Ampeg SVT Classic, new tubes and screen resistors, perfect green bias, no sound

Discussion in 'Amps and Cabs [BG]' started by ZeeGees, Dec 10, 2017.

  1. ZeeGees


    May 10, 2016
    Hi everybody, I’m dying for a solution to this before I throw this amp out in the street.
    I recently had new tubes installed, along with all new upgraded screen resistors.
    Now when I let it warm up and take it off standby the power light turns green, the bias lights are both green, but I get absolutely no sound. Not even noise when I turn the gain and master up.
    I’ve tried different speaker cables, a different bass, I even put my old set of tubes back in and the same sound.
    If anyone can help I’d appreciate it.
  2. Stumbo

    Stumbo Guest

    Feb 11, 2008
    Who installed the tubes and resistors?

    I suggest a new tech take a look at it.
    Aqualung60 and bobyoung53 like this.
  3. If i took it to someone and it came back dead,
    I believe I'd go back and ask something
    like 'Whiskey Tango Foxtrot'.
    SirMjac28, BirrML, Aqualung60 and 7 others like this.
  4. ZeeGees


    May 10, 2016
    He’s a really reputable guy who knows what he’s doing.
    I’ve had this ampsince 2001 and it was perfect up until last year and now it’s just one thing after the other.
    How would the power section be functioning properly and not even pass noise?
    alesreaper9 likes this.
  5. Bent77

    Bent77 Supporting Member

    Mar 6, 2013
    Desert, Colorado
    No offense, but the amp should have been tested before it left the shop
  6. pcake

    pcake Supporting Member

    Sep 20, 2011
    Los Angeleez
    reputable or not, if it's not making noise after he fixed it, and everything is hooked up, he made a mistake and needs to fix it.

    absolutely! and that's true of anything you take to any shop.
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Why did he change the screen resistors?

    Usually, if it's not bad you are better leaving well enough alone...
  8. beans-on-toast


    Aug 7, 2008
    Since the power tube board was worked on, the connecting cable to the output of that board may have not been reconnected. This could be why the amp bias can be set, yet it is dead quiet on the speaker out.

    I’m troubled by your comment that the screen resistors were “upgraded”. These resistors are intended to act like fuses and blow open when there is a problem. Putting the wrong resistor in there could result in a costly repair down the road. The spec is 220 ohm, 1/2W, 5%, flameproof.

    To debug:

    Reset the impedance selector switch, back and forth to where it should be set.

    Plug you bass directly into the power amp input jack. You should hear something if the power amp is working.

    Connect an instrument cable from the pre out to the power amp in. Test the amp and see what happens.

    Connect an instrument cable from the preamp out jack to the power amp in jack of another amp. If you don’t have a power amp in, connect the preamp out into the instrument input of another amp, just for a quick test. Watch the volume levels. This will tell you whether or not the preamp is working.
    Last edited: Dec 10, 2017
    SirMjac28, BirrML, Lvjoebass and 2 others like this.
  9. BassmanPaul

    BassmanPaul Inactive

    I’ll go along with post #6. If the amp is dead when it should be working, take it back to the tech and demand that he fix it.

    You did take the amp out of Standby yes? :D
  10. ZeeGees


    May 10, 2016
    Thanks for the trouble shooting tips...I’ll try all those out later tonight.
    I did plug the bass directly into the power amp in and got nothing. But I haven’t tried running out to another amp yet. It’s also very possible about that cable between boards not being connected. I wondered that myself.
    beans-on-toast likes this.
  11. Jim C

    Jim C I believe in the trilogy; Fender, Stingray, + G&L Supporting Member

    Nov 29, 2008
    Bethesda, MD
    I'm in the "why even touch it if it supposedly came back from the shop fixed"?
    Once you are inside, the tech may claim no warranty.
    And by the way; the tech is not good if a non-working, untested amp leaves the shop as a repaired amp.
  12. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    preamp unplugged or something left unplugged. its kinda a pain in the ### to take that chassis apart
    ZeeGees and bobyoung53 like this.
  13. This is trivial, but sometimes you just forget to do the simplest things, is the cab plugged in, and did you flip the standby switch?
    Wesley R and alesreaper9 like this.
  14. Wesley R

    Wesley R Gold Supporting Member

    A while back (1995 or so) I was convinced my B15S needed new caps, probably some tubes and other work. It had developed a nasty hum and slight crackle. After a day or two of despondency i was going to try one more time and think the issue out so i could describe it to the shop. No audible problems, of course I was using a different instrument cable. Please not that I am not suggesting the issue at hand is that simple, just thinking it could be something relatively simple.

    There is a repair shop in the area that does not test their repairs through speakers, they "scope them out". That testing procedure caused me to return an amp for repair and not utilize their services after that repair was complete.
  15. micguy


    May 17, 2011
    Reputable, to me, means he does good work, and cares about his reputation. Part of keeping a reputation is admitting when you missed something, and apologizing for your error. Give him a chance to fix it. If he is reputable, and knows what he's doing, he'll fix it, and apologize for missing whatever it is he missed.

    If he won't, then I completely diagree with your assessment of his reputation and skills.
    BassmanPaul and agedhorse like this.
  16. ZeeGees


    May 10, 2016
    I reset the impedance switch. I patched an instrument cable between the preamp out and power amp in and got no signal.
    I plugged the bass into the power amp in and got a very faint signal.
    Then I plugged the bass into the normal input on the front and ran the preamp out to another amp and had a normal signal coming out of the other amp.
    beans-on-toast likes this.
  17. SirMjac28

    SirMjac28 Patiently Waiting For The Next British Invasion

    Aug 25, 2010
    The Great Midwest
    Did it work at all when it left the shop?
  18. ZeeGees


    May 10, 2016
    I got it fixed on the road, the tech is across the country.
    He is texting with me.
    I just need this working ASAP.
    It seems like it’s something fairly simple since all the signal flow is there, just no output to the cabs. (Although there’s nothing simple about taking these damn things apart.)
    It’s out of standby and I’ve tried other speaker cables.
    SirMjac28 likes this.
  19. beans-on-toast


    Aug 7, 2008
    Your tests confirmed that the issue is in the power amp.

    The faint signal could mean that a driver tube is not working. On the bottom chassis there are three small signal tubes, a 12AX7 and two 12AU7's. Reseat these to ensure that they are properly inserted into the sockets. If that does't help, if you have spares, try swapping them in. If a driver tube is not working, you might hear a faint signal as you described.

    If that doesn't help, the next step is to open the amp and check the cables on the power board and their destinations.
    ZeeGees likes this.
  20. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    If you get signal form the pre and only weak signal from the power in jack try a new 12AX7 in the PI position. The jacks are just ahead of this tube. If the bias lights are functioning properly the issue probably isn't with the 12AU7s or 6550s as an issue with those tubes would cause problems with the bias LEDs.
    ZeeGees likes this.