Even a Blind Pig finds an acorn once and a while (35 watt push-pull RD30HVF1 amp for HF)

     I spend (waste?) significant time looking for RF bargains on the auction site that you all know and love.  Recently, I found a US-based listing for Mitsubishi RD30HVF1 MOSFETs at a very reasonable price (<$12/piece for 4).  So I bought 4 and thought I'd put together a 35 - 50 watt HF amp for my Hermes Lite 2 that would perform a bit better than the typical IRF510/IRF530 designs that abound on the net.  A cursory search didn't reveal any examples to use for guidance.  So I looked at several RD15HVF1/RD16HHF1 designs that served as a starting point: 

1.  A couple of examples from 60dbm.com

2. KK4DAS's design that had evolved from Skelton's (EI9GQ) design.

I had actually built an example of Dean's design, and while it worked as advertised, I never quite got the input match a good as I wanted.  I figured that another try, using the RD30HVF1's, was in order.  The circuit I came up with lies below:

A photo of the test build follows:

 

Construction is patterned after the ubiquitous "Manhattan" technique, except pad locations were determined by inspection and outlined with an indelible marker.  The 1 oz., 0.062" clad was masked as marked with blue painter's tape and etched with good old ferric chloride.  The "bottom" (non-component side) was completely masked and served as a ground plane.  The only two slightly non-standard components are the output transformer and the bypass cap for the supply line to the transformer's center tap.   The output transformer is an RF600, 1:9 (impedance ratio) from Communication Concepts that I had on hand.  (You could easily build one from copper tubing, scraps of copper clad and FT-50-43 or FT-37-43 toroidal cores.)  The aforementioned bypass cap is a WIMA MKP 4 metallized polypropylene type.   The heatsink is ~5" square and was also a junkbox part.  The input transformer is currently a 1:1 (impedance  ratio) conventional transformer wound on a FT-37-43 core.

     After setting the idling current at 250 ma per device, initial testing showed an input SWR of <2 to 1 over most of the HF range (7 MHz and above).  The Pout vs. Pin curve at 14.1 MHz is depicted below:  

I obtained these results with no tweaking at all - hence the title for this post. Power gain at 28.1 MHz was still 19.5 db. 

     There is, however, more testing to be done. The input match could likely be improved by adjusting the input transformer turns ratio and/or adding capacitive compensation.  Detailed gain vs. operating frequency data need to be generated to determine if the output transformer could stand some capacitive compensation.  Of course, I'll eventually have to measure this pallet's IMD behavior and optimize device bias while keeping reasonable efficiency.

There's no amazing new technology here, just fairly careful application of existing,well-documented designs.  The only unique thing is that it worked reasonably well on the first try....