For the UKAS contests in 2015-16, I was using a very old
A200 amplifier which would only produce around 35W, ok for the AR section and
absolutely fine running off the car battery.
However, once I had bought my generator, I was able to pull an
unfinished project out of the cupboard – the SSPA. I had bought this module from Hellenic RF Lab
in Greece a few years earlier and put together the metalwork but just hadn’t
got round to powering up.
The module is supplied fitted with an MRF6VP11 device, quite
capable of the legal limit. At full
steam, the efficiency is around 80% - so 200W of heat to be dissipated. I used the Marston Ultra-Fin heatsink fitted
with the recommended Papst 3312 fan – this combination should have a thermal
resistance of 0.08C/W implying a temperature rise of only 16C. I have used four long screws through the
heatsink to hold the fan in place.
Annoyingly, the fins of the heatsink flex when drilled so it looks a
little untidy under the fan. Don’t
forget the finger guard on top of the fan.
The fan doesn’t seem to create a massive airflow (compared with fans
used to blow a 4CX250) but all seems to work ok.
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| View inside amp |
The output connector was screwed directly to the heatsink /
groundplane and with the high RF currents, it seemed like a good idea to
maintain this. I’ve used a Perancea CFL3
case which has a fixed base and removable lid.
I made a cut-out in the bottom of the case so the heat spreader sits
flush onto the heatsink. Cutting out
this rectangle was a bit painful with my limited tools (hacksaw and flat file) and
I plan to use the version of the box with two lids for my next amplifier. The
output N connector is screwed directly to the end wall, then the heat spreader
is screwed down to the heatsink. When
supplied, the input was also fitted with an N-connector which I removed and
replaced with a short length of RG174 and a BNC connector for the input feed.
DC at 50V (and 20A) is fed in via a pair of chunky feed
through capacitors and I’ve used phono connectors for the bias volts and fan
12V supplies. The fan supply is fed back
out on an SMC connector, it was easier to use a small screw-on connector to
prevent the possibility of the cable loom pulling the wires off the fan. I’ll write up the details of the main PSU in
another blog entry.
The module arrived from Greece with the bias being fed by a
power resistor from the 50V rail but I wanted to separate this out so I could
turn the amplifier on and off using the PTT line. I installed a little jumper link so I can
select 50V or PTT. The gate needs to be
adjustable between 0V and 3V to set the standing current and there is a
multi-turn potentiometer into the gate feed and this also allows the bias to be
turned to zero for initial testing.
Testing
I was fortunate to be able to use work’s 50V, 20A, current
limited power supply so I could do initial testing on the amp at a protected 2A
and then turn everything up carefully.
The transistor data sheet indicates it should be biased to a few hundred
mA and with less than a Watt of drive should give lots of power and indeed it
did. However, the power output started
to fall off after a few seconds. I
realised the heat spreader was nice and warm but the heatsink was cold. Although the spreader and heatsink were
screwed together they were not connected thermally. A quick layer of heatsink compound soon
sorted that out and the amp is now temperature stable. I also found that turning the standing
current up to around 1A improved the gain significantly and probably did
wonders for the linearity.
So after a quick tidy up and covers on, the amp was ready
for use
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| Completed amplifier |
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