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Author Topic: VHF 2 meter amplifier  (Read 2396 times)
KJ4YZI
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Posts: 38




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« on: March 06, 2012, 06:19:52 AM »

Can somebody tell me what this is, meaning, is this the amp, minus the enclosure and other switches? or is this only the board with the mosfet and a whole lot of other stuff has to go on it.
http://www.ebay.com/itm/140-150MHZ-RF-POWER-AMPLIFIER-PALLET-VHF-300WATTS-with-BLF278-or-MRF151G-/320696233516?pt=LH_DefaultDomain_0&hash=item4aaafc2e2c
Im really wanting a 300 watt amp, and am pretty good at building electronics, and always wanted to build one like this. however, my 160 watt amp draws 22+- amps of current, this board here  states current draw is only 12 amps.. for 300watts, that doesnt sound right at all.. it also says 52vdc , maybe theres another circuit needed to convert 12vdc to the 52vdc needed? any info would be appreciated, i would hate to buy something like this for this much money and not know what the heck to do with it. 73
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W4OP
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« Reply #1 on: March 06, 2012, 06:38:09 AM »

This appears to  be a complete amp but you will need to build the RX/TX bypass switching, a healthy heat spreader and heat sink. (Be prepared to drill/tap copper) and supply the 52VDC. 52V at 12 amps is
Much better practice than a 12VDC amplifier at these power levels. 52 volts @12Amps is 624W input and looks to be spot on for input power.

Dale W4OP
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WB2WIK
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Posts: 20545




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« Reply #2 on: March 06, 2012, 08:27:27 AM »

Or if you don't mind ordering in French, you can buy the same amplifier pallet for less than half that price here: http://www.cafr.ebay.ca/itm/140-150MHZ-RF-POWER-AMPLIFIER-PALLET-VHF-300WATTS-with-out-BLF278-or-MRF151G-/320696234985

 Wink
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K1ZJH
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Posts: 901




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« Reply #3 on: March 06, 2012, 08:34:59 AM »

Or if you don't mind ordering in French, you can buy the same amplifier pallet for less than half that price here: http://www.cafr.ebay.ca/itm/140-150MHZ-RF-POWER-AMPLIFIER-PALLET-VHF-300WATTS-with-out-BLF278-or-MRF151G-/320696234985

 Wink

Half price, but the FET isn't included in the cheaper auction...
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WB2WIK
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Posts: 20545




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« Reply #4 on: March 06, 2012, 08:39:47 AM »

Or if you don't mind ordering in French, you can buy the same amplifier pallet for less than half that price here: http://www.cafr.ebay.ca/itm/140-150MHZ-RF-POWER-AMPLIFIER-PALLET-VHF-300WATTS-with-out-BLF278-or-MRF151G-/320696234985

 Wink

Half price, but the FET isn't included in the cheaper auction...


Good point.  You're right and I didn't notice that at first.  It's not mentioned until you scroll quite far down the page.
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KJ4YZI
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Posts: 38




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« Reply #5 on: March 06, 2012, 08:47:17 AM »

well with this pallet, and FET, how complicated is a TX RX circuit.. the connectors, switches and heatsink are easy enough for me, but being that this is also a 52vdc board, i couldnt use this mobile, could i?
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K1ZJH
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Posts: 901




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« Reply #6 on: March 06, 2012, 09:51:03 AM »

48 VDC switching supplies are widely used and are available surplus.

Mobile would be tough, you'd need a 12 to 48 VDC converter, and the current
draw at 12 volts would be appreciable.

Pete
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WB8VLC
Member

Posts: 116




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« Reply #7 on: March 06, 2012, 04:41:43 PM »

That is simply the amplifier board only and as others said there is no t/r switch for input/output switching and also there is no Low pass filter and you'll need a 48 -50 volt, at around 15 amp, power supply.  It looks like it is adjusted for around 1 to 2 watts of drive with the input attenuator pad on this board.

Additionally the biasing is resistive voltage divider derived from the 48 to 50 volt drain supply so if the drain supply voltage sags under high peak conditions then the devices bias point will also change for the worse.

There is a lot of voltage being dropped in the 50 volt to approximate 3.5 to 4 volts which is required on the gate of the transistor in that ridiculous resistive biasing scheme used.

Also there is no feedback whatsoever in that circuit hence it could go unstable under various loads, the original MRF151 design that Helge did was into a matched 50 ohm load and when those parts were operated into a reactive load  with such a basic design the output power would fluctuate depending on the phase of the load presented to the amplifier and when this happened the amplifiers stability went down, way down.
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