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                                                                              My First Home Brew Linear Amplifier

 

  I had an interest in in home brew projects since I obtained my Amateur Licence in the early eighties. So after messing around with small projects gaining experience and confidence in my own ability I decided to build an HF Linear Amplifier using recycled ex equipment and ex military bits and pieces.

  After looking at a few it circuits was decided to use PL509 old TV line input valves as there was an abundant supply of these from a family member who at that time worked as a Refuse Disposal Officer ( bin man) at the local rubbish tip. I was given an old ex equipment case by a friend so that was my starting point , all I needed now was the rest of the bits to make a start. I used a well tried and tested circuit using 3 PL509 valves in grounded grid.

 

  I now had a case which was to be the basis of my project and I intended to fit all the bits into this one way or another, When I had most of the bits to hand they were loosely placed in the case to see if they would all fit in, and when I was satisfied that the case would fit the bill work was started on it.  As it was my first major project a lot of it was trial and error and tin bashing to get all the bits to fit. After a few months building it and sorting out the bugs (most of them self inflicted) I had the amplifier up and running with the help of a local amateur to get the  Pi-Tank resonant from 80m to 10m.

 Much to my surprise it worked very well after a few minor hiccups and run faultlessly for a few years. The local club used it in SSB field days a few times and was easy to tune and use by members of the club. The amplifier passed through a few hands before I was given it back, as I now had no need for it the amp was sold and went to GI land ..I never did hear of it again, however this whetted my appetite for home brew and have built a few items since some of which will appear on these pages   in time. Unfortunately all but one of the build photos got lost in the mists of time , nowadays I take more care of my build records. The only picture to survive is a view on the underside, not very interesting but I'll include it anyway.

                      



 

                                                                MY FIRST 4CX250B 2m Amplifier BUILD 

   I temporarily moved from HF to VHF, 2m in particular was my choice to try to work DX on SSB and CW rather than FM. As I had a Yaesu FT 101 Mk2A and bought a Europa transverter for not a lot of money, this was found lacking in the hearing department ( stations calling me but not hearing many) so a hearing aid was needed and my choice at the time I think was a SEM preamp made in the Isle of Man. I ran this setup for a while with an output of around 100 watts from the Europa Transverter (QQVO 640 in the final) then wanted a little more power ...4CX250 maybe?

  As I wanted more power on 2m the cost of any thing over100 watts to me was cost prohibitive as we had a young family who soaked up any spare cash. My only choice was home brew and growing in confidence I went along that route. A local amateur GM3KJF SK gave me a few pointers and a lot of good advice also recommended a circuit from a German VHF magazine the name of which has long since slipped from my memory banks.

  Again my same build principles applied, find a suitable case for it then fit all the bits in. By this time I had a good collection of transformers, switches and high voltage capacitors both electrolytic and ceramic, I managed to find a suitable ex equipment case from a rally and the project was started. I found out very quickly that it building for VHF was more difficult than for HF. By now I had most of the parts to begin so off I went to my shed and got started.

                                                              ( I had to used two transformers in series to get a usable HT voltage)

   The RF Deck was made from PCB soldered together (not a good idea as I was to find out later) and plumbers copper tubing used for the tube line with homemade tuning and loading capacitors, the anode blocking capacitor was a strip of Mica using plastic bolts and nuts. The lid for the anode compartment was also made from PCB and held down with anti magnetic anchor nuts and aluminium screws. The build took a few months to complete as I discovered my shortcomings in VHF amplifier building. On completion all voltages and settings were checked and found OK but I could not get it to resonate then, I ran it up without the lid and was now getting some RF output. Lid back on and no output, after a lot of head scratching, many cups of tea and phone calls later it was found that the PCB was acting as a capacitor and detuning the anode circuit This was fixed by directly connecting both sides of the PCB as possible, I finaly got it up and running so I sat back and left home brewing alone for a while and got on with operating and chasing VHF DX.  

                                                             

                       

                       

   



                                                                                                                                        

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