May 21, 2010 I made a new toy: 30W PA for 50MHz with MRF450A
I bought yesterday an old Zetagi 70W CB PA for 27MHz, not working or, I believe that it was more about the standard CB operator… that does not know much about what is he using. I received the PA to repair it, then I told him, that is not suitable for use as PA, neing unstable with CB transceivers exceding 3,5Watts optput. So I bought it from him for the enourmous sum of 50 lei – that means about 12 euro.
I changed almost every component and improved the circuit for class AB operation, and i’ve inserted some new circuit that works well for giving a visual information about the output power. The MRF450A power transistor is working up to 30MHz if you look at the Motorola Datasheet. Really? It gave me better than 10dB gain at 50MHz, althought it is a 13dB gain RF transistor for 27MHz.
This new, improved, YO5PBG🙂 6m all mode PA, gives about 28W (53V/50 ohm) output power for 2,25W (15V/50 ohm) from the transceiver at 12,7V supply. Should be better than 30W at 13,5~13,8V. On the Internet I found another 50MHz MRF450 PA based on old CB amplifier, located here (en francais).
Here is the schematic (click the image to enlarge).