I wanted a bit more power out of my uBITX, especially on the 20m and 30m bands, so I could operate with relatively inefficient portable antennas and still be heard (without the hassle of a separate linear amp).? So, after sifting through the suggestions here and over at ubitx.net, I implemented the following three relatively easy mods:
- As suggested by Howard (WB2VXW): Added a 27 uH inductor in series with R86, and 220 pF caps in parallel with R87 and R88.? That improved power output at 14 MHz and higher frequencies.
- As suggested by Bill (K9HZ): Replaced RV1 with a little daughtercard that uses three miniature relays (triggered by Q17, Q18, and Q18) to switch in a separate drive pot for each band group.? That way you can back off drive levels on the lower frequencies without sacrificing so much power on the higher frequencies.
- As suggested by many folks, I separated the PA and mainboard power supplies, and upped the PA supply to 24 VDC (while keeping the mainboard at 13.5 VDC).
For anyone contemplating those mods, here is a list of the resulting output power level and power supply current level for each dial frequency (into a dummy load):
- 3.573 MHz / 22 W / 2.6 A? (Initially 28 W, so I backed off RV1d a bit)
- 5.347 MHz / 16 W / 1.96 A
- 7.074 MHz / 22 W / 2.33 A? (Initially 28 W, so I backed off RV1c a bit)
- 10.136 MHz / 12 W / 1.58 A
- 14.074 MHz / 20 W / 2.13 A
- 18.100 MHz / 12 W / 1.61 A
- 21.074 MHz / 11 W / 1.40 A
- 24.915 MHz / 9 W / 1.25 A
- 28.074 MHz / 8 W / 1.24 A
Again, all of these tests were into a dummy load while using a 24 VDC supply for the PA , and 13.5 VDC for the mainboard (from a little LM2596 buck converter inside the chassis, so I don't need a second power supply).? I used the tune button on the WSJT-X software to drive the audio input (audio frequency set to 1500 Hz).? The finals got a little warm, but not too warm to touch, while I was adjusting the RV1a, RV1b, RV1c, and RV1d pots.? Nevertheless, I added a little DC fan (which will be switched on and off by a thermal switch, once the switch arrives).
Note that I was getting about 11 W at the lower frequencies, and 3 or 4 W at the higher frequencies, when I first assembled the radio (using a 13.8 VDC supply).
I was initially reluctant to mess with a 24 VDC supply, in that all of my portable power packs are 12 VDC.? But I found that a pair of 7AH Apex AGM batteries would only set me back $35, shipped.? That should be enough power to last a day or two at a far higher TX duty than is typical for me. (Current draw while receiving, with the fan on, is 148 mA.)
I also ordered a DROK boost converter ($11.99 on Amazon) that is supposed to be able to supply 24 VDC (up to 4A) from a 12 VDC supply.? That may make it practical to power my portable setup from a single car battery, assuming that boost converter is as quiet as my buck converters have been.??
To summarize, I now have as much power as I want for digital modes on all bands, even under lousy band conditions.? It has been a blast tinkering with this thing, but I'm going to try leaving it assembled for a while and focus on making some contacts.
73,
? Mark (AE7TO)
BTW, I haven't messed with bias levels on the finals, and don't plan to.? I'm confident those were adjusted correctly at the factory.
Mark, KE6BB