SVT3Pro Bias Setting

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

  1. beans-on-toast


    Aug 7, 2008
    It is a good idea to check the bias of both tube and solid state amps when they are being serviced. Both types can change with time. Care must be taken to follow the procedures properly if an adjustment is necessary.

    People start with the pot at a CCW position to avoid issues with hysteresis related to the pot. It allows for a more consistend and stable setting. It depends on the pot through. A multiturn bias pot is easier to wrangle into position than a single turn.

    But as was mentioned, a slight adjustment of the pot can lead to a dramatic change in the bias which could lead to damage. So you have to be very careful.
    Last edited: Apr 3, 2016
  2. B-string

    B-string Supporting Member

    While true for most there is no note to start with the bias at full CCW on the 3 series. In fact most that assume that to be proper procedure without confirmation wind up buying a matched set of MOSFETS and some resistors. Strange design.
  3. beans-on-toast


    Aug 7, 2008
    Just to be clear, I wasn't saying full CCW. I was simply saying when adjusting, it is common to back off and adjust CW to the desired bias point. ;)

    Matched mosfets aren't a bad idea to keep them operating evenly.
  4. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    If the 3 Pro bias wasn't so touchy and I hadn't been scared off of touching it myself, I probably would if I still had mine ;) But Jerrold and my local tech have made me totally paranoid about doing it because of how quickly you can burn up your amp if you mess up.
    beans-on-toast and B-string like this.
  5. beans-on-toast


    Aug 7, 2008
    Absolutely, it seems that with some amps all you have to do is touch the adjustment pot and the bias goes way off. The problem with this is when the amp is getting bounced around, the bias can change. With a multi turn pot (depending in the pot, it can take 10 or 20 full turns to go from one end of travel to the other) life is easier because the adjustment is fine. Makes it less touchy. They are more expensive so manufacturers shy away from them.
    B-string and JimmyM like this.
  6. bobber222


    Apr 16, 2015
    Yes the readings are in dc my, starting from ccw is were the lowest readings that I had
  7. Crazy John

    Crazy John

    Nov 14, 2015
    This thread may be old, but the information is timeless. I have been a tech since the mid 60's and I am used to touchy pots. My very recent SVT3 Pro, my 40th anniversary gift, came with the bias at 8 to 11mV. It sounded pretty good. With all your warnings I was careful but still managed to hit 44mV. Fortunately I have a very fast "turn back" reflex. I ended up with a range from 21 to 23 mV and decided not to play with the pot anymore. A big THANK YOU to Jerrold for helping me understand the amp, and inspiring confidence because you were there and "done that" at the factory. The 12AU7 driving the MOSFET output is an ingenious way of having a tube amp but getting around the weight, expense and heating that a tube output entails. Thanks Ampeg for making an amp with lots of tubes that is affordable.
    This is my first post.
    dalahorse, beans-on-toast and JimmyM like this.
  8. beans-on-toast


    Aug 7, 2008

    Welcome John! Many of us that had amps back then got into working on them. In my case it was out of necessity. If you couldn't figure it out, there was always the neighborhood radio/TV repair shop.
    Last edited: Nov 15, 2015
    Crazy John likes this.
  9. Crazy John

    Crazy John

    Nov 14, 2015
    It seems our backgrounds are similar! I have watched this forum for a while, I like it.
    beans-on-toast likes this.
  10. greengo67


    Dec 8, 2015
    Hi Folks,

    Just created an account to say that I too am really happy this thread is still active, the information here is excellent. My SVT 3 Pro has been my workhorse amp for both practice and gigging for the last 17 years or so, and it's been rock solid throughout. The only maintenance or work I've really done on it since owning it was to swap the tubes out, probably about 2 or 3 years after I bought it (it likely didn't even need it at the time). I kept those tubes, but I've been using the replacements I bought ever since (a bunch of generic Sovtek tubes - I only realized recently that the first (original) preamp tube is a Groove Tube).

    So, recently I was browsing these forums, and came across this article. I was quite interested in the bias adjustment info, as my own amp had slowly developed some hiss/noise while idling, and a gradual deterioration of tone. I decided to swap back in the original preamp tubes, since I figured they still have lots of life in the. I took care to put the GT in the proper location. Then, I powered it up and checked the voltages across the eight resistors at the output FETs. Like several others in this thread, I found that the voltages were quite low (<1mV across most resistors, 2 or 3 mV across a couple of them).

    Here comes the sad part of my tale. I was not as lucky as Crazy John; my first minuscule 'tweak' of the bias adjustment pot yielded a boost up to the 10mV range or so, for most of the resistors; good so far... the next hair-width twist pushed it up to the point where the resistor I had my meter across at the time went up to 27mV or so. Rather than backing it off a bit, I left it there as I prepared to measure across the others. As I was about to begin, I heard the fan suddenly speed up. I quickly hit the power, waited for everything to cool down, then backed off the pot (I had marked its original position). When I powered back on again, the fuse immediately blew. Not having a replacement fuse handy, I shut her down for the night, picked up the 10A slo-blo fuse the next day, and tried again. This time, I got the smoke and light show :-(

    So, not too thrilled about the prospect of de-soldering and replacing a bunch of components, I started to compile the list of replacement components, and search the web for online sources. As I was searching, I came across, where I found I could buy the entire power amp circuit board, fully assembled (just tubes and fan missing), for $215 US. The individual parts I was about to order were already approaching over $100.00, and I wasn't even completely certain which components (besides the obvious blackened ones) were fried.

    Even with Canadian exchange and duty, it was great to be able to simply purchase the board outright. It arrived yesterday, and earlier this evening, I removed the old board (carefully recording where all the wires go - there are about 20 spade terminals on the power amp board, with various wires connecting to them from all over) and installed the new one. Happily, it powered right up, no problem. Next, just out of curiosity, I measured the voltage across a few of the output resistors, and found them between 15 and 20 mV. No f'n way was I about to start tweaking that pot again, so I slapped the lid back on, and plugged in my bass.

    I'm quite happy to report that it sounds great - the hiss is gone, the tone is immediately and dramatically improved. It was a hard lesson in being patient, but I'm happy to have my amp working again, looking forward to band practice next week, to hear it within the context of the band.

    A big thanks to Jerrold (if he's still here), and to all others who contributed information to this thread. The SVT 3 Pro is a great amp, and it doesn't appear to have changed much (at all?) in the past 20 years, so there must be thousands of them out there. Threads like these will help to keep these amps running for another 20 years.
    JimmyM likes this.
  11. IronSean


    Sep 18, 2013
    Just a reminder to everyone to really BE CAREFUL if you're attempting this. This thread piqued my interest, so while I was swapping some tubes around I decided to check the bias. I hadn't noticed a lot of excessive distortion so I didn't expect it to be far off, which made it even weirder when measuring each side of the resistors resulted in showing ~0-3 mV each. It seemed so low that I didn't even want to touch the bias pot for fear that I wasn't even measuring properly.

    Well, I can't tell you what it was or if it made a difference, because I got too careless while I measuring for the 4th time and I shorted the left side of the top left resistor with the heat shroud... and then the light show began. There are now some char marks and fried components at a few places on the board... and I'm doing the math on ordering the $213 USD Power Amp PCB from Full Compass or seeing if I can get one used for a decent price in CAD.

    So everyone remember to take your time and please don't attempt this unless you know what you're doing.

  12. Bummer, Man.

    Let us know what you do for your repair...
  13. cerrem


    Apr 4, 2006
    San Diego
    Sorry to hear about that mishap in your amp....
    I once biased an SVT-3 for a friend ... He was a lab tech and he brought the amp into work one day and asked me to bias it for him in the lab while on lunch break...
    I did and it was the cross-over fizziness to go away and amp sounded great..... As I was walking away, he decided to monkey with the trim pot some more and next thing I see is a huge fireworks show from across the room...
    The PCB was almost completely blown up.... more than half the components were destroyed.... He was in tears, because he a had a gig coming up in a day.... I loaned him my amp in the meantime..
    Long story short... I wound up rebuilding the PCB board for him with all new parts that he ordered....amp finally all worked out in the end...
    I remember being on the phone with SLM at the time telling them they need to limit the swing on that bias trim pot....So,I installed a resistor in series with the trim pot to limit the bias swing...
    The method SLM used to factory adjust the bias was to use a variac with a power meter on it and adjust the bias for a targeted input wattage.....
    While I do not agree with that somehow crudely got the job done....
    Those power meters are not accurate and have poor tolerance and do not account for power factor which should be accounted for with this type of power supply...
    As for the output FETS... these devices should be matched, just the same as you would do for tube amp...
    I have 3 Tektronix 575 Curve tracers for matching transistors/FETS.... If anyone wants one for cheap let me know....
    Last edited: Mar 3, 2016

  14. WOW!
    Where are You.
    Do you take on work from public?
  15. IronSean


    Sep 18, 2013
    Well, I priced out my options. A replacement populated board for the power section would've been around $300 CAD + taxes and installation if I didn't do the work myself. And I made an offer on a local used SVT3-PRO for $400 that is now in my hands.

    And because I'm an idiot, I decided to bias it. Luckily I had no issues this time.

    It started around these levels

    And got it to (approximately, from memory)

    Which is around 20.5 and 18.5 in each bank. I actually left that top left resistor around 34 when I set the bias, but it was a bit lower when I went back to check them all, but I'm not going to push that one any further because I don't want to push my luck.

    But from my experience, add 2 more from 2008 that are very low biased.
    kcandme likes this.
  16. arnocl


    May 24, 2008
    Wow, been 8 years since I visited this forum, still an active thread, fascinating to see I wasn't the only one with this problem. I still have my amp, still clean as a whistle. Again thanks to everyone for the advice, especially Jerold!
  17. This is the best place to find relevant and helpful information regarding the inherent issues with the Ampeg SVT 3PRO bass amp head. I just picked a used one up from Guitar Center yesterday only after reading about it on this forum. I've been interested in this amp for quite some time (years in fact). I am a fairly competent electronics novice and one of my band mates is an electronics technician who works at NIST in Gaithersburg, MD. He and I have been discussing the attributes and issues of this bass amp for months leading up to my purchase. SO, last night I opened it up and all the preamp tubes are Sovtek (Sovtek WA in V1) and an Electro-Harmonics 12AU7 is the fifth tube. Unloaded I turned it on, let it warm up, turned out the lights to inspect tube glow = all good there, turned on the lights and checked the MOSFET ceramic dealybobs in the middle and they were all between 3 - 4 mV. I located the tiny little blue pot and stared at it with much respect. I adjusted nothing. I hooked it up to one of my Squier Jazz Basses with Duncan Designed pickups and an Ampeg B-Series 115 cab. I'm not convinced I need to do anything to this amp. I was able to get quite a variety of tones. The graphic eq is a mighty and helpful tone shaping tool. I would like the "Tube Gain" knob to impress more character on the tone however. It makes a difference when turned all the way up but, not as much as I was expecting. Which tube position would most effect the input from the "Tube Gain" knob? I have researched just about every 12AX7 tube on the market and I believe I am interested in trying the Genalex - Gold Lion B759/12AX7/ECC83 gold pin tubes in the preamp. Any comments on trying that? Should I even consider having the MOSFET bias adjusted by a trained technician?
    Savage Hippie likes this.
  18. cerrem


    Apr 4, 2006
    San Diego
    Personaly when I used the SVT3 and SVT4 amps the fattest sound I can get is using NOS GE 12AX7A pre-amp tubes... I measure them and make sure the gm is really strong....
  19. Thanks cerrem. I've been evaluating tubes based on warmth, sensitivity, headroom, signal gain and noise. I think I would be looking for a warm tube tone, plenty of headroom to stay away from distortion, a high/strong sensitivity (gm), and moderate to low signal gain (gu). That's why I think the Genalex - Gold Lion B759/12AX7/ECC83 gold pin preamp tube would be a good choice. That way I can let my bass and the pickups do the talking. The gm of this tube is reported to be a 3.5 out of 5. I wonder how significant the V4 and V5 position tubes are? Is there a better 12AU7 tube I should be considering as well?
  20. chria


    Jan 6, 2014
    Ok. I tried to do this job.

    First of all I turned on the amp and measured the 8 resisistor DC voltage. I measured 0mV on every resistor. Is it possible? I didn!'t touch the trimmer because I wondered if I had to replace first the tubes. Please tell me what to do. First replace tubes or first set up the bias?
    Thank you
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