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Ampeg SVT-III Pro Tube Replacement

Discussion in 'Amps and Cabs [BG]' started by Nate Plays Bass, Jan 13, 2012.

  1. EDIT: Biasing fixes the distortion problem. Read entire thread.

    I just recently acquired an Ampeg SVT-III Pro (oldest model). It is very dirty and gritty with lots of distortion, even at low volumes. I'm pretty sure it needs new tubes but correct me if I'm wrong. It currently has 3 Sovtek 12ax7-wa's in preamp and 1 Sovtek 12ax7-wa and 1 unknown 12au7 in driver tubes. I want a warn clean sound, but no distortion. I don't want any
    Chinese tubes. I've been looking into 12au7's as preamp tubes.
    Any suggestions or info would be greatly appreciated. Thanks, Nate.
  2. nukes_da_bass

    nukes_da_bass Banned

    Feb 19, 2006
    west suburban boston
    I owned this head and it always had lots of distortion even with new tubes even at low volumes which is why I got rid of it!
  3. Have you ever had the MOSFET biased?
  4. grendle


    Mar 4, 2011
    Central FL
    I like the EH's in my svt 2. I replaced the tubes in a 3 pro with jj's before sounded ok but I think the EH's are better overall. Stuck a 5751 in the v1 slot as a tester and it cleaned up the input a lot also. Sylvania or ge would do well.
  5. spacerust


    Feb 9, 2010
    South Texas
    I opened my up to see and it has electro harmonix in the au section with a groove tube in the ax. Then it has two jj's and another groove tube in the 3 sect ax part. Just got and havent reslly tried it out only when I bought it. Should I have it biased or something? Or change tubes to be the same?
  6. NKBassman

    NKBassman Lvl 10 Nerd Supporting Member

    Jun 16, 2009
    Winnipeg, MB, Canada
    I think I currently have a full compliment of JJs in mine. Sounds as good as it ever has.

    Not sure where your distortion would be coming from though. Have a tech check out the poweramp section, and replace the tubes (preamp tubes are not thaaaat expensive), and hopefully that'll bring things up to snuff. These are really great amps, so welcome to the club!
  7. Well, I read in a thread about SVT 3 Pros and it said that getting the MOSFET power amp biased fixes the problem. I may take the amp in tomorrow.

    Yeah, hopefully it can be fixed for fairly cheap. Where are JJ Tubes manufactured?
  8. f.c.geil


    May 12, 2011
    I just went through this with both an SVT-3 Pro and an SVT-III Pro. Let me save you some time and trouble:

    Do NOT change tube types.

    The tubes are what you need, as far as type. Get four good, matched JJ/Tesla 12AX7s and a 12AU7. Put them in, BEFORE you get the MOSFETs biased. Tubes make a huge difference when biasing, so biasing for one set can completely ruin the sound with different tubes. With these two changes, you'll be amazed at the sound. My SVT-III is being sold because it is too clean, I can't get enough overdrive from it, and that's with mediocre Groove Tubes in it, but the MOSFETs are properly biased for those tubes.
  9. I just got the amp biased! Sounds so much better! Unfortunately, 2 MOSFETS are bad so it can only be ran at 75-80% power but I don't have the $200 to get it fixed. But it'll definitely happen down the road!! It sounds amazing! He said that the tubes driving the MOSFET didn't make a huge difference in the biasing so I didn't replace them just yet. Thanks for the advice!!
  10. Metal Mitch

    Metal Mitch

    Jul 14, 2003
    +1 except mine isn't for sale :p

    Sometimes I think it's cleaner than my GK. I did the biasing myself, using instructions and pics I found right here in the TB Amps forum.

    Original tubes in mine were Ampeg "Hot Rods". I ditched those and put in Reflektor/Tung-Sol. The 12AU7 was an RFT, so I kept it. :eek: Serious Ampeg goo!

    With the Reflektors it's super clean and hifi. So much I'm thinking about changing them. Those Reflektors are great in my SVP-PRO, but that's a dirtier/grindier amp.

    3PRO is a great amp for rolling tubes. These Ampeg tube pre's really take on the character of whatever tubes you try out.

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