7 basically. High Mu triode gain 100 and medium Mu triode gain 40.
     
  6. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    thanks man! i'll just spend $15 and buy a 12au7. that should do the trick. V1 should be the tube closest to the input, correct?
    i tried a jj 5751 but that didnt help much.
     
  7. gastric

    gastric Professional product tester for hire

    Jun 8, 2009
    Raleigh, NC
    Source Audio BETA tester
    If you swapped v1 already I'd check bias. My MTI doesn't sound any more driven than the CL I just sold. After that try swapping a known good for each of the other tubes one at a time. Process of elimination, take your time.
     
  8. Alex1984

    Alex1984

    Jan 16, 2010
    Vancouver
    Most SVT's I've played have most of the volume control just before 9 to a little before 11 o'clock, not sure if I'd consider that gradual. Have you considered getting a new pot with a different taper?
     
  9. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    I believe MTI amps started out with 12DW7s then switched to 12AX7s. They issued a service bulletin with the schematic changes and it's only a few resistors on each board - the power amp and pre-amp, could be worth making sure you have the right tubes for the right resistors. Also - check out the PI tube and make sure you've got the right one in there.

    [​IMG]
     
  10. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    i have 4 jj 5751 in the preamp and one in the PI postion trying to cut the gain down. thanks a bunch for the info on the mod. i'll take that to my tech and see how much he will charge. it couldnt be much.
     
  11. Check your tube chart, your SVT isn't calling for a 12AU7 in the PI position?
     
  12. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    6c4, 12ax7, 12ax7a, 12ax7, 12ax7 preamp

    12bh7, 12bh7 driver
    12ax7 PI
     
  13. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    should i try the 12au7 in the V1 position or the PI postion? which would be more ideal?
     
  14. If you use it in the PI position you WILL drop overall voltage gain but since the 12AU7 has higher current capabilities you gain headroom and more than likely better low end. The 12AX7 is not ideal to feed the driver tubes due to it's low plate current.

    I would try the 12AU7 in the PI before I did anything else. The AU7 is a smoother sounding tube than the AX7 so unless you like the sharper more gritty sound I think you will find it worth while.
     
  15. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    you are the man! that actually makes since.lol. but if i swap out the PI tube will i have to adjust the hum balance control? i think its behind the grill. or just put in the 12au7 and rock?
     
  16. When you change any tube you may need to adjust the hum balance. The (output) balance control may need to be adjusted but not likely unless your old PI was wonky.
     
  17. DrowningBrian

    DrowningBrian

    May 21, 2009
    Atlanta, GA
    dang, i guess i'll just take it to my tech. im pretty good at messing things up.lol.
    thanks you've been really helpful b-string.
     
  18. You can try it without touching anything, just plug and play. It will work to it's peak performance when properly adjusted though.
    No trouble to pass on knowledge :) I would like to say I hung up the test leads 9 years ago, but I have two amps here now :meh:
     
  19. gastric

    gastric Professional product tester for hire

    Jun 8, 2009
    Raleigh, NC
    Source Audio BETA tester
    I vaguely recall trying a different PI tube type and it either decreased or increased output then read it may be detrimental to the power section due to reduced voltages sent downstream or something like that. Comment?
     
  20. Subing a 12AX7 (higher gain) for a 12AU7 will cause higher gain and early breakup with loss of total power (lower current from the 12AX7). As far as electronically/physically detrimental not that I have ever seen or could prove.
     

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