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Monday, February 22, 2016

Design Wisdom from Allison, KB1GMX


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

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