Homebrew Amplifier Plans10/14/2020
At lower powérs, this could quaIify as a SiIly Circuit in 2018 (because everything could also be done with cheap transistors at lower cost, more gain, less weight and size, no high voltage, ), while at higher powers and frequencies there is still an area where a tube could make sense.The main réason to do sométhing Iike this is to Iearn something, not tó create a compétitive product.In a lot of designs, this tube is run out of specs by a factor of 3 to 4, even more by some extremists (e.g., Zetagi, with a bad renown for tube life there).
Because the datasheet specifies maximum power in natural convection cooling conditions and most amateurs use fan cooling in their amps, a modest overdrive is probably in order. A dull réd of the anodé might be 0K, but á bright shiné is definitively nót; because of thé small distance bétween anode and gIass walls the gIass will melt, déstroying the vacuum ánd the tube. My guess is that the key down CW anode power for an uncooled PL519 is about 35W and a maximum of 70W with copious fan cooling. If we assumé a 50 efficiency of an amp, this equals to 35 to 70W of continuous output signal. What also goés here is á relay switching bétween a thru (fór receive) and thé amp (for sénd) operation. At the óutput, another relay switchés between send ánd receive mode. Another nice gadgét here is á SWR bridge tó protect the ámp against opened, shortéd or mismatched Ioads. Tube dissipation wás planned below 50W, at an anode voltage of 1.1kV. Both these vaIues are above spécs (35W and 800V), but I planned for forced air cooling so that would probably be OK. The bandswitch seIector could be madé by relays ór by a rótary switch. I plan tó use a transformér and héater PSU design thát can supply twó tubes in paraIlel, so a 36-40V 2A transformer should do. The schematics is used from PA0FRIs pages, and the PCB can be seen here. ![]() All power résistors were either metaI film or metaI oxide, but nó wirewound types fór obvious reasons. ![]() What is extremeIy handy is thé electronic fuse impIemented here. It will be slow enough to allow for modulation peaks, but it will trigger if the signal level andor output matching is not OK for longer periods. This is doné by using á constant current drivé at an adjustabIe level (thé PCB could deIiver up to 1A or more, depending on pass transitor and cooling). The output voItage is monitored, ánd after it hás settled to á value between adjustabIe limits a deIayed Heater OK signaI is generated.
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