Monday, December 18, 2017

Adding an External Linear Amplifier Control circuit to your uBitx

Adding some big "shoes' to the uBitx!

So OK the concept of the Bitx and uBitx is to operate QRP and with the new design of the uBitx having 10 watts is an excellent power level for making contacts. After three days of operation on 40 Meters it was clear the uBitx needed some shoes. I have the shoes so the problem is how to control the external amplifiers.
The hack here is not new as I did the same thing about a year ago with the Bitx40 so it is a proven design.
First I looked at the uBitx Schematic and see that one of the pins on the Arduino is "HIGH" and by grounding that pin this causes a series of actions to put the uBitx into transmit. So now the problem is how key an external linear amp and do it in such a way that considers how to accomplish this without interfering with the normal operation of the uBitx and to also have an isolated control of the linear such that there is essentially a firewall between the uBitx and the amp. The hack should also not involve changing the sketch and be completely reversible.
A word here about the uBitx wiring. For over 60 some years my microphones have been wired so that the Sleeve on the microphone plug is ground, the Tip is the PTT control and the Ring is the Audio. All of my microphones are wired that way. The uBitx instructions have the Tip and Ring interchanged. That is not a problem so long as you wire the microphone jack and the microphone plug the same way. The construction photos initially through me for a loop as the Orange and Violet Wires on my Microphone Jack were reversed [in the photos] from the way I wired it. It was at this point I decided to read the instructions and sure enough Farhan made it clear that the microphone input was on the Tip. The sketch I will be providing is based on my standard and not the way Farhan has suggested. So LOOK at that carefully.
So here is the solution to add the linear control and the parts required are: two 1N4148 diodes (literally any silicon signal diode will work), a 12 VDC Reed Relay, some perforated board, two aluminum pillars, four 4-40 screws, two 2 Pin headers, 2 one inch pieces of heat shrink tubing and two lengths of insulated wire about 14 inches long. Finally you will need an RCA Jack installed on the back panel.
The two diodes are connected cathode to cathode and that common connection connects to the TIP connection on the microphone. One of diodes is connected to the Orange wire (anode end) that goes to the Raduino board. The other diode (anode end) connects to one end of the coil on the reed relay. Use heat shrink tubing so that after soldering the wires to the two anodes, the heat shrink tubing is slipped over the diodes and the wired connections --No shorts here.  The +12 VDC end of the Reed relay is connected to the 12 Volt rail at the power switch in common with the red and brown wires that go to the mainboard.
Essentially when the PTT is engaged both diode cathodes are grounded. On the Raduino side the PIN that was HIGH is now LOW and the uBitx is placed in Transmit. On the amp control side the Reed Relay  field coil is energized and the contact closes (short to ground) which is brought out to an RCA jack on the back panel. The two diodes prevent any voltages from being back fed into either circuit and effectively isolates the circuitry.
My uBitx now has some "big shoes". With this arrangement I can run a small solid state amp at 100 Watts or add in a Heathkit SB200 on top of that for 600 Watts to the antenna. I no longer get reports on 40 Meters "you sound really good only not much signal". Now I can use the uBitx to Run with the Big Dogs. On 20 Meters running just the uBitx I can hit about 700 watts using my homebrew 3CPX1500A7. Life is too short for QRP! 

Pete N6QW


  1. Hi could you use an Opto Isolator instead of the reed relay?


  2. Hi could you use an Opto Isolator instead of the reed relay?