"Build this Solid-State Titan..."

So states the June (pp. 27-31) and July 1977 (pp.27-29) issues of QST.

Well.. I received my Communications Concepts AN762 kit.. I also decided to buy another heat sink, too so I can still complete the WA2EUJ amplifier from the 40m amplifier homebrew competition using the vastly inferior (and I'm not kidding for the price) RF Parts heat sink.  That amplifier uses four Mitsubishi RD16HHF RF MOSFETs in push-pull and parallel.  I have that 2/3 built but really I wanted a full 100W for the Flex-1500, the SDR-Cube and all of my other little QRP rigs I have lying around now.

One of my main goals on the SDR-Cube project is to have my own completely home built (rebuilt in the case of my SB-200 amp.  I'm counting it anyway) station.  I've wanted to have home brew HF be a primary (if not the primary)  station since day one of being licensed.

Anyway, the 33.5 year old AN762 (K7ES/OH2ZE) design is still as elegant as the day it was designed and really, it's the grandaddy of all of the newer designs including Push-Pull FET designs.  I've wanted to build one since I saw it in the first ARRL Handbook that I owned after getting licensed in 1986 (1986 Handbook).  I didn't have the money, or skills back then and I also didn't have any thing to drive it with...

Fast forward 25 years, and I have all of that now...

I bought the AN762-140 version, which uses the less expensive, and slightly less power (nominally 140W though I consider the amp to be a 100W design with these finals) rather than the AN762-180 version with the $50 more expensive MRF421's.  I also decided on the MRF454's because of the slightly better gain out of those devices which should better match the QRP rigs I have and the stages I plan on designing for the SDR-Cube's Softrock.  (With little actual noticeable on the air difference, 1 dB isn't worth $50 more for the MRF421's either.)

Anyway.. the Picture of the AN-762 kit is below:

I also bought CCI's larger heatsink, recommended for this amplifier, but probably big enough for the 300W plus amplifiers.  This thing is big.  Granberg suggested that 7.9" of an equivalent (AAvid Engineering 60140) is enough for the 180W version to run 100% duty cycle, CW at 40 deg C ambient.  This thing is the same size with the same fins but is 12" long.  I have a "frame" or an aluminum box that I have surplus that's roughly the same size but only 9" long.  That's still 1" longer than the original Granberg article suggested, so I'll probably cut the 3" off of this beast!

Needless to say this heat sink is bigger than any 100W amp I have on a rig is.  Here is an opportunity to build up something better than I can commercially buy so I am excited.  Usually one can only do that with antennas any more.  (The Heatsink is a BARGAIN for the price.)

The frame I'm going to use is below.  $2 from a surplus store a long time ago.  I'm going to cut a hole in the top where the AN729 board mounts so the MRF454's contact the heatsink directly. 

The dots are a test for mounting.  Those holes, plus four more for the MRF454's themselves need to be drilled and tapped (I think 4-40).  The thicker line is the 8" that Granberg suggested as the minimum for the 100% duty cycle (with no fan at 40C at 180W also) the thinner line is the 9" that the surplus frame is long.  I suspect I'll cut it off and save the other 3 inches.. otherwise I'll need to have 1.5" hanging off of either side.. which would be OK but ugly.  I'll have to decide soon.  With heat sink, bigger is better, tho.

Oh.. all of the toroids on the CCI amp are professionally machine wound.  They look beautiful and I don't have to wind them!  Yay!

I'm also going to mount the very fine HF Superpacker PRO LPF board that I received today into the same box to probably by controlled by an 8-bit AVR that I picked up several boards of ("Sanguino.cc" and the ATMega644 and ATMega644P 8-bit devices.. but I don't typically use them as an Ardiuno preferring to do real "C" instead.. since this is all simple slow control.. the 8-bit devices should be A-OK for this).  I've also homebrewed a VSWR bridge which is pretty simple to do for a power meter and VSWR protection... which will feed into analog inputs on the AVR.

Anyway here is a picture of the LPF board from K5OOR:

It is pretty nice looking with filters designed for the AN762 power level (The Superpacker Pro used basically the same RF deck design as this amp).  If I were to design a filter board, it would look just like this one (but I might have done the 12m/10m bank as normally closed with all the rest normally open in case the microcontroller failed for some reason.. but that's the only thing I'd do differently... This board was designed so a rotary switch could be used instead, so that wasn't an option.)  The filter kits that come from CCI are 300W rated, $14 a band and really too physically large.  This was about $20 cheaper, appropriately sized and has the switching relays for me.  Like I said it is perfect for this project.

Did I say, Yay! earlier about not having to wind Toroids on the AN729?  Well this will make up for it.  In the case of a filter bank like this at power, there is really no choice but to use hand wound toroids.  Ideally (and since I can because I can get access to test equipment) tuning and flattening the pass bands of the filters is a good idea.  So it's hand built and hand tuned anyway.  Toroids with "non-standard" values and hand tuning is needed in this design, so my complaints about being Toroid-ed to death by ham projects do not apply here.  K5OOR has specific instructions for winding that differ a bit from my standard way of winding and has L/C meter and other checks in here that should help, too.

I'm thinking I'm going to build this entire project "backwards" and build and tune the LPF's first.  This way I won't be tempted to run a poor IMD amp on the air even for "testing" purposes.  :O)

Then I will do all of the mechanical work on the amp.  The amp electrical build itself is very simple.  The mechanical work is the critical part (as with any power electronics) and will take the longest time.

I'm a little busier at work and at home now (as expected) so I expect progress on this and the SDR-Cube to slow.  I need to verify TX on the SDR-Cube tomorrow and maybe (if TX works) try it at QRP level on the air.

But this is like my 6 year olds at Christmas... I don't know which toy to play with first.  I did want to inventory and document all the stuff for the project.. but in addition I built the VSWR bridge I'm going to use with the AVR for power output indication and shutdown of the amp (VSWR protection).. just because I found all of the parts in my junk box for it.  (It's a simple "Stockton" design from the W1FB notebook, but I used SMT caps and a SMT Schottkey diode rather than a 1N34.)

I suppose once I test it (tomorrow?) I'll take a picture of it.. but it's a pretty standard design.  I made it on a smaller hand etched (with a Dremel and diamond bit) board that should fit near the amp deck and the LPF.

I do need to wrap up the SDR-Cube (for now to the "version 1.0 state") first though... so many toys.. so little time...

TTFN...