DIY Vacuum Tube Amplifiers

During several months of running listening tests on my 6SN7 Delux all-triode amplifier, I was baffled when my wife commented on how much better the triode amplifier sounds than my Class A amplifier that uses 6L6/5881 output tubes.

My wife has a less common hearing problem; she can only hear higher audio frequencies from 10 KHz and up. Below 10 KHz she has a steep roll-off. I knew the triode design had a much cleaner sound, but I found it hard to understand how she could tell without being able to hear the lower and mid-band frequencies.

What makes sense is that she is sensitive to both even and odd order harmonic distortions that fall into the higher audio spectrum. The difference she is hearing is the level of odd order harmonics coming out of the pentode output tubes compared to the 6SN7 output tubes. For me, it is a more subtle difference as the lower and mid-band frequencies are masking the higher pentode harmonic distortions. Considering her hearing problem, those distortions must really stand out compared to any of the music she can hear.

Harmonic distortions, both even and odd order, are signals that are a multiple of some fundamental signal and are usually higher in frequency than the fundamental frequencies. Objectionable levels of odd order harmonic distortion may be masked by the fundamental frequencies while adding an unnatural edge to the sound. While the odd order harmonics add an unnatural edge to the audio, even order harmonics are musical in nature and will enhance the sound.

If one is striving for the best Hi-Fidelity sound, eliminating odd order distortion would be a prime goal. For both solid state and vacuum tube amplifiers, push-pull topologies are not the answer as they cancel out the desirable even order harmonics while letting the odd order harmonics pass. There is also the problem of push-pull output transformer hysteresis as explained on the Home page.