Allison KB1GMX has helped me out of numerous battles with recalcitrant amplifiers. She provided an interesting contribution on the r2pro mailing list thread that I referenced yesterday:
Interesting thread...
I see Rick as having provided the basis and tools and it up to the collective 'US" to use them to
create that next generation radio. All you have to do is decide the performance and
then go about looking at the means to do so. All the blocks are there.
Dynamic range, how much is enough? When I'm portable or mobile raw sensitivity is
more useful as the antenna is usually a compromise. Overload is easy to handle with
switchable attenuator. The exception to this was a radio designed for contesting in a
hostile environment (a KW user 800ft away) if you burn power you get overload
performance. Its not a battery friendly radio (RX power is over 1A for headphone output).
Look at what you need and not what you want.
TX power is just adding stages. I've worked MOSFETs, LDMOS, GaN FETS and there
are some pretty cool devices out there and some not designated for RF are cheap.
If all else fails the IRF510 gets both raves and derision. At 12V its a tepid device
but at over 20V and at 24V it starts to wake up and really perform. I've run The WA2EBY
design for a few years at 45W level using two of those push pull at 28V and its clean and
solid and the original pair are now over 6 years old! I also run 8 of them (4x4push pull)
at 32V at 6M for a cool 210W with good IMD. I'd add all the good (high gain, low IMD)
power fets perform better at 28 or 50V. For those into CW consider class E as I've
worked with this and using GaN fets have generated 15W with 82% efficiency at
13.56mhz (includes driver and osc) and using the lowly IRF510 at 12V a full 10W
with 85% efficiency. Class E can be amplitude modulated.
As to the thermionic FETs, a 6AU6 crystal osc driving a 5763 for 10W gets a lot of raves
on 40M from a buddy that runs CW. The same deal plate modulated can sound good
at 5-6W AM on 75M. For those that want more a 6C4, 6aq5, 6146 will get you over
50W on CW and 25W AM. Change the bias a little and inject IQ SSB into the driver grid
and be running 50-80W PEP. A 12BY7 or 6CL6 driving a pair of 6146 will get you into
the 180-200W DC input range for about 100W. Remember the hybrid radios solid state
low level and rugged tubes for the heavy lifting. The Pi network (or Pi-L) will load anything
from about 28 to 100ohms more if you use enough taps and variable caps. That and DC-DC
converter for the HV are not terrible at 80% or better (even the 1960s transistor designs
were better than 75%).
In the end it all starts with the receiver. For that you can always start with a 1T4 RF and
a 1R4 converter and a 1T4 as regen driving a 3V4 audio. Power it with 45V (five 9V battery)
and a C cell and go portable. It should run for a very long while. Hollow fets run well at low
drain currents. :)
Allison
Gerard ZS5AAC
Steve Smith sent me a good article on broadband transformers. Reading through it, it occurred to me that perhaps using a heavier gauge wire in that PA output transformer might help. So I rebuilt the FT-50-43 transformer and put it in the final. No joy -- output was still down around 1 watt. Then I tried adding two turns to the secondary (on his web page Farhan advises experimenting with the turns ratio in an effort to improve output). Again, no joy. So I went back to the FT-37-43 transformer with 12 turns on the secondary. This yields the best results so far: about 2.5 watts. Still a bit low, but for some reason, the smaller cores seem to do better. Toroidal transformer tweaking to continue... Stay tuned.
I continue to tweak and peak the JBOT amplifier in the Azores 17 SSB transmitter. On this version I used some FT-50-43 toroidal cores instead of the smaller FT-37-43 cores recommended by Farhan. This morning I was experimenting with the output transformer. I seem to get noticeably more output with a transformer made with four FT-37-43 cores than I do with one made with four larger FT-50-43 cores. 
Over the weekend I built another JBOT 5 watt linear amplifier (design by Farhan). I used a nice piece of copper-clad board that Dave, W8NF, sent me (thanks again Dave). 
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics"http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
Maria wanted to go with a mathematical theme for this year's Jack-0-Lantern. Pumpkin Pi!
There it is -- my version of Farhan's JBOT 5 watt linear amplifier. JBOT stands for "Just a Bunch of Transistors." But I think Farhan is being modest. I really like the design, especially the effort to make this something that hams around the world can reproduce from readily available parts. For example, Farhan's original version used TV balun cores for the transformers. I didn't have any of those around, but I found three "mystery spec" binocular cores in my junk box. They seem to be working just fine. (I tested them a bit: Farhan had written that FT 37-43 cores would work. T1 has seven turns trifilar. Seven turns on my mystery cores yielded 13uH. On an FT 37-43 core 7 turns yields around 20 uH. Close enough -- these are, after all, broadband transformers.)
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics"http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20
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