Fix: Uneven TX output across bands suggests you consider addressing the power fall off issue as frequency increases 

Why do something about the power fall off issue?

One of the issues that many constructors raise about the µBITx is the uneven output across the bands (from 3.5 MHz to 30 MHz).   The output decreases from over 10w on 80m to only 1-2 watts on 10m.   This fall off in power has been found to be caused by:

  • necessary compromises in the design of the push-pull PA stage
  • pre-driver and driver power fall-off, probably due to the relatively low fT of the transistors.

Some constructors will want to do something about the power fall off because the rig may put out too much power on lower frequencies (jeopardising the finals) or not enough on higher frequencies (making it more difficult to make contacts).  At the bottom of the sunspot cycle, every watt counts!

A number of solutions have been put forward including:

Alison’s mod is considered the best as they address the primary source of the problems: insufficient gain at higher frequencies in the pre-driver and driver stages.  There is no need to change out the MOSFETs.  The IRF510s are fine up to 30MHz and are not the cause of the problems giving low power out.

Special warning:  Increasing power output will also increase output of unwanted mixer products. You are advised to NOT overdrive the audio on any amateur band.   It is recommended that you do NOT operate the rig on SSB or digital modes above 17m  without checking your emission levels are compliant.

Some constructors have found unwanted products exceed US emission limits. Operating the rig on CW is not an issue on frequencies up to 30MHz. recommended fix recommends a combination of the following:

  1. Raise the power output on 15m and 10m by using either Howard’s mod (unlikely to give you a full 10w) or Allison’s mod (will give you around 10w output).   Replacing the six driver transistors with metal can 2N2222s in Allison’s mod should not be too difficult for most constructors.  You also need to change bias resistors and it is suggested you change Q90 as well.
  2.  Control the drive level by band or band groupings.   This could involve either of the following options:
  • installing a potentiometer on front or back panel to control drive level manually (i.e. a panel mounted potentiometer directly replaces RV1)
  • use  the relay and potentiometer solution from Bill K9HZ in place of RV1.

Replace driver transistors and some other parts (Allison KB1GMX)


Improve the gain in the pre-driver and driver stages of the TX amplifier chain by replacing the existing 2N3904 transistors with higher performance transistors and adjust bias resistors to suit the change in parts.   You will get much greater power out on all bands.   Changing C81 will flatten out the power curve (reducing power out on the lower bands, but not affecting the higher bands).  It should be possible to get 10w or more out of your rig on all bands.   Be cautious about power output greater than 10w as your heatsinks will need to be replaced.


Allison’s solution involves the following steps:

  1. Replace the 2N3904 transistors in driver stages (Q911, Q912, Q92, Q93, Q96, Q97) with 2n2222 transistors (in a TO18 metal can)
  2. Parallel up R941, R911, R96, and R942 to get 11 ohms each by placing a 22 ohm resistor across each existing resistor.    These resistors can easily be stacked if you use 1206 surface mount parts.
  3. Replace Q90 with a BFR106 transistor.
  4. Increase R81 to between 2K and 2.7K
  5. Change C81 to 470 pF

Relay and potentiometer solution (Bill K9HZ)


Three relays are driven off the LPF trigger transistors.  The relays select one of four potentiometers, depending on the band grouping activated in software to select the relevant LPF.

While it may not be ideal to add in three additional relays and four potentiometers, this 7 component solution is probably the most straight forward automated solution available.  It is pretty much guaranteed to work.

Remember that more than one relay may be activated at a time (depending on the band grouping).  Band groupings are 80/60, 40/30, 20/17, 15/ 12/10).  However, only a single potentiometer is ever in circuit.  This allows output of your TXCVR to be  set within a watt or two across all bands to provide around 10w PEP output as per the intended µBITx specification.

Circuit diagram


You can access the outputs of the relay drivers from the transistor side of each of D11, D12 and D13.   Remove RD1, and replace with four potentiometers switched via the 3 relays (one potentiometer is activated when the relays are all off, the other 3 potentiometers are switched in with the relays).

RV1 (drive control) from the circuit board and replaced it with a tiny board with three small relays and four 100 ohm 10 turn pots.  The relays are controlled by the KT1, KT1, and KT3 drivers.

With the addition of a transistor driver for an additional relay and one additional potentiometer you can now have a tune button that will tune your rig with lower power output.    Pushing a tune button while keying the rig will result in lower power output.  Alternatively, you can wire up a DPDT switch for tune with the second throw on the switch wired to PTT.

See the wiring diagram above provided by Mike N6CMY  which represents an underneath view of the circuit board.   The relays are similar to those used on the µBITX board i.e. HFD27-012-S available on eBAY.  The pots are 100 ohm available on eBAY.   Mike has used a 100 ohm resistor rather than a “pot” for 10M since this band has the lowest output and the pre-driver stage needs to run flat out.

The photo below shows Mike’s final install of the 3 relay version of the K9HZ mod (without the tune control).