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. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Despite the protection circuit, power tubes blow, screen resistors opening are common. The system isn't perfect.

    A plate resistor will act as a fuse. A 10 ohm plate resistor rated at 5W will eventually open with sustained 1/2A. In '77 they used a 5 ohm 5W plate resistor, 1A sustained. There is also a 1 ohm 5W shared cathode resistor. When these blow damage is done, often to the tube and the PCB. The fusable screen resistor also offers some protection. High frequency oscillations are capable of damaging an amp as well. Ideally a plate resistor and a series connected ferrite bead would be good to have.
     
    Last edited: Sep 16, 2019
    agedhorse likes this.
  2. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    I can attest to that. (Fortunately I have the skills to diagnose an open screen resistor and replace it.) At least the protection circuit helped keep the shorted tube from causing more serious damage (like the output transformer).
     
    beans-on-toast likes this.
  3. My classic died at the gig, although I think it's just a tube failure. I just can't seem to catch a break. I will keep everyone advised on the Mti as I get parts.
     
    BassikBrad and Wasnex like this.
  4. Update: got parts for the mti earlier than expected so I'll get it going tomorrow. The classic only had a failed tube.
     
    Wasnex likes this.
  5. 20190922_184805.
    This is bias on k1 after a playing period to see if it would blow up
    20190922_185306.
    This is between k1 and k2 at somewhere around 27v
    20190922_183932.
    And the camera doesn't like the CRT scope so this is the best waveform picture I have but much better than it was.
     
    Wasnex likes this.
  6. It appears to be working. I'll give it a test run Friday and see if it's good to go but it all appears to be okay.
     
  7. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Did you measure the actual DC and AC current sharing between the 6 tubes?
     
  8. It's still oscillating under actual bass guitar use. I'm taking this to a local tech, I've run out of time and money to keep buying test gear. I either need it to work or sell it.

    To reply to your question, agedhorse, I don't have a meter nor a probe capable of handling a 700v differential. I should get one, but I just don't have any more time.
     
    agedhorse likes this.
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    A badly matched set of power tubes can cause the oscillation. Have the tech check this.
     
  10. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Current sharing generally shows this too.

    Could be damage to the screen of one of the tubes too.
     
    beans-on-toast likes this.

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.