Revision 3: 6L6GC SE Amplifier

This was my first serious amp project. Originally it used a 6DJ8 or 6CG7, but when I added NFB (Negative Feedback) I found it needed more gain, so now it uses a 12AX7. Anywho, I had ordered up some transformers from Antique Electronics Supply, a Hammond 269AX power transformer and a P-T31 output transformer (which is made for a 6V6, but works well enough with a 6L6). These form the backbone of the amp, powering it and allowing the output tube to do its business.


Circuit Description:

This circuit is pretty basic so I'll just highlight the special parts. One would be the tone control circuits, which I brewed up from scratch. This goes from full bass to full treble, with somewhere in the middle for listening. Its disadvantages are the steepness and crossover points are fixed; only the ratio of tone to bass can be varied. In the event that flat response is preferred to heightened tone and bass, I put in a bypass; this has an 82pF bypass to boost HF, since the high impedances in this area tend to drag it down. On to the output, why what's that across the primary winding? A Zobel network, which supresses oscillation (in fact this thing makes a nice 5W oscillator without that little bit installed, and that's with or without NFB connected!). At the very end, at the speaker, is where the NFB loop is connected. The 2.2k resistor forms a voltage divider with the 100 ohm at the 2nd 12AX7 cathode, feeding back some output. The result is such that all signals are reduced: gain goes down, but so does distortion; frequency response is flattened and damping factor increased. Overall the amp sounds very nice, and has earned its place as my bedside amp (though it does deserve better speakers and signal source).


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