[Amps] Nonlinear Thoughts

Radio WC6W via Amps amps at contesting.com
Wed Oct 22 11:55:49 EDT 2014


  I've been reading through all of the recent solid state amplifier postings and, having been working along similar lines for around a decade, I thought I'd contribute a few data points.

  After much consideration, I've abandoned the idea of using solid state devices in an Envelope Modulated configuration. 

  I am still pursuing an Envelope Modulated architecture employing a tube final.  The test until will employ a 6146.  The modulator is a standard digital audio part operating at 200 KHz driving a bridge of power FET's.  The 4-pole output filter(s) are at 25 KHz.  Designing a single filter becomes problematic at impedances greater than 1K or so due the self resonant frequencies of available inductors.  Therefore, four bridge rectifiers driving four filters are seriesed on the output of the (switching supply) output transformer.  This also conveniently allows sourcing the screen at 1/4 of the plate supply.  An auxiliary supply and LF amplifier is connected in series with the screen source which may be controlled to envelope correct this final amp.  When/If this design becomes functional a legal limit version utilizing a 4-400A will be attempted.  This thingy is under slow construction, impeded by such unnecessary excursions as an autoswitching dual voltage
 heater supply including soft start so that 6883's may also be used.  And the mechanics for an autotuning system with motors clutched to knob tuned Plate & Load capacitors.

 I'm still contemplating a phase modulated bridge, similar to what's employed in the Broadcast Electronics 4MX series, for a solid state final.  The 4MX employs ordinary power FET's since it only has to operate up to 1.8 Mhz. (Much!) Better transistors will be required for HF.  I plan a test unit using ancient Siliconix VMP4's operating at 50 watts.  Operating at 50 ohms eases the output coupling and filter requirements.  To translate this to the legal limit level it may be best to pray for some appropriate GaN parts.  The input of such a device may be opto, or dielectricly, isolated at the digital input and a simple 1:1 50 ohm transmission line transformer can isolate the RF output from a direct line operated power supply.  A series LC filter is connected in the primary of this output transformer and a double Pi-Network on the secondary will provide output matching as well as harmonic suppression.  At legal limit power, the caps will necessarily be
 small vacuum units.  Envelope correction for this scheme will require a small (50nS / Freq. or so) adjustable delay inserted into one side of the bridge drive.

 Both of the above designs will be driven by a DSP based exciter, capable of AM, FM, CW & SSB, employing some standard Audio DSP parts, a few Microchip PICs, and an Analog Devices Quadrature Up-Converter which can output directly on frequency.  The exciter portion of this design has been stable for about 5 years and is under slow construction.  A receive function is also contemplated with a "conventional" front end, including somewhat less than standard IF gain, and a DSP baseband demodulator.  

  Operating designs of this type to "zero" output in SSB mode is problematic.  I've considered adding a low level signal (say 35-40dB down) "out of band" in the tens of Hz region.  This would be just a curious artifact on the air and be filtered out by anyone listening to the desired signal.   

  Just a thought on heatsinks.  Check out the heatpipes from the Apple G5 (which are currently seeking their place in the landfill).  They have an inch square base and are probably good for a couple hundred watts each. 

73 & Good morning,
   Marv WC6W

http://qsl.net/wc6w/


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