By Toni HB9ASB, 13-May-2014

LF amp

The Ultimate 3 can be used as a CW transmitter, if the modulation type is set to TX and a key is connected in parallel to the second switch. With the 25W to 50 W output you may have some nice CW QSO on the low bands, even with relatively modest antennas.

This PA is easy to set up, has few components and it is fairly robust. If you forget once to connect the antenna, there are no fireworks. The output transformer is a 1:9 unun, wound trifilar on a EPCOS N30 core. The blocking and coupling capacitors are MKP.

PA circuit

The drain current can be set as low as 10mA. Intermodulation distortion is not an issue for modes like WSPR , QRSS and CW, since only a single carrier is sent out. But at 100mA bias current it delivers more power.

The PA is followed by separate low-pass filters for the 2200m and 630m band. They use only one single capacitor value ( 10nF ). I have used 500V mica capacitors (NOS from Russia). But MKP or ceramic should work as well, if tolerances are tight (5%).

The coils are wound on an Amidon T106-2 (72 and 38 turns)


LPF photo

With 100mW of drive from the ultimate and a 13.8 V Power supply, it delivers up to 25W on both bands. If you squeeze the ultimate, as I’ve done it (3 BS170 and 12V supply for the PA) you may get up to 50W.

The efficiency is about 85%.

In order to suppress the tendency to oscillate at higher drain currents, I have added a 2n4 silver mica capacitor between the gate and ground *. So even SSB would be possible on the 630m Band.

73 de Toni HB9ASB