• No, ... - Paul Joppa 19:25:50 03/26/05 (0)
  In Reply to: ok, plse confirm posted by gelatinouscube@hotmail.com on March 26, 2005 at 14:34:22:

  Mike said

  "...the inductive reactance equals the reflected primary impedance..."

  Inductive reactance is 2pi f L, reflected impedance is 2500 ohms in your example. A little algebra, and you get f=19.9Hz at 20 henries.

  That's the -3dB limit on the tube's ability to deliver voltage and current, assuming an operating point optimized for clean power delivery. It says nothing about the transformer.

  It seems sensible to use a transformer that can handle that much power, that is half the amplifier's maximum power (-3dB, remember) at the power band limit (20Hz in the example). That is equivalent to asking for full power at 1.4 times that frequency, 28Hz in the example.

  But all this glosses over the important questions of what your criteria are. An amplifier does not go from sweet to nasty the instant it crosses the line, it just gets gradually worse as you deviate from a resistove load line. The same is true for a transformer as it approaches saturation. Is 8kGauss a good design limit for M6? Or can you go to 12k? or 16k? or 18k (the actual "saturation" point, IIRC)? Same for a tube - what current margin do you really need at the "cutoff" end of the grid voltage swing? These things can easily change the power bandwidth by an octave either way, for the same amp and transformer. And, of course, how low do you want to design for? I have found a power bandwidth of 40 or 50Hz to be good enough for many applications; I prefer 20Hz when I can get it, and I can clearly hear better bass if it is 10Hz.
  
  

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