U3S Output #u3s

Phil Crockford

Yes I have done it. It involves a board with relays driven from the B0 - B5 outputs on the U3S board. Each relay controls the amount of Bias on the PA transistor via a potentiometer, one for each of the outputs.
It works well. If you want diagram and notes let me know.

No doubt there are other ways of doing it but this was done with what i has in my box.

Phil G8IOA

Sverre Holm

There is a bit that can be set on the U3s. It is used to change antennas in my U3,

If one output is attenuated, and the two outputs are then combined and fed into a power amplifier, one could regulate the output power in two steps. In theory this could be done even more accurately, as there are more than a single bit than can be set per frequency.





I concur with the others that running barefoot the ranges of output power is nice. On the middle bands with my verticals, 250 mw gets me half way around the globe with wspr.

For running an amplifier,  check the U3S and 5w amplifier instructions.  I recall Hans suggests a different output transformer design for that.




The answer is still no.  To get a level output you need a totally different circuit that has
uniform gain from lowest frequency to the highest.  The BS170 device can do that
but the circuit its in is not designed with that in mind.

The easy way is to live with what the BS170 does at 10M for your amp, then use a relay
or manual switch to insert an attenuator for the 80M (or several switches)  to make
it right levels for any lower band.  This is by far the easiest way.



Hi Vern,

While I haven’t tried using it myself, the U3S firmware has the ability to control auxiliary relays via the “AUX” setting.  Maybe you could use a relay switched attenuator to level the output power?

See application note A003: HERE



Vernon Matheson

Allison thanks… I guess I may have worded the note not quite right.


I am looking to have the same output from each band to drive an amp.


Presently my single BS170 will give me say 125mW on 10m and 350mW on 80m into a dummy load and that is with the bias the same for both bands.


I would like to be able to have say 200mW output from each one with the same bias setting into a dummy load.


I believe I can change the # of turns on the 25 turn toroid but I do not have the knowledge of anything else that can be changed.


My current problem is that if I run the U3S and amp with the 10m settings it’s far too much when you get to the lower bands and I have to keep changing everything.


Almost need 2 different setups.








Yes it can be done, no its not simple or cheap to do.

Even if it were, then you have antennas of varying performance.
For example a dipole at 16ft height on 10M is good, to get the
same general performance at 80m it would have to be 66ft up!


Vernon Matheson

Hi all, 

Has anyone been able to successfully balance by whatever method the output of the U3S so that the basic output is the same on 10m as 80m.

Is it possible?