1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

Biasing mti svt, am I doing this wrong?

Discussion in 'Amps and Cabs [BG]' started by poorbassist15, Sep 9, 2019.


  1. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I don't think that method works (or is reliable) with a typical sampling DMM.

    From what I remember, the balance is essentially a symmetry adjustment. If you have a traditional analog meter this may work ok, but I doubt anybody here doing this has one.

    The way I posted is essentially the most foolproof because it verifies that the currents through each bank are the same before moving on.

    The way Ampeg posted is faster and easier but if there is a condition where there is circuit unbalance, it could give a misleading result.
     
    MAXSPINRUN likes this.
  2. moogieotter

    moogieotter Custom Title

    Jun 16, 2009
    Duluth, GA
    Results? Does it slam af?
     
  3. Raw N Low

    Raw N Low If I can't hear it, hopefully I'll feel it Supporting Member

    Jul 16, 2009
    Denver, Colorado
    I too am curious to see the results. This is a really big bucket list Ampeg item for me. Something about the MTI SVT's seemed more aggressive from other variants. The breakup reminds me alot of the brighter 2 pro.
     
    moogieotter likes this.
  4. somebrains

    somebrains Supporting Member

    Feb 7, 2017
    That should be a write up and a sticky.
    Maybe a YouTube video.
     
  5. Wasnex

    Wasnex

    Dec 25, 2011
    I see the balance control as a symmetry adjustment as well :thumbsup:.

    Next time I setup one of my SVTs I will have to try the balance adjustment with the meter set to AC to see if the results are better. My meter is a rather cheap DVM. The readings were stable and didn't bounce around when I setup my SVTs. I also got consistent readings if I removed and reinserted the probe, but I think the results were similar to @poorbassist15 ; I.E. the best sound was not exactly at the null.

    As far as the bias. I think the Ampeg procedure will guarantee the voltage at K1 and K2 is equal. What it won't do is ensure the voltage is 72mV, because as you increase tube current in the second set of tubes it will cause the power supply to sag, which will throw off the bias in the first set of tubes. The amount of error will depend upon how cold the second set of tubes is running when you start the adjustment.

    I usually adjust both bias controls incrementally to get K1 and K2 very close to 72mV before I take the differential voltage reading between K1 and K2. Then I go back and fuss with it until I get final readings to exactly 72mV with the differential as close to 0V possible. I also tend to leave the amp on the bench for a few days to make sure the bias holds. But I have all week to play with it if I want :woot:.
     
    bobyoung53 and agedhorse like this.
  6. Wasnex

    Wasnex

    Dec 25, 2011
    @poorbassist15 if you still have your amp on the bench, please try the balance control with your meter set to AC and let us know if the results are better. Also, if you don 't mind, try setting the control at different frequencies like I did. I am pretty sure you will find the balance setting is only really perfect at the frequency you set it at.
     
  7. cerrem

    cerrem

    Apr 4, 2006
    San Diego
    The SVT bias adjustment is really not good.... The cathode currents are not a real indicator of balance, since the screen currents are not the same with these tubes and has variance... also the current sense resistors in the cathodes are like 5% or 10% tolerance on top of that...SO by the time you stack up your tolerances it's doesn't look to good..... I use the DVM to set just one side.... Then I use my ear to adjust the other bias pot...you adjust to minimize or null the 120Hz signal with your ear at the speaker.... Then adjust the HUM pot to minimize the 60 HZ also coming from the speaker.... If you play into a dummy load a 100Hz to 400 Hz signal with volume around where you play.... You can turn the BALANCE pot till the transformer stops ringing at the applied signal..... When the AC is off balance the signal will resonate the windings in the OT ....quite normal... WHen you have true AC balance in the output signla the OT will stop resonating at the exact point of balance... Been doing these around 40 years.. I have verified this method against distortion analyzers and looking at the signals on Scope....
     
    bobyoung53, Wasnex and BassikBrad like this.
  8. BogeyBass

    BogeyBass

    Sep 14, 2010
    well yeah. having a scope and distortion meter could be very effective...but

    ding ding you win the internet lol.

    major benefit, if any to run small percentage of negative feedback with a tupe amp is lower noise or hum reduction.

    and ironically by ear you can also balance a phase inverter by simple listening for least amount of hum.
    or with scope least amount ripple zoomed in to hell on the output. assuming the topology is using negative feedback. or likewise if running hard into a dummy load you can balance by looking at how even the clipping is on the rails

    no need for dummy load with ear test. basically need hear the thing.

    if null is out in outer space to balance things. either tube set. or just need a new phase tube or the actual followers to drive the tubes needs replacement. worst case is resistor drift in multiple places. and that is never a huge surprise and common

    i mean technically its bad practice to " shot gun" a amp. but at no time i dont find it ridiculous to recommend changing both tubes on the power board or chassis on a SVT lol. it will never hurt and isnt ever waste money. basically the phase and driver tubes. usually often overlooked on tube changes and or simply old and beat up
     
    cerrem likes this.
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Without identifying the actual cause, why would you change out what may be perfectly good parts. IMO it is reduculous and a waste of money.

    One issue with measuring total current per push-pull side, and it's one that can really trip up a careless tech, is that if a single tube is operating at much less current, the remaining 2 tubes will carry most of the current resulting in them being overbiased. This then skews the balance calib. because the thansfer function is now different on each half.

    This is why I always include a way to perform a current balance test on my tube amps. The same applies to linear solid state amps, techs who discount the current sharing test don't grasp the importance.

    Now on the SVT, Bill Hughes provided a way to separate the plate current from the screen current as well as to measure the plate current for each tube individually. Just beware that you will be doing a low voltage differential voltage measurement in a circuit with almost 700 volts common mode.
     
    Wasnex likes this.
  10. You're doing this with a cab:roflmao::roflmao::roflmao::roflmao::roflmao:?? I thought I was insane. I bought some power resisters years ago, 16 ohms, 75 watts I think, I parallel them to 4 ohms, they get hot pretty fast too.
     
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Back when the SVT was designed, people used analog meters. It's easier to measure across K1-K2 and adjust for a minimum than it is to measure a small K1-GND, then K2-GND voltages. That's why they suggest using K1-K2.

    The cathode currents of each trio are combined and the voltage is measured across a one ohm resistor. (I like to use a pair of matched resistors on the cathodes.)

    Anyway, 72mA through three tubes is a pretty cool 24 mA per 6550. That's under 16W per tube (660 plate voltage in curre t amps, 695V on the vintage schematic is an error). Even if one tube failed, went into cutoff, and the other two were matched and biased to 72mA, the two remaining tubes would still be safe at 24W each. I think that the SVT was designed with that safety factor in mind. Fault tolerance is a good thing to provide.

    It's important that the amp be warm, at a thermal steady state so that component values are not drifting, when setting the bias. Warm it up for half an hour, standby off - playing mode. Set the bias on both sides. Recheck the bias. Let the amp run for a while. Recheck the bias. Always read the AC line voltage as a reference when taking readings. Things drift. You don't have to go nuts trying to get everything perfect. I fine tune bias by ear so that it sounds the best and check with test equipment to ensure that it's within a safe operating region.
     
    Last edited: Sep 11, 2019
    bobyoung53, BassikBrad and Wasnex like this.
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    If you use an analog meter, the stated method may be fine (still doesn't identify a current sharing problem). Any good tech will insure that each tube is carrying its share of the load. Otherwise, why go to the expense of using 6 output tubes?
     
    BassikBrad likes this.
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There's value in a properly matched set of power tubes.

    I suppose that many assume that each trio is made up of tubes running evenly. I start with an IR thermometer to eyeball it but still like to measure the current of each tube in the amp just to be sure.
     
    BassikBrad likes this.
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Current sharing tests identify tubes that are either defective, failing, or have aged much faster than the rest of the set. That's important to know when doing any kind of repair or service imo.
     
  15. It most definitely does, the mti is a hot one, it's slamming. This thing on my stack will make you realize you can feel your kidneys move. I immediately put my ge long plate 7025 in it. I see why these are crucial to the sound of the amp. It's like they're made for each other.
     
  16. moogieotter

    moogieotter Custom Title

    Jun 16, 2009
    Duluth, GA
    Thanks for the follow up.
     
  17. I will give it a try tonight. I will also have a scope Thursday so I'll try to use that as well.
     
  18. Even though the classic is about as loud given the same tube brand, there's something about old svts.
     
  19. I honestly use my hand. I realize this is quite stupid but I can tell if one is hotter than the rest. Same end result? Yes but with more pain.
     
    beans-on-toast likes this.
  20. moogieotter

    moogieotter Custom Title

    Jun 16, 2009
    Duluth, GA
    I never understood SVTs until I played an older one, 76 Magnavox one - blew my face off, mainly because of the feel and warmth control in the plucking hard.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.