Screen voltage higher than plate

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by boneshaker335, May 23, 2006.

  1. boneshaker335

    boneshaker335 Member

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    In my DIY Musicmaster Bass, the screen voltage is slightly higher than the plate voltage (336V screen, 333V plate).

    Is that a problem? Usually the screen should be >10V less than the plate.

    I use a choke in that amp. I also installed 470 Ohm screen resistors.

    Peter
     
  2. boneshaker335

    boneshaker335 Member

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    Mar 5, 2006
    I use a Mercury Magnetics upgrade PT that also supports 6l6 tubes. Voltages are slightly higher.

    The only other changes I made to the original circuit:
    -MM Choke instead of the 1K resistor between first and second filter cap
    -Rheostat instead of fixed cathode resistor for easy biasing.

    Peter
     
  3. John Phillips

    John Phillips Member

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    Yes, it is potentially a problem, especially with many modern 6V6s - it will likely cause premature tube failure.

    The cause is that the choke has virtually no resistance at DC, so the screen voltage is then the full B+ and in fact higher than the plate, since that goes via the OT primary resistance which is higher than that of the choke. The solution is simply to increase the value of the screen resistors until the voltage drops back below the plate.
     
  4. donnyjaguar

    donnyjaguar Member

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    Some hi-fi designs feed the Sgrids with zeners, which when you think about it isn't a bad idea. If you are a tube purist you won't want any silicon in your amp, of course. :crazyguy

    In all ultralinear output stages (common on vintage, and not-so-vintage hi-fi, rare on MI amps, although Fender 75 and Sunn come to mind) the idle plate voltages are slightly lower than the Screen grids. This is unavoidable. Its not a death-sentence to output tubes though, provided all the other operating parameters of the tube are met.
     

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