SVT3-Pro bias extreme drift

Discussion in 'Amps and Cabs [BG]' started by tchristian, Feb 28, 2017.


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  1. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    I decided to single myself out with this thread rather than continuing the 3Pro bias thread that addresses "normal" MOSFET output adjustment:
    SVT3Pro Bias Setting

    Quick synopsis: I've had to readjust the bias on my 3Pro 7 times now over the course of 3 months. Five of those times it drifted down to a few mV and gave me the well-known dirty output at all volumes. Each time readjusting it brought the output back to clean and strong. Twice it drifted up, once into the 50's and once into the 70's. The symptoms for that are high fan speed and hot air exhaust. Again, readjusting brought it back to clean, strong output. For some of these adjustments I warmed the amp up, for some I didn't. Doesn't seem to make much difference. (I'll do it warmed up only from now on.)

    This last time it drifted down from ~25mV avg to ~3mV avg overnight. And today, after resetting the highest MOSFET to 35mV, I've watched it drift down to 15 mV in an hour. (I plan to leave it on the bench all day with the probes attached.)

    I called LOUD tech support. What I got there was an "applications" guy. He referred me to LS Electronics in St Louis, who Ampeg has contracted to handle actual service of the SLM era gear. I called them but they're so busy they don't take phone calls - only email.

    So I'm appealing to y'all!

    My inclination is to replace the trim pot.

    Many thanks for your indulgence. theo
     
  2. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Might should have put this in "Repair"?
     
  3. ERicks

    ERicks

    Feb 22, 2014
    Good luck! I've had mine in 4 different shops and no one can fix it. My solution has been to buy different amps and use those. I have long considered taking my SVT 3 into an empty parking lot and throwing it as high into the air as I can. It's sad too because I really like how it sounds when it is right.
     
  4. cornfarmer

    cornfarmer jam econo

    May 14, 2002
    Hmm I just posted about this today, luckily so did you............! I guess I'm in the same boat, I was trying to figure out why I couldn't get a clean sounding output. I'll be checking out the thread you linked - good luck to us all...
     
  5. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    The bias pot is very sensitive, move it a hair and the bias changes dramatically. Bounce the amp around a little while moving it and the bias can change. One reason why I like multiturn pots in amps so afflicted. They are less sensitive, it takes turns to change the bias slightly. Retrofitting them into an amp is not always easy, it depends on the pin spacing.

    I'm not aware of a bias pot retrofit kit (similar to this Adding a Bias Pot to a 70's B-15N) for the 3pro.

    Not all amps have this sensitivity issue and in those cases, a single turn pot is fine. It depends on the design. It could also be that the pot in your amp is defective in some way and it turning too easily.
     
    Last edited: Feb 28, 2017
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    It should be noted that the problem may have nothing to do with the bias pot, but things changing within the circuit and the changes not being tracked accurately resulting in bias drift.

    Is the bias transistor Q2 in contact with the heatsink? If the design is sound, this is the temperature compensating element that governs the tracking (within the limits of the design).

    I am not familiar enough with this amp to know the design details of the bias tracking circuit, but it may not be the pot.
     
    crguti, so tuff and Bassmec like this.
  7. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    My pot turns easily. After 4 times I placed a tiny spot of hot melt glue on it. These last 2 times it turns only with gentle resistance. I feel like I've got a vibration resistant pot now.

    Doesn't look like Q2 was meant to contact the heat sink. Maybe 30 thousandths gap between and I don't see visual evidence (with strong magnifiers) that they were ever in contact. Q2 isn't bent but standing straight up on it's legs.

    Still maintaining ~15mV after several hours.
     
    beans-on-toast likes this.
  8. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    In days past, people would put a dab of red nail polish to lock a pot in place. Much better to use an easily reversible goop. I use GE general purpose silicone from a hardware.
     
  9. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Here is an image of the bias transistor (Q2) relative to the heat sink. In this case, they changed the pot, see post 14. Ref: NAD - Ampeg SVT 3 Pro


    BTW, dust can affect the performance of the circuits. What is seen here is not good. Best to remove it with a soft brush.
    Q2 SVT3Pro.jpg
     
    JB1962 likes this.
  10. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Thanks for that! My bias transistor sits about the same - possibly a little closer but not much. I have a lot less dust but I'll do some cleaning. That's a 22 turn, 1/2 W, 300V pot. I've been looking for a 10 turn. (10Kohm trimmers commonly come in 1, 4, or 12 turn, or more.) Was worried about lead spacing but I'm going to just order something similar to this one and install it.

    This is not the first time the UK (including NZ) has put a smile on my face. Thanks again beans - I'll be back with results in a few days. In the meantime I'll put my amp into service with mostly missing cover screws and no ATA case and keep an ear on it. :)
     
    beans-on-toast likes this.
  11. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    You want a linear pot. Note that the lead spacing varies with different Bourns models so examine the technical specs carefully.

    Check out suppliers such as digikey, mouser, allied electric, and newark. Stock and shipping costs varies.

    The 3296w is a 25 turn, 1/2w, 300V working voltage. The leads are in-line. The 3296P, Y, or Z might be closer to what you need with the center lead offset. Bourns also makes a wirewound trimpot that has better specs, 3250. They cost about $22 each, a lot more expensive than the 3296.
     
  12. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    I ordered a 3339P (for the round form factor) and a 3296Y. Lead-free (assuming that's compatible with a mid 90's pcb).

    This morning when I switched the amp on I had ~110 mV. I turned it down to ~30. I guess I'll gig my backup amp this weekend. Geez looeez.
     
  13. BoogieZK

    BoogieZK

    Sep 28, 2008
    Toulouse, France
    Look around near the pot in the circuit if there is any electrolytic cap that start to inflated on the top like this:
    capacitors22.jpg

    If yes, replace it.
     
  14. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Lead free means that the component is RoHS compliant. No problem, most all components meet those requirements today.

    BTW, for my work, I only use leaded solder. I find it easier.
     
    tchristian likes this.
  15. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Quick update while I wait for parts, just for those few who might be watching this thread. I monitored the bias (on the bench) for 2 days - it decreased a few mV each day down into the mid-teens. So I buttoned it up and gigged it Friday night (4 hour bar gig). Saturday I pulled the cover again to get ready to work on it. Just for the hell of it I hooked up the multi-meter leads and turned the amp on. Bias was over 200 mV. I grabbed a screwdriver and backed it down quickly but the amp shut itself down anyway. After a few minutes it started and seems to be OK.

    I wasn't aware of a shutdown mechanism. Just shows my lack of genuine electronics knowledge I guess. I was almost wishing it was fried and I could divorce Ampeg SVT3 "Pro". Still, I'm hoping a trim pot fixes it. Back in a few days. Thanks for listening.
     
    cornfarmer and beans-on-toast like this.
  16. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Parts arrived so I replaced the trim pot this evening and played the amp for an hour or so. So far so good - bias at idle remains steady (+-1mV) after drifting down from 28 to 25mV. I'll monitor it this week and check it again after next weekend's gig and report back. (That 200mV overshoot and shutdown doesn't seem to have caused damage. Fingers crossed.)

    Notes:
    - I measured the original after removing it and set the replacement to a similar value, then backed off (CCW) a half turn or so. When I switched the amp on for the first time I had 0mV. Easy to roll it up to 25 or so. Safe.
    - This 4 turn pot is still a little sensitive but nothing at all like the original. A 25 turn pot would be fine.
    - Amp is switched off in that last pic. That's why the meter reads zero.

    Ampeg VT3 Pro (ted) - 13.JPG Ampeg VT3 Pro (ted) - 12.JPG Ampeg VT3 Pro (ted) - 14.JPG Ampeg VT3 Pro (ted) - 15.JPG Ampeg VT3 Pro (ted) - 1.JPG
     
    beans-on-toast likes this.
  17. Bim1959

    Bim1959

    Apr 15, 2009
    Naples Florida
    Sales and electronic tech/piano tech: England Music Center - Clinton IA - now closed
    Good luck!

    I had a 3pro that I loved but it was always giving me problems - the bias would drift, the relay would kick in and shut down the output section but let the preamp work, it would be skimpy on power until I read that you have to turn on the graphic eq and turn it's gain all the way up - so I'd fix it until the next problem. Then I finally got tired of it acting up so I turned the bias all the way to the left to turn it off and just used it as a preamp and fed the output into a power amp. That worked great until I was getting ready to play a benefit and the 3pro went up in smoke - actually the transformer burned up. I was going to fix that also until I found out another (universal) transformer would set me back close to $350, so I buried it in a storage shed.
     
  18. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Bummer. You're tenacious though! I understand - this amp sounds awesome when it works.

    I've read that low bias = low power. Could be people are compensating for that by cranking the EQ gain when the underlying problem is a 2 dollar trim pot. (closer to 5 bucks actually)

    Thanks for wishing me well.
     
  19. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Low bias does not equal low power. Either you read wrong, or got bad info on the web.
     
  20. tchristian

    tchristian Supporting Member

    Nov 18, 2011
    Missoula, MT
    Probably read wrong - wouldn't be the first time :unsure:
     
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