New PCB available for WA2EBY linear amp #linear-amp


Ken K0KV
 

After running my uBITX barefoot for the past 18 months (with 1700 FT8/FT4 contacts) I decided it was time to add a little more power.  The ever-popular WA2EBY linear amp seemed like a good route, except that nobody offered the circuit board anymore.  On a whim, I sent an email to amateurradiokits.in to see if they had even one board left over.  Sunil, VU3SUA responded that the old boards were gone, but they were just a few weeks away from releasing a totally new board for this design.   I asked to be their first customer, and very shortly had the new board in hand.  Here's my review (unsolicited, uncompensated): 

Having seen pictures of the earlier board in past construction articles, I can safely say the new board is a big improvement.   Cosmetically, it has all the professional elements that were previously missing: plated-thru holes and vias, solder masking and comprehensive silk-screening.   The size is a few tenths of an inch longer and narrower than the original, which turns out to be an excellent match for an inexpensive 150 x 60 x 25mm heatsink that is widely available on EBay as of this writing. 

Before you start building, you need to make two design choices.  First, your input attenuation – pads and traces are provided for a standard pi-network attenuator, but it’s up to you to choose the amount of attenuation needed for your setup and then compute the appropriate size/wattage resistors. Plenty of websites can help with these calculations.  Likewise, space is provided for a single band low-pass output filter, but the L and C values are up to you.  Again, no problem –  go to any WA2EBY article/website/blog published in the past 11 years and you’ll find them listed.


A few notes related to the photo:
  •  yes, there are still surface-mount capacitors – 10 of them.  And yes, mounting them seemed scary at first, but turned out to be much easier than I thought.  Plenty of YouTube videos are available to walk you through that process.     
  • the IRF510 pins need to be bent up 2-3 mm to allow the MOSFETS to mount flush to the heat sink while still maintaining adequate spacing for the board itself.  
  • the board has mounting holes for SMA connectors at the RF input and output.  I don’t generally use SMA’s on my projects, and the holes didn’t match up with the ones in my junk box, so I just soldered some mini-coax directly to the pads.
  • two other connection pads allow you to route the input signal to an external attenuator and back, if desired.  Otherwise, these points (J4 and J5) need to be jumpered. 
Because this is a mature design, you’ll find a wealth of published articles available to provide detailed advice on everything from component selection to toroid winding to expected performance.  It’s like having multiple assembly manuals to choose from.  Construction went smoothly, and the amp worked perfectly the first time I applied power.   As always with this design, tune-up consists of simply setting the bias current on the IRF510’s.   Stability is excellent, and very little drive is required to obtain full power out.  At some point I’ll do more comprehensive testing, but for now I’m having too much fun operating with it.
 

As a final note, the original QST articles (Mar, Apr 1999) provide an excellent analysis of the thermal challenges that come with running a pair of IRF510’s at 40+ watts.  Based on this, I've decided to run my amp at 30 watts output for the time being, until I have time to construct a better cooling system.  A couple of dB output power just seemed like a small price to pay to spend my time operating instead of replacing IRF510’s.

The WA2EBY PC Board can be ordered from amateurradiokits.in.  It is currently available as a bare board only, but will eventually be sold as a semi-kit as well.  Until it’s added to their website, you can email an inquiry to the address shown on their Contacts page, they’ll respond with the details.     

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