Major steps on the Gu74B 1 Kilowatt amplifier

Some major steps has been taken since the last post. The structural part of the chassis assembly is finished. (When you build an amplifier; actually half of the work is in the workshop). What is missing now are the holes in the front panel for switches, shafts and metering.

I will also start posting parts of the circuit diagram below.

Overview picture of the amplifier:

I have started with parts of the Mains wiring, mounted the blower and the socket in the pressurized sub chassis. The air intake is in the upper right corner, just to left of the Power Entry Module. The blower the then drags air through the PS compartment, and then through a hole in the wall between PS and the RF deck. The air exhausts through the tube cooling fins and up through a chimney and finally through a hole in the top cover.

Detail pictures:

The blower is obtained from the very helpful people of Airflow UK; http://www.airflow.co.uk/. It is rated 31 CFM at 0.5 inch back pressure, which is 50% above the tube specification. I hope this will be enough considering that the airflow will be somewhat hindered by all the components in the PS compartment and in the RF deck.

All wiring follows European standard; blue for Neutral and brown for Live!

Parts of the Mains wiring. All connections are shrink tubed to prevent accidental contact!

Safety interlock switch. If the top cover is removed. HV is instantly disconnected. But of course; you should never take of the top cover on a live High Voltage Power Supply! Still you also have to think of the capacitor bank which can take up to a minute to fully discharge. But more on that when we come into the filtering section!

Heavy Duty relays in the Mains circuit to cope with high current surges!

Circuit diagram of the Mains wiring:

Here is the first part of the schematics. I will add more as we get further into the project. Here is how it works:

When the Mains switch in the Power Entry Module (The Power Entry Module contains switch, fuses and EMI filter) is turned on power will be present in the control transformer, blower and mains indicator on the front panel. In this stage the amplifier rests until the HV ON switch on the front panel is activated. When HV ON is switched, 12 volts DC will be present on the first relay. When the relay is actuated 230 volts will be present on the HV transformer primary. But the current will actually start flowing in two steps. For the first half second the current will be limited by the resistor in series with the Live lead to the transformer. When the capacitor is charged, after about 0.5 seconds, the second relay will actuate and bypass the resistor. The purpose of this step start is to take stress off from the large capacitor bank in the DC circuit of the Power Supply.

At this stage; grid voltages will be applied before the cathode is heated. I will have to adress that later with some kind of delay circuit.

The circuit diagram is done by the help of TinyCAD. An Sopen Source software available at: http://tinycad.sourceforge.net/

Update on the Kilowatt amp project - bench testing of voltage doubler

Here is a picture of the voltage doubler board under bench testing. For the test, the transformer primary is fed with a 11 volts AC source.

NEVER do a bench test like this with full High Voltage!!!!!

The transformer is rated 900 volts at 0.8 Ampere, with 230 volts on the primary. With 11 volts AC on the primary the calculated DC output after voltage doubling and filtering should be 133 volts. On the picture you can see the DVM reading 126.8 volts. Very close to the predicted value!

The total capacitance of the capacitor bank is 47uF and the voltage rating is 4000 KV. Under no load conditions, that will give a safety margin close to 50%. I'm contemplating reducing the number of capacitors to 8 in order to save som space. In that case, I would have a safety margin of 18% under no load conditions. I'd be happy to take comments on that!

I have also continued with the metal work of the cabinet. It is almost done, and I will update with pictures in the next few days.

A weeks work on the chassis for the Kilowatt amp project

The first step in building an amplifier is to make the cabinet. The cabinet is made from scrap aluminium sheets and angle stock. The total cost of the aluminium was about 40 Euros. The size 17x15.5x7.5 inches. It took about a week of evening work to complete it.

In the upp right corner you can actually see the very first component fitted! Yes!!! It is a double pole mains switch with EMI filter

Update on the Kilowatt amp project

The most important update is that I in the final moment decided to scrap the double triode project based on GI7B and go for a tetrode design instead. The main reason is that I felt that no one knows about the supply of GI7Bs in the future. It is no doubt a powerful and cheap triode. But at the same time, very few comercial builds use the tube. So I decided to go for the more common tetrode GU74B/4CX800 which is used in many comercial builds. I ordered one from Dr. Alex in Ukraine. The service was excellent and the price fair. The tube arrived only a week after order. Here it is:

The tube arrived in a sealed plastic bag with original documentation in Russian. The document was stamped 1986/01/16. In the same package I also bought a socket and a ceramic chimney:

Contact with FT5GA - Glorioso DX Expedition 4th mot wanted DXCC

I've heard FT5GA on many occasions on 20-17-15, but I have not had the time to chase them. But today I managed to log FT5GA on 17 meter. The Pile up was huge, but I got through after about 15 minutes. I think I was lucky because many strong signal were calling everywhere 5-20 up. I just selected a spot about 7 KHz up where it was empty and suddenly I came through. Now, lets see if I can log them on 20 and 15 too! And, perhaps 12 and 10 opens up?

This is how my "score-card" looks: