I had the fun this past weekend of a very interesting get together with some guys and some cool equipment.
A friend has a preamp, that began life as an Audio Note D to A. After adding and reconfiguring, it had become a line stage in addition and eventually lost the D to A completely. So it became a line stage based on an Ampex circuit.
Now the fun part. It now has a solid state rectifier PS AND a tube rectifier PS both. At the flip of a switch, after a delay, you can pretty immediately hear the difference. Also, it includes a built in Variac to allow matching or purposely completely different B+ voltages. Both supplies are well done, choke input, not shorted technically in anyway to purposely make one better than the other.
The conclusion is, at matched voltages, that the tube supply is much nicer than the SS. About a dozen people heard it and I didn't hear any differing opinion. Not a conclusion in all cases maybe, but interesting to hear with your own ears!
Also interesting, is the varying of B+ voltage. As you first reduce from the nominal B+, it gains a bit of a sharper, crisper edge. Not totally bad, particularly on some music that can sound ok with drive and aggressiveness. Go more, and to me it just falls apart into harshness. We're talking 250 normally, first reduced to maybe 225 and gets bad below 200.
Measurements (on the audio output) on this reduced voltage showed that it was 3rd harmonic that came up, producing the observed hardness. So it was an opportunity to really hear what a 3rd harmonic dominated distortion really sounds like. Pretty cool for an audio geek like me!!
Also, with relation to the above comment about 2nd and 3rd harmonic distortion, it's a big difference in the effect between the two! THD is almost a completely useless number! Its the higher harmonics, the odds that are so unnatural to the ear. The second is not too disturbing, but the third is much more so.
A story I've heard being talked about, but I've not found reference to it yet, is that the 3rd harmonic is out of phase with the fundamental, and blunts the leading edge of the audio. This seems to make sense. A friends DIY tube amp, measures about 50db down on the 2nd and below 90db on the 3rd, and its the nicest sounding amp I've ever heard! The clarity on highs is just astonishing.
Sleepysurf, with regards to your comments about voltage sag. That's a desirable characteristic on a guitar amp, but no well designed audio amp, should have an issue with that. The soft clipping is something often mentioned for tube amps with regard to the output stage when over driven. It isn't so much to do with the PS part of things. I've also had the chance to experience the difference in PS rectifier tubes. In an original Dynaco ST70, going through a hand full of different tubes you could hear a difference. That was into big Voice of the Theatre Altec speakers that are efficient enough to not use much power and over run the amps notoriously under filtered power supply. Pretty cool, hearing the change between tubes. The best we heard, was the original Mullard 5AR4 in that situation.
OldMonolith
