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Ampeg SVT3 , advice needed

Discussion in 'Amps and Cabs [BG]' started by Stitchz, Apr 13, 2006.

  1. Hi all!

    I've just bought an used Ampeg SVT-3 and Ampeg 115 classic cab.

    I'm really enjoying the sound, it's a big leap from my previous rehersal rig. But after a rehersal where I was really pleased with the bass sound (and the amp was all flat!), I noticed how the amp always had some distortion...

    Now I know a tube amp should give a nice smooth distorted sound that lots of people enjoy, I find it strange that no matter how I change the amp settings I cannot get a pure, clean tone. I tried 3 basses, a warwick 5-str corvette $$, a tune 4-str bass maniac and a cort 4-str acoustic. All of them distorted the amp.

    So, I'm asking you fellow TB'ers for help. Please :(

    Since I hear the distorted sound even from the amp DI, I guess all the distortion is coming from the (tube) preamp stage. The previous owner never changed the valves and, according to the pcb, the amp is from 93.

    But, the previous owner said he tested the valves (he has some sort of valve testing machine) and they are all ok. But I have talked to him and he aggrees that there may be a problem with the valves being old, and barely used.

    Well, since the previous owner had to remove the tubes to test them, it's possible he mixed them up... So, heres a pic of the amp.


    There are 4 12AX7 valves, and one 12AU7 which is in position 2. Is it correct? If not,where should it be?

    Or maybe this isn't the problem at all. :(

    Next thing to do is to replace the tubes, but I don't have the replacements yet.
  2. On a similar note, can 12AX7 an 12AU7 valve position be switched without harming the amp itself?
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The 12AU7 in "position 2" is correct.

    As far as the distortion, set the DI to "pre", and see if you still have distortion.

    That position takes the signal directly from the input jack, passing through only one amplifier part. See if there is still distortion. If so, then try using the "PAD" switch next to the input jacks.

    It is possible that a weak first stage tube could cause some distortion. A tube from 1993 is only 13 years old, and for a preamp tube that is not necessarily bad.

    You can always try a new tube...

    The 12AU7 should BE a 12AU7. It needs the higher current and drive capability to properly drive the output mosfet devices. A 12AX7 might work, but is likely to increase distortion and lower output power capability.
  4. Thanks for your help Jerrold! :)

    I've followed your instructions carefully. Switching from pre to post changes the tone and volume but the sound is still not clean.

    Using the pad switch and pre/post DI still had the same results :( other than volume/tone changes, the sound isn't clean...

    I'll try to describe the distortion a little better. It's kind of a trebly gritty sound that almost seems like it's frequency is independent of what I'm actually playing on the bass, but only appears when the bass is actually producing output. Kind of like a digital clipping sound, but nothing is clipping as far as I can tell, since the amp DI is connected to an headphone amplifier and I can see the line level while I'm playing.

    Since if I use the pre option for the DI only one stage of amplification is used, what valves are being used? Maybe I could switch them aound and find the faulty one. Let me know if I'm just being silly :)

    Thanks again for your precious time.
  5. 8mmOD


    Mar 20, 2005
    I endorse & use Tech 21 pedals, Eminence loaded cabs, EMG pickups, Jim Dunlop picks & Ernie Ball Strings, BC Rich Basses.
    I dont know whats wrong with your amp, but my 3 pro was one of the cleanest sounding amps I have owned, esp with the tube gain all the way up. it sounds like something is wrong with yours

    Sorry I dont have an answer for you, but i do have a...

    BUMP :)
  6. bassman blue

    bassman blue

    Aug 22, 2002
    have you tried the head with a different cab just to be sure the SVT15E isn't part or all of the culprit?
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    When you use the "pre" output for the DI, NONE of the tubes is being used.

    The signal is going from the jack to the DI out, passing though one active device only (an op-amp used to prevent "loading" the bass").

    I forgot to suggest also using the "mute" button.

    If the distortion is not gone when you are using the DI out "pre" and have the "mute" selected, there really isn't much inside the amp left to cause a problem.......

    Then, if the noise isn't related to the note, and the amp is generally working, I don't know what it is.

    Just about any noise/distortion I can think of either would be present all the time, or would be directly related to the note somehow.
  8. Thanks everyone for your help.

    I've solved the problem.

    No tubes to blame, just the local electrical power supply company. The headphone amplifier wasn't working properly, and now a keyboard just died. The lights at the studio always seemed to be inconstant... I guess another amp drawing power was enough :\

    Still, I'm extremely happy with the amp and cab. Unfortunately now I'll have to figure out a solution to this power problems instead of actually playing bass :rolleyes:

    Thanks again for your help :)
  9. svtb15

    svtb15 Inactive

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig. Q+
    I had a very similar thing happen to me.... it was making me nuts.. some young punk at GC said that is the way an ampeg supposed to sound.. Oh really! all of a sudden i said..
    No it isn’t… It was clean sounding up to 2 weeks ago….The distortion sound just started recently … I had the amp for 2 years at that time…
    I will not recommend you do this yourself.. maybe Jerold can guide you.. But i found it was able to be fixed by adjusting the small blue micro pot just to the right and lower area of your picture..
    I do not know what the spec is... I did it by ear..which is not the correct or safe way .. but it worked for me until I got it on a scope..
    It only took about 1/32 of the rotation of the pot to cure it..
    if you turn too far the amp sounds horrible… very cold and brittle... if you go the other way the amp gets real hot and the fan goes into high speed... not good and it may melt down….
    I turned it to where the amp sounded crappy then backed it to where it stopped that square wave sound and smoothed out... I have experimented with that setting and the amp has been sounding GREAT ever since.. That was 4 years ago and i use it all the time.. about 3 gigs a week when i am not using my 72 SVT the other days and when im feeling strong enough to carry the real SVT..
    I am not a tech but I believe that trim pot may adjust the bias to the mosfets.. if you do not have the right tools or know how I would think that you should let someone that does fix it and bring it to spec.. But you asked and I am telling you my findings..
    I hope i was able to steer you in the right direction… because I was told it was the tubes.. so I retubed the 12ax7s and 12au7s 3 times and there was no change… and then I realized it was not the tubes…. I actually realized after I retubed the first time but the guy at GC said he guaranteed that is what it was…… wrong answer….
    the distortion that i got was in the back ground even at low levels... even with a passive J bass plugged in and the red input light not even close to blinking... the input overload light was OFF
    I am only saying this so that people don’t ask "IS THE CLIP LIGHT ON?" or is the Tube voltage on the front panel turned up.. NO…
    So that little potentiometer did the trick…
  10. Thanks svtb15! I noticed the same blue pot and wondered what it did :)

    I'd love to hear Jerrold's take on this. If it is some sort of mosfet bias adjustment, it may just be the ticket for people who think their svt3 pro sounds either to hot or cold, or have an always distorted sound.

    Talkbass is just an awesome forum. :smug:
  11. sedan_dad


    Feb 5, 2006
    I too would like to know the purpose of that blue pot.
    I can bet that "Jerrold's take on this"
    will be to don't touch it.
    Face it ,Ampeg can not condone unqualified persons tampering with potential lethal voltages.
    And you could cause serious damage to the amp.
    Best to take it into a qualified service center.
  12. svtb15

    svtb15 Inactive

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig. Q+
    I agree with sedan_dad
    I know how to and almost always service my 1972 SVT by myself.. That can be very dangerous and lethal if you dont know what you are dealing with.
    If you do not know what you are doing , leave the tweaking to a pro... The SVT3 pro does not have the voltages that the tube version SVT has but it can still kill you.. we dont want that to happen....
    I remember opening up a 1969 SVT, 25 years ago,,,,that was before I knew what i was doing.. just wanted to see what made it tick inside...
    the next thing i knew i was telling myself to breathe,, breathe... everything went black... it was not good... and i was lucky to have had a strong ticker...
  13. Don't touch that pot! It IS a bias control, and some years ago, I also wondered what it did and gave it a try... "Turn, aha, turn, sounds good, turn, hmmm, lots of punch, one more turn..."
    Suddenly, the fan went completely overdrive and BANG! :bawl: Half my power amp to heaven...

    edit: it's just as svtb15 said, overlooked his post :smug:
  14. Skel


    Jun 19, 2005
    Boulder, Colorado
    I didn't know MOSFETs had a bias voltage, or a bias trim pot -thanks for sharing this info.
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Yes, it sets the mosfet bias. It is kinda sensitive, and aside from it not being a "user service" item, you require other equipment to properly set it.

    Unless you don't need to ask any questions, I wouldn't mess with the trimpot..... if you set it wrong, you either get distortion, OR you get a very hot or damaged amp (shorted mosfets).

    As predicted, my advice is not to get in there and set it.

    OTOH, if you know what "app 20-25 ma per device" means, be my guest, as that indicates you probably are a qualified technician.
  16. soapwater


    Aug 9, 2006
    Hi, I'm new here and I also have problems with my SVT3-pro. I get a distorted sounds that seems to be 'under' my bass-sound when I'm playing at low volumes. I can't get it as clean as my cheap Park-combo. I figured out that it must be the power-amp because when I use only the preamp with an other poweramp it is clean. I checked the tubes and there are 5 12AX7's in it. Do I have to replace tube n°2 with a 12AU7 or doesn't have it any effect on the distortion? I measured the voltage over the source-resistors of the mosfets and that varies from 0.8mV to 1.1mV over the 0.47Ohm resistors so that's about 0.47mA. Is this normal? Can I just turn the trimpot untill I get 20-25mA or do I first have to try an other tube? Does someone have a schematic of the amp? That would really help me figuring it out...
  17. soapwater


    Aug 9, 2006
    Nobody?? :meh:
  18. svtb15

    svtb15 Inactive

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig. Q+
    it is most likely the bias.. i had the same problem... i did it myself. jerold tiers had the best explaination how to fix... i dont know enough to explain how to do it correctly....
  19. xshawnxearthx


    Aug 23, 2004
    new jersey
    mine was clean too when i played at a low volume. if i tried to get loud enough sometimes w/o pa help, it can def get distorted. thats why i sold it.
  20. soapwater


    Aug 9, 2006
    Is it certain that it has to be 20-25 ma per device and do I imply measure over the 0.47Ohm (source)resistors (So that has to be 9.4 - 11.75mV?) Can someone please tell me if it's correct because i don't want to ruin my amp... Does someone knows how I can get in touch with Jerold Tiers?


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