Simple QRP PA 10W

This QRP PA was originally developed for use with the TC60 M160. PA published OK2BHW, now OK2HW, on Amateur Radio 6/1987. Because of that, that old Amateur radios are mostly unavailable to young amateurs, I would like to revive this simple tried and unpretentious PA.

The amplifier is in double-acting connection

The amplifier is in double-acting connection, which has its advantages. Especially in that, that odd harmonics are naturally suppressed by a balanced output connection.

Input transformer Tr1 (4:1) transforms the input impedance 75 ohms on 18 ohms. Tr2 excites the bases T1 and T2 symmetrically. The third transformer Tr3 provides a symmetrical load T2. Thus, each input transistor has an input impedance of about 9 ohms.

Tr4 at the output serves for symmetrical DC supply of collectors T1, T2. The two-hole nuclei will not be saturated, because the passing current in the windings connected to each other creates a magnetic field of opposite sense. Tr5 transforms the symmetrical circuit T1, T2 on asymmetric 75 ohms. I.e, that there is on every collector 37,5 ohmu.

So much for the principle of involvement. As for the actual design of Tr1, 2, 3, 4, 5 they are always wound on a two-hole core 6 threads bifilar, two wires with a diameter of 0,4 CuL twisted to be 2-3 threads on 1 cm of length. When connecting, the beginnings and ends of the winding must be observed, in the diagram marked with a dot.

Mechanical design of PA

I performed the mechanical PA without a printed circuit, so-called. by air method directly into a box soldered from cuprextite. PA is powered from a source 13,6 V. The effectiveness of PA is 50 to 60%, The tip is therefore used in a variety of regula 1 to 1,5 A. The final result of the power PA depends on the used transistors T1, T2. When used “better” transistors can be included 1,8 – 30 MHz. With the original KU611 transistors, the tip is suitable for 1,8 – 7 MHz. When using 2SC transistors 2078 it is necessary to pay attention to waking up. It can be solved by an attenuating element at the input of the amplifier.

One more piece of advice at the end. It is very practical to take the shaft of the VOX potentiometer out of the box. In case of relay control directly from TRX, We will release VOX completely. It would probably be appropriate to include a switchable L cell to accommodate the LW antenna (or a small transmatsch).

Used parts

– Tr1-5 transformers: see described above
– T1, T2: KU611, BD series transistors, 2SC or 2SC2078
– C – ceramic
– C el. – at least at 25V
– R – 1 W
– D1-3 KA501 and similar Si diodes
– Relay – any at 12V which has my two changeover contacts. ( QN…, so-called. Teplice (Cube of sugar) and similar types.

Article by Jindra, OK1XR. TNX!