![]() ![]() That would result in lower gain and a cleaner amplifier. The 5672 can also be used for the preamp stages, if the filament pins are inverted. Load lines are shown in figure 2, with the class A curve in red and the class B curve in orange. ![]() The output stage consists of two 5672 tubes running at 90V, biased at -11V with a 32k plate-to-plate transformer. To achieve a logarithmic response, better for audio, a 200k resistor is connected at the lower pads of the potentiometer. The dual-gang potentiometer is used as a post phase inverter master volume. A trimmer is used adjust the signal going to the 4th tube, so that both output stages are driven by signals of similar amplitude. The circuit consists of two pre-amplification stages using the 5678 tube in triode mode with gain and tone controls, followed by a paraphase inverter, also using the 5678 in triode mode. The Hammond PP 125A transformer will also work as an alternative (22k : 8 ohm). You can use anything that gives you a similar ratio (32k plate-to-plate : 8 ohm). Regarding the output transformer, I use a 100V line transformer (10W, with 2.5W and 0.625W taps). normal wire and stereo shielded wire (choose the thickness you like to work with).1x mono audio jack (or 2, if you want to add an additional speaker output).Not included in the list are more common parts, such as: To keep it short, I added a PDF containing all the components. The circuit can be divided in three main parts, concerning the high-voltage switched mode voltage multiplier, the inverting switched mode voltage doubler and tube amplifier circuit. Bolts and nuts (3 mm, different lengths).8x2 inch (200x50 mm) metal sheet for the faceplate.20x8x1/4 inch (500x200x5 mm) wooden plank (for reinforcing the box and the frontal panel).tolex (optional, you can finish it with other paint, or any other material).8x8x4 inch (200x200x100 mm) wooden box, or about this size.4 inch, 8 ohm full range speaker (or an oval one).I used a perfboard with circular solder pads) Ferric Chloride etching solution (optional, in case you are etching your board.access to a toner printer (optional, depends on desired faceplate and PCB)Īnd some basic materials that you can add to your toolbox:.clothes iron (optional, used for toner transfer).The goal here is to use only two batteries and not only supply filaments and high voltage, as also provide a lower bias voltage at the grids.įirst, let us start with the tools used to build this amplifier: By combining more of them, one can easily reach voltages that can be elevated with simple switching circuits, making them the battery of choice. Every phone charger or USB powered device can supply the required voltage for charging them. Rechargeable LiPo batteries, on the other hand, are found everywhere. Although 1.2V batteries for the filaments are not a problem, 45V to 90V batteries are not a common thing nowadays. ![]() Tube radios, such as the Koyo Parrot, used three different batteries (filaments, high voltage, biasing) to work. This tubes, however, have direct heated cathodes, which makes the design a little more tricky.
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