Tubes Headphone Amplifier

Posted on Jun 2, 2012

The amp is built according to the Morgan Jones design. In the spirit of DIY, I made a few modifications. I did not change the circuit topology at all, but used Svetlana 6N1P tubes, as noted. I also used a conventional power supply instead of the wall wart/filament transformer combination, because the 6N1P has a higher filament draw (2 amps) than 6922s, and I wanted plenty of high-voltage supply, too. 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.

Tubes Headphone Amplifier
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The power supply shown for the original Morgan Jones amp is unusual and clever, but the 6N1Ps need a higher voltage for the plates and more current for the filament heaters than the original supply can produce. [Editor: The 6N1P needs a higher plate voltage than the 6922/6JD8 to reach its optimal operating range.] The heater current is 600mA for the 6N1P versus 365mA for the 6922, a significant difference. The clever wall-wart scheme, which is barely adequate for the 6922s, clearly can't provide enough current for 6N1Ps. So I used a more traditional power supply. Tube amps traditionally have a power transformer that has a separate winding for the heater filaments in addition to the main winding. As it happens, a transformer is available that provides usable voltages for this project, the Hammond 269AX. Do not use the center tap (red/yellow) on the high voltage secondary winding of the 269AX. Also, be careful if you run this power supply for very long without a load on it, since voltages will quickly accumulate into the 400 volt range. The amp will draw about 27 milliamps of current. If you want a dummy load to use when checking the power supply, you will need to cobble together a 13k ohm load capable of dispersing 10 watts. The power supply output network has one 1K-ohm/2W and two 1K-ohm/2W film resistors in parallel. You can trim the output voltage by adjusting the value of the second 1K-ohm/2W pair slightly up or down...

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