Meet the linears.

Being a VHF operator the big snag with linear amplifiers is that you generally need one for each band. You can get ones which cover two bands, but then you either have to use a compromise antenna or switch between the two. Also, I use three different rigs so there really is no help to be had there. I just have to accept that I need one for each band.

Therefore for those three radios covering five bands I have six linear amplifiers. Clear as mud?

Just to get this out of the way, I would rather have no linear amplifiers. Firstly I am not a fan of high power and I have always resisted running more than about 200W. This is just a personal bias. Anyway, I have found that I do fine with that sort of level. Secondly, I keep blowing them up. I can see why so many manufacturers shy away from building linear amplifiers. A lot of them are unreliable. Or is it me? Surely not.

I still resist running much power. In any case, there are all sorts of restrictions which would prevent me from doing it. The overall UK limit for 70MHz is 160W. With my present antennas at their current heights the OFCOM ERP rules limit me to 300W on 144 and 432MHz. 1296MHz is about to be affected by the new limits to prevent interference with commercial satellite systems, and for me this in practice means 150W. It is on this basis that I have set my limits for some time.

Recently OFCOM altered the rules for some bands but that did not affect the ones mentioned above. However, it did mean that I could run more power on 50MHz. Previously I had various linears for 6m, with power outputs of 100W then 350W then 500W, limited at the time to 400W. More recently I had been using 200W on 6m. When the limit went up to 1000W on 50MHz it began to dawn on me that I could loosen my corsets a bit. I dithered for quite a time before deciding to return to the days when I had a 500W potentially available on the one band I use where this is possible. Not that I use it all, but I could if I needed to.

50MHz - ACOM 500S

Acom 500S linear amplifier at GM4FVM

Although I could have gone for a much larger beast within the new limit, I decided that 500W maximum was a good target to aim for. I had good results at that level in the past. I decided on this one after having the loan of an ACOM 700. The 700 had 700W available which was more than I needed. Sadly it did not fit into the shack, but the 500 had the advantage of being much smaller and lighter. In addition the 500S also covers HF and 70MHz, should I ever need to use it there.

I was very impressed with the Acom 700. It worked with great ease, compared with the 500W Gemini which was my last 6m model with this sort of output. While the Gemini was screaming its head off and making the shack very hot, the Acom 700 quietly got on with its job. Happily, the Acom 500S has proved to be every bit as good. It has been here now for a couple of months and I am very pleased with it so far.

Immediately prior to using the Acom I was using my Gemini 4 on 6m. While the Gemini is rated at 300W, it could deliver only 200W on 6m. The extra power which the Acom can run is very helpful in adverse conditions.

70MHz - Gemini 4

Gemini 4 linear amplifier at GM4FVM beside its bandpass filter

I have had this Gemini 4 since 2016. In the meantime I have had three other Gemini amps for various bands but they have all gone now leaving just this one. Rated at 300W maximum, perhaps because it has mostly been run at just over 100W on 70MHz and for a short time at 200W on 6m, it has survived for a long time.

Unlike my other Geminis, this one is passably quiet. Loudest noise is the Tohatsu clunk when the relays engage. It does not have the "advanced cooling" of later Geminis, which also endears it to me. As it is under-run it keeps cool and works away without much attention. While it was away on 6m duty I missed the extra power on 4m, so I am glad that the arrival of the Acom has allowed me to bring the Gemini back to 4m.

144MHz - RM Italy LA-250

RM Italy LA-250 at GM4FVM

My previous Gemini 300W has gone for parts or repair, followed not long after by its replacement which was a Gemini 500W. My 2m linear is now an RM Italy LA250. Capable of 200W output, the RM is unusual in my 2m linear history in that it actually delivers what it claimed to be able to do. I installed it, it runs. It does not overheat, in fact it stays cool and does the job without fuss. There is a display which shows power out, SWR and temperature. The manual says that if the temperature goes too high the fans speed up - but they never have.

There are a few niggles. It is a very odd shape and it does not fit on any shelf I have without hanging over the edge. The relays sound like they are not high spec and it has PL-259 sockets rather than my preferred N-types. The output power meter really only works on continuous modes like data or FM, on SSB it shoots all over the place. None of this seems to get in the way of steady operation.

There is a cheaper version without the fitted fans which I thought about quite a lot. However, I might try meteor scatter or even EME with it so better cooling was required. So far it has never got warm to the touch.

This is my second LA-250. After the 300W Gemini had tried to incinerated itself (I think the fifth time that happened but I am not sure) I got fed up and bought an earlier LA250. I used that until I was moved to fix Gemini, which was about six months, and it never missed a beat. Then I sold it and regretted that ever since. So when I had a gap again the 200W LA-250 replaced the 500W Gemini and I have not yet noticed a difference in reports.

432MHz - Tajfun 1000

Tajfun 1000 linear amplifier at GM4FVM

The Tajfun has been here since 2020. I managed to blow a diode up thanks to a spike from my Icom IC-7100. I had excellent support from VH Electronics in Slovakia. Over the years the Tajfun has delivered what it was claimed to do. Rated at 500W for SSB and 300W for data, it runs this with ease. I use it almost exclusively for data. It has a turbine-like whine when under strain. Like the Acom and the RM it has proper advanced cooling.

The display covers PA voltage and current, forward and reverse power, SWR and it can be configured for various working conditions. The linear can also be set to run under contest conditions, which runs the fans continuously. 

At the moment I am having a problem with the display in that it sometimes freezes on transmit. I think I can fix that by reinstalling the software on the display memory card, which I must do soon. Not that it matters much, it still works fine.

The standard Tajfun 1000 runs 1000W with versions for 6m, 4m, 2m, or 70cm. The version I have is switchable between 2m and 70cms and only runs 500W maximum. On 2m it needs a bandpass filter (though I know one amateur who uses it on 2m without an additional filter). However, I almost always run it on 70cm.

1296MHz  - 2 x W6PQL homebrews.

 

150W W6PQL linear amplifier at GM4FVM

100W W6PQL linear amplifier at GM4FVM

My first 1296MHz linear was intended to raise the output power of 23cm to 100W. That linear was based on a W6PQL board and it came from Riccardo IK5CON, sadly now Silent Key. Although the Motorola XRF-286 should be capable of 100W output it only proved able to turning out 50W. That was a very useful improvement over the 10W produced by the Icom IC-9700. However, thanks to Sid G8SFA, I was offered a second amplifier also based on a W6PQL board. This amp has two devices but also failed to achieve the hoped for 150W output directly from the IC-9700 as it needed about 30W drive to achieve this. The answer was to produce a two-stage amplifier using the smaller one to drive the larger one.

The devices used on these boards require 28V DC and at first I used ex-LED light power supplies available second hand on e-Bay. When one of these failed I went instead for a new S-360-24 PSU which is rated at 360W at 24V. These generic PSUs offer good value but can have noisy fans. The ones with an S prefix usually have good protection against switch mode power supply noise and so far I have had no problems with this one.

W6PQL boards are in many of the home built 1296MHz linear amplifiers. My experience of them is that they are very effective and work well. The effect of the recent restrictions on 23cm output power appears to be that my output power of 150W is just about right for this band, though W6PQL offers boards with higher ratings and for various bands ... https://www.w6pql.com/600w+_33cm_amplifier.htm

Finally

I view linear amplifiers as an unavoidable necessity. Often they are unreliable so reliability is more important to me than having the highest power. Barefoot is generally not enough, but nor do I find the need to use a kilowatt. Being a VHF enthusiast I need one for each band, The solutions I have come up with work for me and at last seem to be settled for a while. Or so I hope.

73 Jim

GM4FVM