The G0MJW Pages


2m Station.

In some ways, 2m is my favorite band. When I started out, it was the lowest frequency I could use as a Class B licensee. I certainly have more rigs for this band than for any other. Rather too many in fact. I tend to acquire them. I am active on all modes, but mainly SSB and CD. My antenna is an 8 element moonraker, closely spaced along with the other beams (see the 4m page).

The home station has been upgraded recently, I am now using a transverter and the Ekecraft K3 rather than the IC7400. My main power amplifier is a Tokyo Hi-Power HL-350VDX that puts out around 300W PEP. I do have much more available using a pair of 4CX250Bs, which I use on special occasions. It is not practical to run very high power most of the time. All my high power amplifiers use valves and they tend to be noisy. The amplifiers get used for contests and special expeditions.


I enjoy building amplifiers and have made several for 144 MHz over the years. E.G. I have built

A single 4CX250B - Producing 250W linearly, 400W in class C. Handy size. I have sold this to make room.

A pair of 4CX250Rs.

The W2GN design - Capable of around 700W in linear service. It once produced over 1kW when I had an RF feedback problem during a sporadic E opening - the PSU protection cut in and prevented damage - but the valves were perfectly happy at 1kW output. It still can't run this power though, because the PSU would not last. It is a home made compact design, weighing only 30kg. This system has survived several contests at 400W output. The 4m PSU can also be used with this amplifier.

A converted AM6155 amplifier from the USA. This uses a single 8938 (A fat 4CX250B). It produces 300W. More if I push it. This one gave me a lot of trouble, I followed the modification instructions from the internet. If you have one of these, make sure you modify the grid circuit to increase the negative grid current capability. If you don't negative grid current, if it happens, drags down the grid voltage, causing even more grid current and a valve saturated at maximum anode current, though probably not for long before it expires.

There have been several previous amplifiers, including a Henry Tempo 6N2, a Nag144 XL, A pair of 4CX250Bs, a a 4CX1000 and a pair of 4CX350As. The later reminds me of the dangers of EHT PSUs. The first EHT PSU I ever built had 2500V and 6 1100uF 450V capacitors. I ran it up for the first time at a safe distance at the end of a long extension cable and hiding arond a corner. All was well. In order to be safe, there was a crowbar circuit with a HV relay and a 33 Ohm resistor. One day it tripped when the PSU was at full EHT. The bang was deafening and the resistor, a large ceramic unit, broken to pieces. Never again - next time I used a much higher value resistor.


Last Modified 24 August, 2008