[AMPS] SS amps

John T. M. Lyles jtml@lanl.gov
Wed, 16 Feb 2000 10:37:35 -0700


A few years ago we built a solid state amp (tubes are still my 
favorite, but sometimes you cannot even find them to do what you 
need) that outputs 2 Kw CW into 50 Ohms, at 805 MHz. It never has 
been connected to an antenna, instead it drives a 1/2 mile long 3.125 
inch EIA coax line, which drives a dummy load at the far end. Along 
the line are 44 tap points with directional couplers, each feeding a 
small solid state amp to the input of a klystron. For 20 years we 
used an RCA cermalox 8501 tetrode to do this, but they discontinued 
the bottle. Its very difficult to find tetrodes for 5 kw levels at 
800 MHz ranges anymore. Low Power TV is all solid state for this 
range.

It took 32 transistors, bipolar MRF899's, to do the job. Running at 
22 VDC, this class AB amplifier is water cooled, in one 19 inch full 
height equipment rack. Each MRF899 contains a pushpull bipolar pair.

The first unit has been running continuously since about 1996, 
without any failures (except one solder joint opened on one 
transistor tab, which was hardly noticible to the output). The second 
unit went online in 1997 and also has an exceptional record of 
reliability.

I am convinced that a solid state amplfier can be built to replace 
all but the largest tube system (50 kW and up), at a cost. 
Reliability can be high, if protection and conservative design is 
followed. For instance, the '899s were rated for 150 W PEP, and we 
ran them about 80 Watts. Each is combined with a Sage Wireline 
hybrids. A radial 14 way combiner takes it up to the final output 
port. Lots of sensors on temp, water flow, collector current, VSWR to 
make sure that there isn't a fault to eat all that silicon at once.

The amplifier design (with photos and block diagram) was presented at 
the 1998 RF Expo in San Jose, also at the 1998 Linear Accelerator 
conference in Chicago. Those proceedings are available. I will email 
an 85K PDF version of the paper to interested amateurs.

John
K5PRO


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