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.