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
Click here to download the full size of the above Circuit.

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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