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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