Author Topic: 15 watt HF Amplifier  (Read 110 times)

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Offline k7rmj

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15 watt HF Amplifier
« on: February 07, 2017, 05:33:43 PM »
I started resurrecting an old project that I started a few years (27) ago. It's a final amplifier stage for a home brew 15 watt transmitter. I want to build an amplifier for the little CatsCan-3 transmitter. The circuit and details are in an old book from the ARRL called QRP Classics dated 1990. That should have been my first hint. It is based on an article that appeared in QST magazine dated August, 1978.

So, first I discovered that the PA transistor, MRF449A, is no longer available so I checked around the normal places for old power transistors. VIOLA! I found one at RF Parts Company in San Marcos, CA. It costs $27.00. not too bad, but they currently have none in stock and no ETA when they might have any more.

Hmmm... Just out of curiosity I checked the stats to find a modern substitute transistor. Good old Mouser Electronics had lots of substitutes but none fit exactly. The least expensive sub they had was only $29.95 plus $8 shipping and it didn't have the power rating I wanted. It was only good up to about 10 watts. The others they had started at $69.95 and up! Geeez, I almost swallowed my tonsils LOL.

The specs on the transistor are:
= Stud mount (screw type)
= 12.5 volt
= 30 MHz
= 30 Watts max output
= 12 db gain (collector/base gain hfe
= 50% efficiency
= VEBO 20V
= VCEO 40V
= Collector continuous current 3.0 Amps
= Collector peak current 5.0 Amps

The reason I want a replacement with Stud mount is that I have already built the circuit board for it and wound the RF chokes, transformers and filter inductors. They are all toroid type inductors.

Does anyone have one of these old MRF449A transistors laying around collecting dust that I could beg, buy or steal?  8-)

Or, should I just build a small 5 watt amplifier for this little QRP transmitter? It only puts out about 300 mW so a 5 watt amp would not be real difficult to build.

73 DE K7RMJ  Frank