Like the original Morgan Jones amp, the original 6N1P OTL had trouble driving low impedance headphones. This version of the 6N1P OTL amp features a re-balanced output stage that gets all of my headphones plenty loud.
If you can read an op-amp circuit diagram, you should be able to read this tube circuit with no trouble. The 6N1P family are “dual triodes,” each tube containing two separate amplifiers in the same device. Each triode has three circuit connections: a source of electrons (“cathode”), an output that receives the electron flow (“anode” or “plate”), and a “grid” which controls the flow. A separate wire filament (the orange glow in tubes) heats the cathode to make it give off electrons.
Figure 2 shows the tube pin layout. The layout is shown from underneath, from the bottom of the tube. This makes sense, since that is the view you have of the tube socket when wiring it. Pins 1, 2, and 3 are the anode (A), grid (G), cathode (K) for the first triode (T1) inside the tube. Pins 4 and 5 are for the filament heater (H). Pins 6, 7, and 8 are the anode, grid and cathode for the second triode (T2). Pin 9 is for shield (S) between the two triodes in the tube, and in this amp should be connected to ground. In the schematic, all the cathodes are shown on the bottom (the little hat shape signifies the heater filament), grids in the middle, anodes on top.
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