måndag 14 december 2009



I've got some comments about the RF deck being empty so I thought I might upload some pictures of things that are going in there in the next couple of months. The main reason for the RF deck being empty is that I'm bent on having completed the PS 100% before starting with the fun parts. Back in the days when I built 813 amplifiers I always started withe the big tubes and capacitors and when it finally was time for the PS I was so eager to get it working that the PS ended up being a very hasty work. But I have learnt that a good PS is really one of the most important things both from a performance and a security stand point. Hence, I will not go into building the RF deck until the PS is fully operational. But, in the mean time, enjoy some pictures of the "big things":



Tank coil for 10-20 meters. I wound with 4,5 mm copper tubing



Very nice NOS ceramic band switch with silver plated contacts. I got from Ralf at QRO-shop in Germany. (http://www.qro-shop.com/)



Tune capacitor. 100o pF, 1,5 mm spacing. I bought it second hand from a amateur in Skåne.



Custom made Plate capacitor from Ham Center in Italy. 23-320 pF, 3 mm spacing.



söndag 6 december 2009

A weeks work on the GU74B Kilowatt amp project

The work has slowed down somewhat since the last post due to other things. But last week I felt that it was necessary to start again. I'm starting to realize that this is actually a hugh project! Some things just takes a lot of time and you don't really see any progress. But this undertaking is like any big project; it is about commitment and persistence! It is not always a perfect day to do some work on the amplifier. But when you feel like that you have to go there and at least do something. Otherwise there is a big risk that the project ends up on the shelf. I'm, however, determined that this is not going to happen.

Ok, let's g over to pictures and tell what has been going since the last post!


Overview of the amplifier. PSU compartment and RF deck.

From left to right:
The first boards are the two stacked G3SEK tetrode boards. Mounted on the sides are two power resistors that are part of screen grid circuit.

The board in the middle is the new redesigned voltage doubler. When the voltage doubler was completed the first time i realized it was too big and heavy (see earlier post for picture). With 10 400 volt NOS capacitors it was so heavy that the circuit board was bending under the weight. The new board is designed around 8 450 volt low-profile capacitors I found on Mouser. They were expensive at almost 6 Euros a piece. But it was well worth it, given the fact that the result made it possible to build everything smaller which in turn will make it possible to accommodate a second HV transformer later on.

To the right you can see a smaller toroid that carries 18-0-18 volts for the control boards and relays. Also here you can see two hefty relays with associated power resistor for HV control and soft start.


Side view of the PSU

On this side view you can see that everything is actually built on two floors. The two stacked G3SEK boards are clearly visible with a large heat sink beneath. Then the voltage doubler and then, to the far right transformer for control voltages and power entry module.

On the second floor I have bolted the HV transformer to the bottom of the chassis together with terminal blocks and 230 volt mains wiring. As you can see here, there is room for another toroid exactly like the first one, which would make it possible to upgrade the system to 1600 VA input instead of the current 800 VA. 800 VA input is basically fine for 1 Kw+ PEP in CW or SSB. But should I like to operate in 100% duty cycle I need more iron! I don't know if I ever go there, but it is nice to have the option.

Next step now is to hook up the G3SEK boards to 18 volts and start testing it. Now there is also an awful lot of boring wiring to do. I think next post will come sometime during the christmas holidays.