![]() ![]() Whilst doing this test I dragged the Hexbeam ant' coax over and used the FT450 to drive the linear to get a 5/9 and a good report from John (EA7BA) in Spain on 14.156. Input swr was 1.4 Output meter showed 290 watts and plate current was 350mA. Linear drive was 50 watts two tone from TS520. On left is trapezoid pic whilst testing at 14.2 Mhz. The 35Mhz Hameg scope is fed with demodulated RF from a "Clean-RF" RF-D unit and a loop coupling from the coax switch on the way to the 1.5 Kw dummy load, out of sight. Yes - the TS520 still works a treat and allows not only the two tone input via the mic socket but adjustable pwr level using a nine volt battery and pot fed to the final tube bias. The two tone genny is sitting on top of the old boat anchor with the broken meter. The linear, a KW103 pwr/swr meter for output sitting on top of a Kenwood AT SWR meter to monitor input. From left to right you may recognise the lumps. The pic on the left shows the testing effort. Having used the single tube linear to test all the newly acquired 813 tubes I will dismantle it underneath, fit the newly made tuning variable, add a tube socket, replace the heater tranny and rewire it all as required for two tube operation. This new tuning cap will align with the existing front panel vernier and chassis space allotted. These steel bits and pieces are zinc plated and so not a bad combination with aluminium in a dry environment. The spindle is 6mm threaded rod and the two stator supports are 4mm. Next time I will measure every one and select equal thickness. The spacing is generous but allows for possible lack of precision! It was a surprise to measure the variation in thickness between 6mm nuts even from the same batch. ![]() Plate spacing is 3mm and plate thickness is 0.9mm. #Homebrew amplifier plans plusThe new variable is quite large at 100 x 110 x 180mm plus projecting bolts etc. The 9pF minimum is nearly half that of the smaller section of the existing tuning cap so is a worthwhile step in the right direction. This is mostly important on the small section of the capacitor.įinal result seemed quite good at about 9pF to 57pF and the bigger section 16pF to 157pF. A good gap between rotor and stator is arranged when minimum C is set. The stator of the smaller section is spaced away from the earthed frame with a block of tufnol. To assist with a low minimum C the shaft is offset. (Anyway - I have enough left for yet another tuning capacitor). There is no way was I going to attempt making those by hand. They cut 26 rotor and 26 stator plates for me. I drew up the plates and sent a DXF file to the laser cutters. It would be another two section variable. The minimum capacitance needed reducing to better tune the 10mtr band so I decided to make another tuning capacitor. The voltage taps I ended up with would allow for some adjustment. A voltage doubler circuit would be used, like everyone else uses. I would wind my tranny with 3000 volts in mind but with a few taps. So I happily hung my hat on that idea and reinforced it with tales that 813 tubes were "rugged". I seem to recall from too many years ago that a linear amplifier derived huge benefit from as high a B+ as possible and linearity was much dependent on a low impedance power supply. I must disclose at this stage that several people said that I should use two 813 tubes rather than one. My junk heap had produced a 60V 1KVA 'tranny' which I could rewind for a perceived 2800 V supply. In consideration of modest power needs, the neighbours, the legal power limits and even the availability of tubes, a single tube version became the initial aim. The principle of this bifilar solution was also shown in SSB for Radio Amateurs. The claimed performance and especially the 813 input circuit arrangement used by ZL1AXB seemed most attractive. That success encouraged me to try and make an 813 linear. My first homebrew linear used used four PL519 tubes in parallel and works very well. ![]()
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