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• Mesa Boogie biasing - steven oda 08:43:03 11/03/02 (0)
  In Reply to: Re: Are Mesa Boogie EL-84's good in Audio Amps? Thanks!NT posted by Dman on November 02, 2002 at 06:33:28:

  Dave,

  Not familiar with the stereo version. But, most MB run a FIXED BIAS, with no pot for adjustments. That's why their admonishment to buy "matched" Mesa-Boogie approved tubes. Most of their 6L6GC amps have a separate bias supply which puts on anywhere from -51 to -56 volts, for 6L6GC amps.

  This setup is a lot like the old Black Faced Princetons. You can add a pot to the bias supply. The first resistor in parallel to the filter caps may be a 10-K ohm resistor. Replace it with a 10-K-ohm linear pot, but be sure to keep the other fixed 10 K-ohm (or it can be a 68 K-ohm) resistor in place.

  If the resistor is only one fixed 27 K-ohm, for instance, replace this resistor with a 10-ohm pot and a 22 K-ohm fixed, in series.

  You will still need to buy a match quad of 6L6GC---as the pot can only adjust the negative voltage to all power tubes. But, at least you'll be able to compensate for weaker or stronger sets of tubes.

  One note: If this is one of those "Simul-Class" amps which can take either 6L6GC or EL34---I believe that these have some type of auto-biasing circuitry. Plus, a hybrid cathode biased/fixed biased setup. Adjustable bias would not be as simple.
  
  

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