SVT3Pro Bias Setting

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

  1. Primakurtz

    Primakurtz Registered Nihilist Supporting Member

    Nov 23, 2011
    Denver, Colorado
    I know it is an old thread, but it seems the most appropriate place to say, THANK YOU to Jerold and the rest of you.

    I scored a minty 1995 iii-pro. The resistors all measured from 0.0 to 4. I have biased many tube amps, but this was my first mosfet. Thanks to y'all, I know own a monster of an amp! :)
  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Yes, an old thread... but it's easily found by a quick search and has lots of useful information, so it's a good place to continue the discussion.

    I've had my 3Pro for almost ten years now, and I've generally been pretty happy with it. But I was using it at a low-volume semi-acoustic practice outdoors on a slightly cool day, and it was pretty obvious under those conditions that the bias needed some attention. The usual complaint: a kind of low level fuzzy distortion, and generally a little weak sounding.

    With the good information and advice from everyone here I had no trouble c-a-r-e-f-u-l-l-y adjusting that notoriously sensitive bias pot (yes, it really is that touchy). No signal, no speaker connected.

    It started out with no measurable voltage on most of the resistors, no wonder it didn't sound good. I'm kind of wondering about the voltage readings I ended up with, though:

    10 12
    17 11
    22 35
    11 7

    Yow! I'm not going to get anywhere near an average of 25 mV without driving that one way too high! What's up with that? Should I be concerned?

    Still, it sounds a whole lot better now. Not only is that fuzzy distortion all gone, but the bottom end seems a lot more solid and present. I have a feeling I'm gonna really like it when I get it up to gigging levels!
  3. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The question is whether yours has any paint dots on the devices. It might, but you have a rather wide variation there, and it could be either the worst possible set of matched parts, or a set of unmatched parts.....

    Paint dots, if all same color in a group of 4, indicate matching or "grouping" was done. No paint dot means they were not matched, in which case you "done good" to get as close as you did.

    I suspect you will be OK as-is. if you did it correctly, all should be well, and I would not try to do any better.
  4. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Like a little pale blue dot in one corner of each MOSFET? (I think they're pale blue, hard to be sure with my LED flashlight.)

    So maybe they're "the worst possible set of matched parts," or maybe a couple of them have drifted as they aged, or ... what? Bad solder joints somewhere?

    I'll count myself lucky to have gotten it to where it is now. It sounds a lot better than it did before. If it ain't broke, don't fix it!

    Thank you for your advice!
  5. Hi,

    I am new to this form as a user, however I have been reading the great posts with lots of info for a number of years. Thank you to all who contribute their knowledge and experience!

    I read earlier on in this post where someone said that the bias pot is very touchy and that pyrotechnics may occur if not careful….. Well I can say that I am one who has experienced that 1st hand. Long story… however, adult beverages and being used to tube amps most of my life, I am (or was) used to having equipment that takes a beating and keeps on ticking.. Anyway , yes the mosfets and the resistors do light up pretty good when you turn the bias pot fully clockwise, trust me…. Lots of smoke too!

    To my problem… I had some mosfets and resistors replaced on mine :rolleyes:… and the tech did the bias adjustment per another process, if I recall it is in the wiring diagram for the amp. Anyway when I got it home, I plugged in and that #@$% distortion was still there. So I saw this post and did the adjustment and voila! Back to great sound….. except… about 30 min later, the distortion is back :(.

    Is there a possibility that the bias pot moves? Should it be locked in place with silicone or locktite after adjustment? Should the amp be running for 20 min before the adjustment, like with the SVT 2 (yes I know there are tubes in the SVT 2, just wondering if the 20 min makes a difference) I am going to do the measuring and adjustment process again, however if it has changed, is there a remedy?
  6. I have been getting this fuzz for a long while now and have read this thread many times, so last night, armed with the knowledge, I decided to have a go at mine.

    Initial reading...
    1.6 - 3.0
    3.7 - 2.8
    3.6 - 2.4
    3.2 - 3.2
    3.025 - 2.85

    and after adjustment...
    14.9 - 26.1
    30.4 - 23.5
    27.3 - 20.1
    22.5 - 23.4
    23.775 - 23.275

    Hooked it up to my cab and it is as clean as a whistle. Can't wait for the next gig at the weekend.
    I've always loved this amp, I love it even more now. :hyper:

    Jerrold Tiers = Top Man.
  7. Now having done a couple of gigs using the 3Pro all I can say is it sounds like a different amp. Awsome.
    Will it stay like that :hyper: or is it likely to drift off again :crying:, saying that, from memory it always had that slight fuzz from new so was it never correct in the first place?
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Most likely it will stay as it is.

    From what I have heard about lots of these with low bias, I think there must have been a factory problem that we never knew about. Possibly a procedure issue in the setup, difference in temperature, something.

    Your comment that it has always been the same is support for that.
  9. sincity


    Oct 16, 2002
    Hello Tbers,

    My 98 SLM SVT 3 Pro is starting to have a slight distortion at the end of a note on the higher registered notes above the 12th fret.

    I am very interested in adjusting the bias of the amp, but I would very much appreciate a step by step tutorial for doing the work.

    I have a multimeter with probes only, no gator clips.

    I am looking for the following questions answered:

    1) Where do I locate the ends of the probes to take the individual readings?

    2) Can each mosfet be biased individually, or does the trim pot raise or lower all the mosfet voltages as a group?

    If someone could post a video, I think the TB community would be forever grateful.

  10. Bassmuse16


    Aug 17, 2012
    What, in specific terms, happens when you over bias and it blows up? Years ago (when I was younger and stupider) my first svt 3 pro was having problems and I thought I would open her up and poke around in there. I saw the bias gain and turned it up with the power on and KA-POW!!!... definitely dead now. I still have it in the shop for parts for my current 3 pro and I'm wondering what it would take to get her going again.

    p.s. This is such a great thread, thanks so much to Jerrold and everyone who contributed.
  11. B-string

    B-string Supporting Member

    I would PM Jerrold Tiers, just left click on his name above. Many things fry when you do that. That bias control is VERY sensitive to movement.
  12. BassmanPaul

    BassmanPaul Inactive

    Agreed. At a minimum you will need a complete set of output MOSFETs. these will need to be matched to each other in order that they share the work load equally. Matching is a fairly easy process - IF you have a sufficient quantity of devices.

    Edit: IIRC you should be able to get a matched set from Ampeg.

    Edit2: There well could be other problems with the amp due to the shorted outputs. If you are skilled in diagnosing and repairing SS electronics fair enough. if not I urge you to take the amp to a good tech.
  13. B-string

    B-string Supporting Member

    +1 I think there is also source and/or drain resistors?
  14. grumpyjfc

    grumpyjfc Supporting Member

    Sep 25, 2003
    Mount Pleasant SC
    Hi everyone. I'd like to ad a comment here...
    I am a marine electronics technician full time and I have lots of experience with electronics but have never serviced an amplifier. I followed the specs on Jerrold's procedure and my amp is now performing perfectly.

    However, as has been stated before in this thread, if you don't know (at minimum) basic electronics you should NOT try this yourself. Not knowing simple things like what "measure across" means, or how to identify a resistor or potentiometer, or how to use a multimeter, you can cause serious damage to this unit. If you have any hesitation whatsoever, then don't skimp, take it to a tech who knows what they are doing.

    Thanks Jerrold for the continued Ampeg support.
  15. prd004


    Dec 3, 2010
    Anyone out there care to explain to a noob exactly what "measure across" means, and specifically how to do it?

  16. Shimmidybone


    Jan 13, 2014
    Houston, TX
    Your multimeter will have two probes. To measure across a resistor, place one probe on each end of the resistor. Set your multimeter for DC voltage in mV. You'll be measuring the voltage drop across that resistor.

    Effectively, you're measuring the current through the resistor. Measuring voltage across a known resistor is a simple and safe way to measure current.
  17. RoosSQ


    Feb 7, 2013
    Huge thanks to Jerrold Tiers for all his insight on this thread! It has been a long few days chasing down this bias issue and without the help of this thread I'd still be playing what sounded like an overdriven electric Guitar.

    After exhaustive research I determined that my bias needed to be adjusted. (I had significant distortion on high frequencies at low volumes) I brought it into a local shop and asked them to replace the tubes as needed and adjust the Mosfet bias. I have gigs on Friday and Saturday so I paid extra to have it looked at right away. When I picked it up I asked how far off the bias was and he told me he did not and typically does not adjust the bias. He replaced the tubes and said he cleaned the amp and sent me out w a 140 dollar bill. I was pretty upset, the only thing I asked him to do was adjust the bias and he didn't even touch it. I know how to change tubes etx, but at advice of all you, I didn't want to adjust bias w out proper tools/experience.

    I got home, plugged in my head, and as expected still had a distortion on the highs (especially with the tweater activated and tone knob rolled off). I brought it back in and said that I still have distortion. I brought him sound clips of the same cabinet and bass swapping out the head in question w a B2re, clearly illustrating the distortion issue with the Pro 3. From here it just got increasingly frustrating. After asking several times to adjust the bias, citing all the things I have learned from these threads, he assured me that it was not the bias and that it was probably my bass or cabinet and that the threads are full of misinformation. At one point he hooked up a CD player to the head to show me that there's no distortion. Ugh...

    Feeling defeated, I came home and decided to take the risk and turned the bias pot EVER SO SLIGHTLY cw. No more distortion!!

    Knowing that this is not a safe method and I can fry my output, I went to RadioShack and picked up a multimeter. Got home and realized the multimeter doesn't read mv. Trip back to RadioShack...
    The first tests were as follows:
    3.5-4.6 (mv)

    I tweaked it incrementally and got to an average of ~23mv.

    Sounds SO much better. thanks everyone for your contributions to these threads, they are a lifesaver. Anyone who is tweaking their bias pot, as stated in these threads, it is incredibly sensitive - be careful. Cheers!
    Last edited: Jan 3, 2015
    B-string likes this.
  18. bobber222


    Apr 16, 2015
    Hi, first time poster long time lurker...
    I changed the tubes in my svt 3 pro and thought since I was in there I would check the bias that I have read so much about the results were as follows
    1.9 2.3
    2.1 1.9
    2.8 2.1
    1.1 3.0

    After slowly adjusting the trimpot starting from ccw the first 50% reads 0 then I get a reading maxing out at
    5.6 5.6
    5.0 4.2
    7.9 7.4
    4.9 8.1

    Then for the last 25% returns back to 0 reading. All mosfets have a yellow paint dot. Readings were done with a dmm no load.
    I checked resistance on trim pot and it tracks properly. Any help would be greatly appreciated thanks
  19. B-string

    B-string Supporting Member

    WHY would you start from CCW? Have you read this thread?
    This design is a special case and large movements of the bias pot can cause severe damage. Are your readings DC MV?
  20. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    You should never touch that bias unless there's a problem, bro. It's not a tube amp.
  21. 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.

    May 26, 2022

Share This Page