SVT3Pro Bias Setting

Discussion in 'Amps and Cabs [BG]' started by arnocl, May 25, 2008.


  1. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    No, but I believe you have the magic number wrong by a factor of 100.
     
  2. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Your line voltage doesn’t affect the internal voltages and settings. It won’ affect the bias setting as others have said. Your power transformer takes care of that.

    There is a big BUT though. The power transformer can be wired for 100VAC, 120VAC, 230VAC, or 240VAC. If your line voltage is a true 240VAC and the label on the back of the amp says 240VAC, you are fine.

    If your line voltage is 230VAC and the amp is set up for 240VAC, or vice-versa, voltages inside the chassis will be lower or higher. It may affect the optimal bias setting. In that case, configure the power supply selectors to best match the line voltage.
     
    Last edited: Jan 12, 2021
    keatz likes this.
  3. keatz

    keatz

    Jan 19, 2011
    Thanks. I will not be attempting this myself. I just want to collect all the best info I can to pass on to my tech savvy pedal builder friend. I guess we can at least measure it, and if it needs adjusting and he doesn't feel comfortable I will just have to take it to a tech. From reading this thread I fully understand how touchy and critical it is.
     
  4. Wherestheone

    Wherestheone

    Feb 19, 2019
    OK.....fine friends--- The POINT----When people say this trim pot is delicate and touchy----HEED THE ADVICE!!!!

    I have succumbed....I bought a used, one owner SVT-3 Pro after renting one for a couple of days, and thinking: "yup, nice balance here of tone, power, weight, etc etc".

    It's got one Groove Tube (first tube position, I think) and EHX 12AX7EH's and an EHX 12AU7A in the remining spots. It's a USA made SLM model.

    It's in pretty darn good physical shape for its age too!!

    Of course I've been drawn into the rabbit hole of reading as much as I could here on these amps...and it wasn't long before I found all the MOSFET biasing material.

    I popped the top off, unplugged everything (cables, speakers), set the Master volume to 0, and flipped the amp on. Warmed it up for a bit, and started measuring mvdc's across the 8 widgets.

    Came to the conclusion that the voltages were a bit low....so .....steadied my hand and ...

    Back to the point here----HEED the WISDOM----I've got the average across all of them now sitting around 24,-------AND I COULDN'T EVEN FEEL THE POT MOVE/ROTATE to get the numbers up from averages of less than 10 to 24. I'm content with 24....I don't dare tempt fate.

    But that trimpot....It is SO SENSITIVE!!!!! Don't expect to even FEEL / SEE it move!!

    And thanks Talkbass!!
     
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  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Yes, we aren't kidding, leave it where it is... not tempting fate is the right approach. You can see why some folks have blown up perfectly fine amps by giving it a tweak.
     
  6. I got a prob bias my svt3pro when i turn the adjusting pot i can go from 0mv to 12mv but theres no way i can go higher any idea whats my problem? Thx in advance
     
  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
    There are about 20 possibilities (including measuring wrong), but you need to know what you are looking at and how the bias network is supposed to work in order to go farther.

    Be sure you are measuring the correct way, common mistake that can result in large amounts of magic smoke.
     
    jastacey likes this.
  8. okwestern

    okwestern

    Jun 1, 2020
    Sorry, for resurrecting this old thread, but it has a lot of useful information about bias adjusting.
    I wonder if it's normal that the current through the source resistors jump 10x during power up?

    Some background:
    I found my SV3Pro to be quite low (and poorly matched, spanned from 2.1 to 13mV), so I bought 25 of each P and N MOSFET. I managed to get fairly ok matching even with such low number of chips to choose from. I used the description of Pass Labs to match the MOSFETs (15V through a source resistor, drain+gate bridged, measured values between source and drain), but used 150R (ohms) for source resistor instead of his two examples of 560R and 2.2K. The four N-ch MOSFETs with the closest readings gave me a voltage of about 2.44V over the resistor and the P-channels 3.69V. However when I put them in the amp, each P and N bank reads almost equal (average for all N is 10.75mV and P is 9.79mV), even though the P group tested almost double of what the N group did.
    I know the four MOSFETs in each bank have some current sharing between them, but didn't think that worked across the two banks.

    Anyway, I hooked my multimeter into the source resistor giving me the highest reading (and dialed back the pot meter to about 15mV - to have a safe starting point). But 5-6 seconds after turning on, the N channel bank jumps to anything from 60 to 350mV for a brief moment (duration 50-200ms not sure, but not long), then down to 15mV again. Is this normal or is it because I selected two P and N banks with such big difference (when individually measured for matching)? They were 2.44V and 3.69V when matched with Pass Labs test procedure.

    PS. I also replaced the horrible pot with a 10k multi turn pot which is a dream to adjust. I suppose it doesn't have anything to do with my problem, but saying just in case.
     
    Last edited: Apr 26, 2022
  9. 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 depends.

    First of all, there’s current matching specifics… they need to be within say 50% at idle and low AC values but needs to get closer together as the current increases (part of the AC current sharing for current balance). At very low levels they may be different by 100-200% yet be entirely fine if the curves grow together as the current increases.

    Also, If any of the source resistors are out of spec, the numbers are meaningless.

    Another thing, if you bought your MOSFETS on eBay, Amazon, Ali Express, etc., there’s a better than 50-50 chance that you have counterfeit or knock-off parts.
     
  10. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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