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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. Wasnex

    Wasnex

    Dec 25, 2011
    It's been a long time since I worked as a technician, so I am not 100% certain how to interpret the distorted section of the waveform... IMHO, it looks more like ringing than clipping
     
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  2. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    I was finally able to watch the video, that's instability, high frequency oscillation. Also check C7, C4 and R46.

    Does it do this with 400Hz as the test signal?
     
    Wasnex likes this.
  3. No sooner had I looked up oscillation waveforms did I see your post, that's exactly it. No it doesn't, but I don't think I'm driving it hard enough.
     
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  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I’d first look for any (often) brown ceramic disk capacitors in the power amp’s signal path as the source of the oscillation. Sometimes they are on the board and not on the schematic. Otherwise, every cap is suspect.
     
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  5. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    It's possible that low frequencies are causing some internal microphonics within one of the tubes (could be the PI serction). If you probe the signal before the PI, (say the output of V1, pin 1) is there oscillation?

    It could also be an issue with the feedback/compensation.
     
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Speaking of checking the plate resistors...here is a visual that you might find helpful. The amp has a push-pull imbalance. Note, they test the plate to plate resistance with the tubes out from socket terminal to socket terminal. This tests the entire path, not just the resistors individually.

     
    Last edited: Sep 15, 2019
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  7. Got the lcr meter out, I'm checking it now, specifically c7 and r46. Both are okay on the lcr.
     
  8. C4 is okay too.
     
  9. 15686019323229028032229228658531. what are the odds I have the exact same problem as that guy?
     
  10. All others are 10 ohm across 2 5.1 ohm resistors as indicated on the schematic
     
  11. 20190915_225629. that would never happen again in a million years lol have the same problem as a guy on YouTube it's basically a guided tutorial on fixing it
     
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  12. Wasnex

    Wasnex

    Dec 25, 2011
    I would say the chances of an open plate resistor are pretty high, given the symptoms. Check the screen resistors too.
     
  13. I can Amazon prime some flameproof metal oxide film resistors and have it in time for the weekend, but should I go for the ceramic resistors?
     
  14. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    THIS is why troubleshooting theory is so structured, and why I detest simplistic approaches.

    The current sharing tests would have showed this up before going down the assumption path.

    This may be only part of the total problem though. Be sure to check things systematically.
     
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  15. I am, but I think I need to upgrade my meter before going near the plates. It's only rated for 600v. The other resistors are so far checking okay but the verdicts out on all the others. I'm going through everything as we speak.
     
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    So far, so good with your efforts.

    Wire wound cement block resistors are standard for this application. Buy extra and match them. I find it best to mount them a milli-meter or so proud of the PCB to keep any heat off the board. Also rotate them so that the adhesive on the bottom is facing sideways. When they fail, flames tend to shoot out the bottom. You don’t want that facing the PCB as it causes damage. Do not exceed the 5W rating, you want them to blow open in the event of a problem.

    Rather than installing plate resistors, Ampeg currently uses a ferrite bead instead of a plate resistor. This is good because it can help suppress high frequency oscillation. The ferrite bead that I use is a Murata BL01RN1A1F1J. The resistance increases at high frequencies and helps to damped oscillations. This is the bead that I use: BL01RN1A1F1J Murata Electronics | Mouser

    You want a screen resistor that will blow like a fuse when required. This can help protect the transformer when a tube has runaway plate current. There are resistors called fusible links. This is the fusible link 220R 1/2W screen resistor that I like to use: NFR25H0002200JR500 Vishay BC Components | Resistors | DigiKey. Note that the value is 220R, not the 22R that was used in the vintage amps. The higher screen resistance is what Ampeg uses today. It helps extend tube life. You won't hear a difference.

    Also, if you have the 1N4007 diodes in parallel with the screen resistor, remove them. Ampeg does not place them in their current products, they haven't in decades.

    I'm wondering if the blown plate resistor is on one of the two power tubes that is next to the transformer. It is possible that the close proximity of the blown part to the transformer is inducing the oscillation that you are seeing. If possible, a tube next to a transformer is not a good design choice. A severe imbalance in the tubes, which is what you have, can also result in the oscillation. Fixing that issue may clear up the oscillation.
     
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  17. It still does have the diodes, all the screen resistors are 22 ohm, and I don't suppose I'll have a better chance to replace them. And yes, the failed resistor was on the side close to the output transformer
     
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  18. Got all supplies on the way
     
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  19. Going to do screen resistors, the 47k grid resistors are all okay
     
    agedhorse likes this.
  20. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    I think current SVTs all have a protection circuit that detects a tube that's drawing too much current and shuts off the B+. That means they don't need a plate resistor to act like a fuse in those circumstances. So in an older SVT without the protection circuit, wouldn't you still want the plate resistors?
     
    beans-on-toast likes this.

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