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power bandwidth, help

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ok read voltseconds guide to reactive load lines.
Can anyone provide a little more insight on how to calculate the -3db full power lo freq point, as opposed to the small signal one?
Presumably this is the saturation point, too?
Finally, as well as tube distortion, caused by the shifting load line, we also get transformer core distortion lower than this point too.
thanks in advance.

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Re: power bandwidth, help - mqracing 15:43:30 03/25/05 (2)
gelatinouscube asks:
"...how to calculate the -3db full power lo freq point"

the minus 3db point (in a large signal model) is acheived when the impedance of the inductive reactance equals the reflected primary impedance.
And the phase angle of the load impedance will be 45 degrees.

msl
- ok, plse confirm - gelatinouscube@hotmail.com 14:34:22 03/26/05 (1)
  ok, so using one of Paul Joppa's eg's, presumably this is correct?
  with a primary of 2500, this makes 22500x0.707=1768 ohms
  now 1768=2pi f L
  if L=20 henries, quite large, then the large signal point will be 13.4 hz, that seems very low?
  Certainly too low for a single ended saturated core?
  - No, ... - Paul Joppa 19:25:50 03/26/05 (0)
    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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