My Greek 70MHz amplifier died a few months back and after a rebuild and a new transistor, I was unable to keep it stable so it got consigned to the junk/scrap box. Around Xmas, someone mentioned that Enigma Components in London were producing medium power amplifiers for 144MHz and when I looked on their website, they had also produced one for 70MHz. It uses the MRFE6VP6300 device, so 300W output, no problem with bad VSWR and a reasonable price. The amp is supplied in class C so suitable for FM but Nick at Enigma confirmed it could be put into class A by adjusting the bias potentiometer. The unit comes fully assembled and with detailed instructions on how to set up etc.
They supply the board mounted onto a heatsink so this turned
into a boxing exercise, shown in the following pictures. I used another Perancea tinplate case, same as my
other amplifiers. This is smaller than
the heatsink so I used 3 pieces of ally angle to hold it in place, the holes
are drilled and tapped into the heatsink.
A similar arrangement holds the fan on the top.
Bias and fan power are fed into the case via two (colour coded) phonos. The main fan supply is then fed out via an SMA - complete overkill
I think the only negative with the Enigma is the output connection, it’s not easy to get a really short-lead RF connection onto the pcb. At 70MHz it’s not too critical so I used a 25mm long piece of UR58 inner for the RF. A short length of braid and a solder tag link the groundplane to the case and the outer of the output N-type.
The amp is designed for the bias to come off the 50V rail
which I’m not too keen on so I cut the tracks and removed the associated Zener. Now my externally switched 12V bias feeds straight to
the on-board 5V regulator. As supplied the bias
was set around 0.6V but it was easy to readjust to the 2.5V required.
As usual, first switch on was using my current regulated power supply set to
30V and a limit of 1A. I brought the
bias volts up and set the current to 500mA – all ok. Then I tried a sniff of RF and got a few watts at the output – more importantly the amp was nice and stable. Then I slowly wound everything up, setting
the bias current to 1A. Eventually, the
amp was peaking at around 300W out, the transistor hardly gets warm so I’m
rather pleased!
Drive is 300mW, output is 300W, power input is 50V @ 8.45A = 71% efficiency