W6JL's 50 W Power Amplifier
Updated 14th March 2015, PCB foils added for DIY builders.
PCB foils (pdf file actual size, Set print to no scaling. If anybody wants the vk3pe Gerber files for having the PCB made at, for example, www.elecrow.com, then email me.
The original 50W PA design was the winner in the QST Magazine homebrew competition for a home-brew 40M 50W PA to be built under US$125.00. In fact, W6JL, built his winning design for under $30.00 This page is a slight adaption of that design.
The details were published in QST in June, 2010 and if interested, you will need to chase up a copy of QST as (I assume) it is copyright. The original article and further information is on the QST web pages in the "in-depth" section, but you need to be a member of the ARRL to access this information.
NEW Jan 2016, A search of the web found this DOC file for the project... W6JL 50W PA Word doc file
The design uses two low cost FET's, type IRFZ24 in the usual push pull configuration with bias being obtained using a constant current supply and adjustable. Peak power is about 65W at the design centre of 40M but slightly lower power levels are obtained at 80 and 20M. Typically 45-50W. Gain is about 10-12dB.
By the way, the QST web page "in-depth" section mentions even higher gain, in the order of 20dB, being obtainable with some alternate devices.
I obtained the IRFZ24 FET's from Farnell at about AUD$3.00 each but W6JL says they are available in the USA for under US$1.00
I made a hasty PCB design and had the PCB's made using an eBay vendor at US$63 for 10 boards. (about AU$80) [sorry, no PCB's available] The original design used "Manhattan" construction (no pcb) and had a relay for switching the Amp in and out. I didn't need this feature. I also added provision for an input attenuator. In my haste though, a couple of gremlins got into the PCB so I had to cut a track. Like most PA's, it must be followed with a suitable low pass filter. W6JL detailed suitable designs but as I wanted to try the Amplifier in my PICASTAR, (home brew HF SDR transceiver designed by G3XJP) as it already has LPF's in it.
W6JL suggests that IRFZ34's can also be used and better power output will be obtained at 160M also. So, since I had plenty of PCB's I made one of each type although not having the correct output balun core for the IRFZ34 version (version at left in view of both PCB's), I stacked up FT37-43 toroids. At one stage I could see over 80W output at 80M (with no LPF) but the toroids were getting VERY hot ! The input toroid is also an FT37-43 core.
You may have noticed the diodes fitted over the FET's. They are used to provide some bias reduction with high temperatures. Like all power amplifiers, a good heatsink is required. W6JL used a fan assisted heatsink obtained from a PC. My initial assembly below left, did the same, although the fan is not shown here. The diodes should be used with heat sink paste, not shown here.
PCB gremlins ! I had to slightly modify the PCB at the gates of the FET's as I had inadvertently placed the coupling caps. in the wrong place ! %*(&(^*&^%&% Likewise, the trim-pots needed to have their leads joined. (The pdf file at top of this page for DIY, has these errors corrected)
2nd August, 2010
I ordered another (specified) ferrite for the output transformer of the IRFZ34 version and have now fitted the PA to my home built Picastar transceiver, based on design by G3XJP.
I did find though, that the input of the amp showed very poor SWR especially over 40M when measured with my N2PK VNA. So, I removed the FT37-43 toroid specified and fitted a BN43-202 wound with 3 and 2 turns. This gave a much better result.
Since the driver amp being used is capable of 20W (G6ALU design) I fitted a 3dB attenuator at the input of the 50W amp. I had designed the PCB with this in mind.
On 80M I get an easy 50w output and made several QSO's
< click pictures below for larger view >
50W to left, G6ALU 20W driver on right, LPF a top.( Only 80M band LPF fitted in this picture)
The FET 50W PA board. Note input now uses a BN43-202 Balun core.
My Picastar all band transceiver (one of 7 I have built). Front panel is temporary, until I fit my colour display.
Page created on 3rd July, 2010 by VK3PE (e-mail via QRZ.COM)
Last updated on January 19, 2016