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SVT-3 Pro - We've got issues!

Discussion in 'Amps and Cabs [BG]' started by Winchuckster, Nov 22, 2018.

  1. Winchuckster


    Nov 22, 2018
    Hello to all and thanx for any insight, instruction, and assistance in advance!

    I'll try and keep this short but add what info is prevalent...

    199something svt-3pro, used it for rehearsal and gigs maybe 4 times a week for a good 3 years. then it went on a shelf, for at least 10 years. Hooked it up a year or so ago no sound, nada, didn't mess with it.

    This week got a new tungsol tubes, quad 12ax7, 12au7w.
    Biased the mosfets (matching groups(red dot rt side facing, green dots, left facing))
    left side average 22.275mv right side average 21.9mv.

    Now back to my problem, I get almost no volume from the speaker cab 2x15(4ohm)unless Master Volume,Gain and Tube Gain knobs are MAXED out! Zero volume with Gain and Master maxed, only while adding tube gain does the volume come up. We're talking minimal volume, like barely a 2 on a scale of zero to what this amp can put out.

    The Peak light works as it comes on when i hit the mute button but with all knobs maxed it never flickers.

    12ax and 12au seem to be glowing pretty good, front 12ax7's seem dimmer but lit.

    Cabinet, cables, etc are all good as proven by connecting my v4bh up and things are copacetic.

    I cant tell if any of the big caps are bad cuz they have a plastic cover of sorts on top. all smaller caps appear ok as i see no evidence bulging.

    I'm somewhat competent in electronics (biased without light show) but not in diagnostics. i can follow directions if someone can walk me thru it.

    So ....*** !!
  2. All I can think of...
    The 3pro is notoriously difficult to bias when noo toobs go in.... that's 1

    2. Have you cleaned the fx loop?
  3. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Perhaps your problem was unrelated to the tubes?

    If jumping from tbe effects send back into the return doesn't work, it's time for a proper diagnosis. So far, guessing hasn't worked that well and cost you a set of tubes.

    Hint, it's almost certainly not a bad cap.
  4. BogeyBass


    Sep 14, 2010
    As others mentioned possible effect loop corrosion from sitting so long.

    Or corrosion in general somewhere.

    Bad caps don't always swell

    Or any numerous solder connections could need touch up. Specially jacks or tube sockets
    Winchuckster likes this.
  5. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Solder connections don't fail due to age or inactivity.
    BadExample likes this.
  6. Can
    Can age AND general usage bumps etc, cause solder joints to fail?
    AlexanderB likes this.
  7. BadExample


    Jan 21, 2016
    Usually only if the soldering is sub-standard to begin with. Seems unlikely on a quality product.
  8. Winchuckster


    Nov 22, 2018
    Thanx for the responses. I will check the effects loop in the am. Too much Turkey day food! cant see straight!!

    What's the best procedure for cleaning the effects loop?

    Most of the time I've had this amp I've had an Alesis 3630 compressor,and Korg DTR-2 tuner connected to it. Including the 10 years it sat dormant. I'm really hoping the effects loop is the issue.

    How can i bypass the effects loop?

    Also a couple of things I noticed, could be normal,im not sure ...
    1. tapping the front most ax7 with a pencil produces very loud sound from the speakers that sounds like a tube being hit with a pencil.
    2. with the volumes up all the way i get a good pop from the cabinet when i power it off.
    Last edited: Nov 22, 2018
  9. BadExample


    Jan 21, 2016
    Just plug an MI cable from your FX send to FX return.
  10. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Very, very unlikely with a quality product.


    First, just connect a shielded cable between the effects send and return jacks. If this solves the problem, we can talk about the proper way to clean a jack's internal normaling switch.

    With the levels all the way up, it wouldn't surprise me if the tube makes microphonic noise when tapped. It's also no surprise that it makes noise when powered down.
    okcrum and rodl2005 like this.
  11. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    New tubes have no effect on the bias for the MOSFET output stage in this amp, barring a component failure. The tubes on the power amp board are AC coupled to the output stage, and OP's already got the bias set.

    This sounds more like a clean all controls, jacks, and tube sockets, and reseat/clean all internal connectors type of thing. It's what you'd do first anyway if the amp has sat for ten years.
  12. Winchuckster


    Nov 22, 2018
    Ok. Patch cord jumped across send and return, no change. Touching end of patch cord plugged into send = no effect, touching patch cord plugged into return = normal buzz. Patch cord plugged into footswitch = no change in sound, peak\mute light illuminates. Shorting patch cord = no change, peak\mute light goes out.
    One thing I have noticed, again could be normal, not sure, but all new tubes have very little glow to them. I swapped out the rear, looking from the front, ax7 a au7 with older used ax7 and an at7 in the au jack, and the at was far far brighter of a glow than the new au7. 20181123_072331. 20181123_072614.
    Also I noticed the icb, the whole amp for that matter was far more sensitive to my tapping with a pencil. I.e. tapping components up near the front ax7 near the rear au7, the small "transformer balance line output" board especially, even the mosfet heat sinks gave a good crackle in the speakers with a tap of the pencil. New tubes back in I got no crackle from any tapping really. Seems obvious that was strictly tube related but I know enough about electronics to know obvious doesn't always mean what it looks like.
    A few other observations ... When plugging in the returen side of the patch on the effects loop partial insertion of the jack lights up the green EQ light in 2 places but is off when fully inserted. And EQ turned on and levels all the way up does increase the overall volume, as meek as it is.
    Not for nothing, but it almost seems like, have you ever had a car stereo with a mute button but pot volume control. If you hit the mute button and crank the volume all the way up, the sound bleeds thru. It kinda feels like that.

    Anyway, I'll await any more suggestions before I get the ball peen hammer out and start persuading it to work correctly! Lol.
    Last edited: Nov 23, 2018
  13. MartLTB


    May 17, 2014
    Have you tried plugging your bass in the fx return and if possible run the send to another poweramp? That way you could at least figure out if the problem is in the pre or poweramp.
    Omega Monkey likes this.
  14. ficelles


    Feb 28, 2010
    Devon, England
    Clean the input jack socket, check your instrument cable, check all cables, make sure the power supply is good. Double check your bass is ok through another amp!
  15. vvvmmm


    Dec 6, 2016
    Sompin's dirty.
    rodl2005 likes this.
  16. BasturdBlaster


    Feb 19, 2012
    Crandon WI
    Preamp out and power amp in loops are just as susceptible as the effects loop, try jumping that as well.
    rodl2005 likes this.
  17. Winchuckster


    Nov 22, 2018
    OK, so I cleaned out all tube sockets, wire connectors, input jacks.....NO CHANGE!

    Things I did notice after restarting the amp. with all volumes down all the way, tapping on the board with all the rear input jacks produces very large speaker crackle. Just finger tapping the jack holes without cord plugged in produces speaker noise.
    Also, as previously stated, the only volume i get is with Main and Gain maxed out, but when i hit the mute button the light comes on but has no effect on the volume. Kinda like i was saying about the car radio. Its almost like its stuck on mute.

    So, since there was no change from cleaning and loud noises from banging on it, deductions leads me to believe it could very well be bad solder joints.


    So, the question is, will those loose screws, which i assume completes ground for various circuits on the ICB, cause my issue of little to no volume??

    I went ahead and pulled the board anyway to examine the solder joints and all appear to be good.

    Should I just hit them all again with a hot iron or leave them alone? Cuz if its not the green wire being loose or a loose solder joint I'm out of caveman ideas....
  18. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Leave the solder alone.

    Tighten the screws (snug) and hope for the best.
    BadExample likes this.
  19. arbiterusa


    Sep 24, 2015
    San Diego, CA
    You're tapping a live amp with your finger? You're pulling boards without draining caps? Nice knowing you, man.

    If you live, take that thing to a tech. You are in over your head.
    Omega Monkey likes this.
  20. Winchuckster


    Nov 22, 2018
    ya, didn't mess with the solder. connected all the wires, fastened all the board screws. rechecked the bias(i'm aware its not related to my problem) which came up from 21mv to 26mv, from just the cleaning i assume. Plugged it in. Same same. No change.

    FYI-I did not tap inside the live amp with my fingers. I tapped the input jacks, which im pretty sure is a safe thing to touch. I tapped inside the live amp with a pencil, which last time i checked, doesnt conduct electricity. and just because i didnt say i drained the caps doesnt mean it wasnt done. thanx for your concern.

    Still getting a pretty good pop on wiggling the au7 tube. regardless of that something is still not getting thru. maybe the tubes arent getting enough voltage to be driven?? although the little sound i get is fairly clean, not broken or distorted.

    how do i hook up my v4b to check whether its preamp or power amp problem?

    instrument to v4b input, v4b preamp outpput to svt3 power amp input, svt3 speaker output to cab? checks svt3 power section?
    instrument to svt3 input, svt preamp out to v4b power amp input, v4b speaker out to cab? checks svt preamp section?

    BTW --- thanx for everyones suggestions. i appreciate those taking time to help me find a solution! ya'll kick a$$ !

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