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Single Ended Triodes (SETs), the ultimate tube lovers dream.

RE: OK, Great ........................

Mike, on the Magnequest forum here on the AA, used to host the paper by
Voltsecond but it's not there anymore.

All output transformers have a limited amount of inductance. Single ended output transformers need to have a gap so the idle current for the output tube doesn't saturate the core. When a gap is placed in the core of a transformer the inductance goes way down. The inductance can be brought back up by winding more wire around the core but there are limits. Too much wire and you will began to lose highs because of the winding capacitance. The end result of all the this is that SET transformers do not have a ton of inductance. The lack of inductance means that the reactance of that inductance will be small for the lower frequencies and that is what causes the elliptical load line.

If you allow the lower frequencies to reach the output transformer the load line becomes elliptical instead of the straight load line we think of when we're dealing with a plate resistor. An elliptical load line causes the tube to operate in a less linear part of the operating curve for part of the wave form.

The part that most people miss (and the part that Voltsecond points out) is that once there are low frequencies present in the output transformer all the frequencies have to follow that elliptical load line, not just the bass frequencies. If you can keep the low frequencies out of the transformer the load line will be much more like a resistive load line and there will be less distortion.

Even if you don't follow what I am trying to explain or if you don't believe me, it costs very little to try this and it is totally reversible.

After reading Voltsecond's paper, I placed caps in front of my SET 300b amps driving my midrange and tweeter and was shocked at the improvement.

Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 03/27/21

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