Progress with the GU74B amplifier

After the summers antenna work it was time in October to pick up where I left with the amplifier last spring. I'm in to the second year of the construction now. But that is OK. The original time plan was two years. I have a small kid, and a lot of "ordinary" work to do. So the construction has to be done on my spare time.

The picture above shows the RF tank nearly completed. The coil mounted vertically on the bottom of the chassis is the 30-80 meter coil and the one above is the 10-20 meter coil. The latter is wound with 5 mm soft copper tubing from Biltema. Not visable on the photo is the L-coil mounted on the back of the band switch. The L-coil consists of two T200 ferrite cores with teflon insulated wire.

On 80 meter the tank was measured to Z=50 with X=3 on 3.750 MHz with capacitors set to calculated vaules. So the tank seem to resonate just fine. The only problem now is to overcome the limitations of the plate capacitor. The minimum capacitance jumped up to 40 pF when stray capacitances were added. This will present me with problems on the upper HF bands where a capacitance of 20-30 pF is needed. I played with different solutions like adding another L-section before the 10 meter coil or switch to a vacuum varaible. But I finally decided to find another air variable capacitor with two stator sections.

I'm also not happy with the loadline calculated for the tube when I was new to this. The original loadline translates into a plate load resitance of 1700 ohms. A plate load resitance of 2200-2400 ohms would be more correct for my project. So, I will most likely end up redesigning the tank circuit anyway. This higher plate load calls for a capacitance on 10 meter under 20 pF. A new capacitor is needed for sure!


Input network. Componentes are soldered on the underside of the tube socket. The tube will be grid driven with grounded cathode. The cicuit is very simple. I expect instabillty on the higher HF bands. But I will deal with that when time comes.

Connections to the grid box are made with feedthrough capacitors. RF input signal is to be fed through the SMA contact.

View of the RF deck from above. The empty space in the upper left corner will be occupied by the 160 meter coil.

Close up view of the 30-80 meter coil. I use pieces of delrin rod as insulating spacers.

Close up view of the RF-output. The RG-142 leads to the RF relays. The RF choke ensures that the antenna output is kept on DC ground. The choke also short circuits DC to ground if some component failure results in high voltage at the output.

Environmentally friendly radial system at SM0NOR

Environmentally friendly radial system at SM0NOR

In preparation for the contest season it is time to go over last years radial system in the lawn. According to plan all the wires layed out last September"disappeared" in the grass and became invisable by midsummer the following year. So, no problem there. However, recent remaking of the garden cut several wires and thus making the radial system inadequate. Above is a picture of the new biodegradable radial pins from DX Engineering. They claim them to dissappear in one year... in time for the grass roots capture the wires! Amazing, I'm proud to be the owner of the first environmentally friendly radial system in the neighborhood.

Thursday May 27th.

Now everything is ready for the concrete truck. The truck will arrive Monday morning at 7 am. Wow, I'm excited!

Here is a picture of the hole with rebar cage and support for the sleeve. The sleeve is a piece of bended sheet metal in which the ground post will slid in. It absolutely essential that the sleeve is positioned exactly vertically, or the mast will lean.

Plate and load capacitor installed.

Toroiods for 160 meter (the big one) and L-coil

And now a little side track. This is my new 12 meter antenna mast to replace the roof mountings. Eventually I will have a Steppir in the top.

The 7 meter bottom section lying on the ground. The 5 meter top extracts by means of a winch.

Rotor cage

Ground post. When retracted, the mast fold over the ground post for easy access to the antenna mount.

The dig! It will be 1x1x1 meter when ready!

Work in progress. The making of an air wound pi coil!

It has been a while since last post. The build has not stalled but the process has been very slow lately. The reason is that I have had too much work to do the last months. I have not been able to spend any time in the work shop. However, this Easter weekend gave me the opportunity to to do some work. The goal for this weekend was to make the 30 to 80 meter pi coil. The raw material is 3 mm copper wire which should be more than adequate for 1 KW in SSB service. Further on, this size should be able to withstand continuos service as well

The completed coil mesures 70 mm OD, 84 mm in length and has a total number of 18 turns. The measured inductance is 11,18 micro Henry and the calculated value is 12, 54 micro Henry. The needed inductance for the Pi-L tank is 11,04. So the result is perfect!

First step of the coil making. About 40 turns coiled up on a 60 mm steel tube. I used only 18 turns.

Four polycarbonate ribs are cut to size. They will support the coil and keep the pitch of the turns through out the length of the coil. Polycarbonate (Lexan) is a very good plastic materiel, far superior to the more common acrylic. It is a lot easier to work with machine tools since it does not splinter as easy.

The supporting ribs are drilled to accommodate the coil.

This is the time consuming part! The polycarbonate ribs are threaded on the coil, turn by turn!

The coil is ready! Nice and shining. It almost looks factory made.

After making this 11 micro Henry coil I realize that the amplifier housing does not have room to accommodate the 160 meter pi coil nor the L-coil for all bands if I make them as traditional air wound coils. For this I need to investigate how to use toroids in high power coil design!

Gu74B amp project progress!

It was I while ago I had some updates here. This is because of several reasons; one of them I will tell about further down. The other reasons is that I had a hard time finding meters for a good price. I searched Ebay and other sites hoping for some nice looking meters to show up for a bargain. Finally I gave up and bought two brand new SIFAM meters. Althoug they look nice, they were also very expensive. The other reason for the delay has been my fear of making square holes! Back in the days I never succeeded, but this time I actually got it right! The secret? The right tools and patience!

Here is the front panel with almost all holes done. What is missing are three shaft holes for the tank capacitors and band switch. As you can see, I succeeded very well with the square holes. The meters are beautifully aligned and so are the rocker switches. The over all finnish of the front panel may look ugly in this phase. But you should understand that the grey surface with blue lines is a protective plastic film on the aluminum. Once taken off, I expect a shiny and bright aluminum surface. I plan to anodize and print labels on it. Does anyone know where I can get the sheet anodized in the Stockholm area? Another alternative is to order a custom made front panel from the guys at Front Panel Express. It is very easy, you draw your panel with their software and you get the price instantly. You then upload the file to there workshop in Germany and about a week later you get a beautiful anodized front panel with holes and prints for about €120. But, I don't think that will be necessary!

Of course the panel meters are back-lit! (I will customize the scales later).

Detail picture of the wiring. I only use teflon or silicon rubber insulated high temperature wire. It is wonderful to work with, no accidentally melted plastics while soldering. But more costly!

This is the 900 volt AC post. I could not find high voltage rated posts, so this is a 600 volt post that I elevated from the chassis by two pillars. Will I have a flash over between the connectors here? I don't think so. Here it is "only" 900 volts AC and about 8-9 mm between the cables. But I'd be happy for any comment on this. Also the teflon wire is only rated at 600 volts. To increase the insulation factor I have led the cable through a piece of teflon tubing. Maybe this is over-kill, but it feels good!