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Biasing mti svt, am I doing this wrong?

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


  1. I'm a bit scared to try the mti, because it's so hot. The prior owner had a 12au7 in the first position to tame it, not that it helped.
     
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    When using a scope to set up the bias, my experience shows that it's better to drive the amp with a ~5kHz signal, and you don't need to drive it as hard. Usually 10VAC on the output is enough to clearly see the crossover distortion. The crossover distortion and symmetry tend to go hand in hand with both bias and balance, but you will need to go back and re-check your bias. Do not go past where the crossover symmetry just disappears.

    I don't recall how interactive the crossover distortion and symmetry are on these amps, it's been quite a while since I played around with them on the bench. I also re-emphasize to be freekin' careful. Many scope probes have a ground ring around the tip of the probe, if this contacts anything related to B+, it will get ugly in a hurry. Do NOT use any of the laptop, tablet or phone based scopes on this amp, the voltages are way beyond that they are designed for and it's a quick way to damage (permanently) an otherwise good device.
     
  3. I would agree with that, it's a much angrier sound, a bit like mel schacher with the tone all the way down.
     
    Raw N Low likes this.
  4. I have a Chinese digital scope and a jdr dual channel on the way. They should be good, I hope.
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    A good analog scope is the tool of choice for the job. Be sure the probes and inputs are capable of at least 750 volts DC. Most are, but not all. Be sure you understand what you are looking at also.
     
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    What's missing from that video is that on most amplifiers, you are not concerned by a line fault, but because the chassis and audio ground is eventually solidly connected to the chassis, the scope's ground reference to the chassis will be ~700 volts below what you are measuring. The secondary voltage itself is the danger.
     
  8. Wasnex

    Wasnex

    Dec 25, 2011
    I think what you are saying in this instance, is the ground is not the problem, but 700 volts is very dangerous?
     
  9. Raw N Low

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

    Jul 16, 2009
    Denver, Colorado
    G.A.S. for MTI era SVT confirmed.
     
  10. BogeyBass

    BogeyBass

    Sep 14, 2010
    yes correct

    current sharing is important. and knowing where each tube sits is very important and usually overlooked or just misunderstood.

    unfortunately it will never be perfect of course. there will always be the weaker tube.

    but yes each side needs to be balanced.

    the real problem comes about if yes there is 2 weaker tubes or stronger tubes on one side and yes current sharing will be much higher on one device.

    as mentioned it will never be perfect, but very important to know where all the tubes sit. and with basic math you can start changing tube positions. or if done on a separate test rig. calculations can be made and everything installed and balanced from the start.

    of course could rehash the classic argument of how bad or good individual bias and balance adjustments for each tube pair could be.
    or again how important current balance is anyways with or without
     
  11. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Yes, it's the fact that between the scope probe and scope/probe ground, there can be ~700VDC. Therefore, if you accidentally touch the scope prob's ground to anything related to the high voltage supply (regardless of AC power isolation) the results are usually catastrophic to the probe and sometimes to the downstream equipment.

    With laptop and tablet devices, it's not uncommon to find the maximum voltage above ground in normal operation to be <300V and if you connect the probe to ground, it's unlikely that the internal circuitry can support a 700V transient.
     
    Wasnex likes this.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It never hurts to understand the basics of using a scope and grounding. People will do things that you’ve never expect them to do.

    A big issue when using a scope probe inside the amp is having the ground clip slip off and come into contact with something that it shouldn’t. Sparks can fly.
     
    Wasnex likes this.
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    That's exactly what I am alluding to, though often I will ground at the generator and disconnect the scope probe's flying ground. What people forget is the little ground ring that remains exposed (unless you put a bit of heat shrink over it) above the probe tip is awfully attracted to high voltage ;)
     
    jastacey, beans-on-toast and Wasnex like this.
  14. For my purposes I should clarify I'm using it to visualize a waveform from the output of the amp. But, I'm curious, should I need to probe inside, would I earth ground to the chassis?
     
  15. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The chassis and circuit ground are tied together as part of the amp's design. It's also part of the scope's design, connecting the probe's ground to the scope's safety ground. There nay be ways to open this ground and use the probe's ground exclusively, but this starts getting into the specific features of different scopes and their different applications.
     
  16. cerrem

    cerrem

    Apr 4, 2006
    San Diego
    Here is a photo of when I set the balance control..... The scope shows the 400Hz sine wave just on the hairy edge of symmetrical clipping..top and bottom..
    I use a Diff prob to avoid ground loop issues... I also put the DVM in the sockets just to see where it is at ..just for curiosity...

    IMG_7966.JPG
     
  17. Wasnex

    Wasnex

    Dec 25, 2011

    Since you DVM is reading -3mV, I'll take this as proof that @poorbassist15 is not crazy :laugh:.
     
    poorbassist15 likes this.
  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The Fluke 87 reading -3mV will not be super accurate. It's close enough to 0.
     
    Wasnex and cerrem like this.
  19. I'm using the mid level harbor freight meter, so mine isn't probably accurate either
     
  20. Wasnex

    Wasnex

    Dec 25, 2011
    Back when I was an AF technician, I think the Simpspon 260 was the standard issue analog multimeter. The thing is, the lowest range on a 260 is 1V. With this level of sensitivity, I don't think I would be quibbling over 2.5mV :cool:.

    [​IMG]
     
    Last edited: Sep 12, 2019 at 5:57 PM

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