Hit me with your best shot baby...

Sunspots got  you down?  Having trouble being heard?  Feeling inadequate?

That can happen when you are running your little QRP rig, even if you have a good antenna.  However, there may be a cheap fix that will put some "boots" to your mini signal.

Introducing a tiny little amplifier that will pump up your QRP transmitter like Arnold working you out at the gym.

70 watt amplifier parts

Check out the parts all laid out.   You have two unidentified final transistors (they have their faces buffed off), transformers waiting to be wound, 2 SMA connectors for your input and output, and the advertisement that this amp will put out a maximum of 70w.  The cost of this amp?  About $17.  That's correct, if this claim of 70w holds true, then you will take your QRP transmitter and make that little mouse ROAR.

So the proof is in the pudding, and this amp kit which I just received in the mail is still just parts in the bag, however, there are some youtube videos of these amps built and putting out power, albeit, not the 70w that is advertised, but still, significant power.

Think about it this way, my Ten Tec Century 21 "novice" rig has an input of 70 w max.  Output power is about 40 watts due to the fact that I recently swapped out the old monster capacitor for a new one.  That 40 watts from the Century 21 is plenty to be heard.

Ten Tec Century 21 70 watt input transceiver 

The issue with using the amplifier above as an add-on to a QRP transceiver is the need for a relay circuit.  You need to key the amplifier BEFORE you start pushing RF out of your QRP transceiver.  As I mentioned in a previous blog, with my 140 watt amplifier (AN-762), I built a 2 relay (SPDT) to switch the amp prior to the RF hitting the amp from the exciter (in that case the exciter is an Elecraft K2).

So now you have your two options for an exciter.  1) a QRP  Transmitter which will allow you to switch on the amp the old fashioned way with a TX / RX manual switch  or 2) using a T/R circuit like the Versa T-R mentioned in a recent blog here.

Which do you choose?  Well that depends on your skill level and your desire for a "commercial" style rig.  There is NOTHING wrong with using a manual TR switch.  You will use a DPDT switch to switch over the input and output of the amplifier as needed between transmitting and receiving.

Hams have been doing that for 100 years, and it works just fine.

What is the point of adding this amplifier, or any amplifier to your QRP rig?  The answer is self-evident in that sometimes that 4 or 5 watts just ain't gonna get the job done.  In about 6 or 8 years from now, you will be working the world on 1 watt, but right now we have what we have.

In the next few days or weeks, I will be building the little Chinese (claimed) 70 watt amplifier.  When it is ready, then this blog will give you an opinion as to whether the amp is able to really get the job done, and if it has a clean output.

KEEP IN MIND, with this kind of power output, you must provide a 5 or 7 element low-pass filter for each band that you are going to work.  There will be some losses, but they won't be detrimental.

If you are interested in buying this $17 amplifier, then look on ebay and/or aliexpress, and you will be able to search around for one with free shipping as well.

One last and very important thing is that you must provide a heatsink to keep from cooking your final transistors.  There are appropriately-sized heatsinks on aliexpress.  You MUST use heatsink compound to create a thermal bond between the transistors and the heat sink.  The heat sink will need to be big enough to do the job.

Completed 70w amp with aluminum heatsink

The aluminum heatsink will need to be about 50mm x 100mm...maybe a touch larger, and should be fairly thick with cooling fins as seen in the picture below.

A 100mm x 50mm aluminum heat sink is more than adequate

The bias voltage needs to be set for 2.7 v.  This will be discussed in a later blog.

There is no real limit on how pretty you can make your little amplifier look.  You can add nice lettering on the faceplate of a custom cabinet along with a volt/ammeter.  The sky is the limit in terms of how fancy you want your finished product to be.

So that is it for now, next step:  building it.

73, Mark 4x1ks