Saturday, 24 July 2021

Enigma Amplifier for 70MHz

 

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.






On the Enigma website there is a link to the 144MHz version built by Dave G4FRE and he recommended cooling of the output filter.  So I have put a couple of small fans on the end wall to blow a little bit of air around.




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


Thursday, 4 March 2021

2021 Lockdown contesting

 For the 432MHz AFS contest on 7 February, I operated for the first time from home.  



Mast on the drive with 28 element M2


  

The joys of living in a three story house - this is the view looking NE from the bedroom window and with the mast retracted


Finally, the operating position next to the front door.  Feeder and rotator go out via the letterbox



And after all that, I won a certificate - leading fixed station running 25W or less