Read Part 1 here.
Well, its been a while since I started this project and I see that many folks are visiting my first post, so here is an update of what I did lately with this amplifier.
I short words first, I put the board all together, set the bias current, attached a heatsink, built LPF board and did about half of the mechanical construction on the enclosure.
20W Amp Status
Most of the time my amp stood on the shelf dismantled because I couldn’t manage to find a reasonable 1 or 2 Amps scale ammeter to measure the bias current. Most of the cheapo multimeters have 2 mA, 20 mA, 200 mA and then suddenly 10 or 20A scale and I wasn’t feeling confident measuring the 2 x 300mA with it. Luckily, a few weeks ago I was able to put my hands on this Romanian old school “piece of art” multimeter, called MAVO-35, which can measure 1 and 5A DC current.
So, after I finish assembling the board, MOS-FETs attached to the heatsink, 50 ohms dummy loads on both input and output, I have set up the currents to 300 mA each side. With the pots turned full CCW the board only drawed 50mA, so now with the bias set it, the total current consumption in stand-by is 650 mA. Instructions can be found also on Kees, K5BCQ, website.
I see now that Kees has a new version of the amp. Main difference is that he replaced the two output binoculars BN43-202 with one single transformer. He says the first version was getting to hot at 20W, so I will probably get the same issue…Anyway, now he published the schematic of the amp on his website, so go check it out.
K5BCQ’s 20W Amp Kit Assembled
This is a first version of the Low Pass Filters board. It is a six filters unit, with five poles, switched by Omrom G5V1-12 type relays. They are initially designed for 160m, 80m, 40-30m, 20-17m, 15-10m and 6m. Here in the picture are missing the 160, 80 and 6m filters. I got the silver-mica caps also from Kees, ordered separately. For the missing slots, I couldn’t find the necessary values yet. After the RF tests I will publish also the PCB layout and the schematic. I still have to follow if is this board will work properly, in terms of heat dissipation, RF loss etc. As you can see I have used Amidon T50-2 for the lower bands and T50-6 toroids for the higher ones.
Low Pass Filter Board
These are the initial panels but I will change their actual design. I have made them a long time ago, but since then a lot of ideas came and gone away. To my first design I have added LED power and swr meters with LM3914, as you can see down bellow. Later on I did a separate stand-alone SWR meter, and this days I have in mind to add a small LCD, 8×2 characters maybe, or an old Nokia graphic display driven by an Arduino Nano.
Same to the back panel. In my first design I actually forgot to add a PTT socket. Even if the amp has a SOX circuit, it is better to have this one also. And, optional, if can be done, I will add an attenuator switch, to toggle between 1 and 2.5W input, or 1 and 5W.
20W Amp Panels
The SWR bridge is a kits-and-parts.com model. One of the toroids is missing in the picture. It is used in the other SWR meter.
20W Panels Inside
To this last additions, LCD, ATT etc, I will concentrate later. Now the next step is to put some RF signal to the board and see how it works. Hope to do this soon and keep you up to date.
Read Part 1 here.