Adding a Linear Amp Hack to the Bitx40.
The Bitx40 is an amazing platform and VU2ESE has even included an option to select the power output level simply by changing what voltage is fed to the IRF510. This easily done using the on board power connector that supplies "juice" to the IRF510. At 12 VDC you have the typical 5+ watts (more like 7 on this end) or by upping the voltage at this connector to 24 VDC the Pout is now 20 watts. However at the higher power level you will need to increase the IRF510 heat sink size (significantly). Bill, N2CQR has done this and you can see this at his blog http://soldersmoke.blogspot.com
Here is an option to keep the Bitx40 at the normal power level (12 VDC) but switch in line an external linear amplifier. It takes but few parts and the modification is totally external to the Bitx40 board. I have an SB200 Linear Amp (heathkit workhorse) and with the Bitx40 as a driver the Pout is 100 watts. More than QRP but really great when 40 Meters is somewhat marginal.
The theory behind this hack is the panel mounted Push To Talk (PTT) line is modified to include two isolating diodes. The diodes essentially separates two functions, where one line, which connects to the PTT1 connector on the board puts the Bitx40 into the transmit mode and the second line engages a small reed relay to provide a closure to ground to turn on your external amplifier. The parts required include two 1N4007 isolating diodes (yes a bit of overkill but in the junk box) a 1N4148 "snubber" diode across the reed relay coil, a terminal strip, the 12 VDC reed relay (low current draw on the coil ~ about 10 ma) and a panel mounted RCA jack. You will need about two feet of insulated wire to make the connections.
The actual hardware installation part includes using a small piece of scrap PC board that serves a base for the Reed Relay. Essentially I super glued the relay upside down to the PC board and then mounted the board to my case (10X6X2 metal chassis). For the ground side of the linear amp contact closure, I simply soldered a wire to the copper PC and the other end to the reed relay. The other end of the relay has a single wire (blue) running to the rear mounted RCA jack. In my initial install of the Bitx40 I used a 5 lug terminal strip for the power connection/power switch. That only used up 3 of the terminals so the remaining two lugs were perfect for the linear amp switching hack.
Whether the linear amp is in line or not is controlled at my linear amp itself. An additional hack might be to include a panel mounted toggle switch at the Bitx40 that would "open" the amp control line. Since I had a bypass capability at my amp I did not install the additional toggle switch.
One advantage to this hack is that does not require you to have 24 VDC plus bigger heat sink to get more Pout. I guess another advantage is that it keeps the rig pure at QRP levels especially when you feel guilty about being QRO. There is also the advantage that with a bit of padding on the Bitx40 Pout, you could drive an LDMOS amp to 600 watts out. It just plain gives you a few more range of options.