Loadlines and distortion (6S33S/6S41S)

[Kaidanovsky]

Hullo all,

I'm working through Morgan Jones's books and Steve Bench's webpages, using as my example a 6S33S from which I got ~10% distortion at the second harmonic and 35% from the 6S41S. I've seen a number of amplifiers around the Infobahn with these valves in and was intrigued about their predicted performance.

I've checked and re-checked the calculations and got the same answers, so my first question is are the people who are listening to these amplifiers great fans of distortion or am I getting entirely the wrong end of the stick?

Secondly, though websites like GoodSoundClub recommend 1.2kOhm loads and around 220V/150mA quiescent operating values for the 6S33S (which results in these figures) they'd surely improve with higher loading and voltage?

Note I'm only talking about the figures here as an example to improve my understanding because of course everyone likes different sonic characteristics! However surely figures as ridiculous as 35% are way, way out if people are listening to these and saying they're accurate?

[DF96]

There are audiophiles who object to the use of the term 'accurate'. Baffles me!

Two possibilities:
1. You have misunderstood the charts, and the distortion is not as bad as you think.
2. You are about to tell the Emporer that he has a wardrobe malfunction.
I am not familiar with either valve, so I am not in a position to judge.

[MrCurwen]

http://www.mif.pg.gda.pl/homepages/f...18/6/6S41S.pdf

Well looking at the curves, there seems to be two distinct areas of operation, with grid voltage under a 100 volts minus, and over 100 volts minus. If you operate the tube so that parts of the signal go over this line, you will have massive distortion, just as you calculated. However, if you stay firmly in the lower region, it seems to be quite linear. At least not horrible by any means.

It seems to be a relatively low voltage tube.

[artosalo]

using as my example a 6S33S from which I got ~10% distortion at the second harmonic and 35% from the 6S41S.

What power level, load impedance, operating point and supply voltage ?

I think most big (DIY) SE amplifiers without negative feedback are performing very poorly with respect to distortion.
But like most "home made" amplifiers, those sound perfect to people who have built those. I have seen many such examples.

[MrCurwen]

Quite unusual in itself - most curves I've seen seem to bunch closer together at higher voltages, these seem to widen to about double. Peculiar.

[Kaidanovsky]

In the 6S33S's datasheet the normal operation point is 120V. However it seems accepted that this is for voltage regulation (its original purpose) so people seem to be running it at ~220V.

For the 6S41S I chose 150V/120mA (rather arbitrarily) because it seemed to be a tangent to the power curve and in a middling position with some headroom to change the load a little. I went for Mr Bench's "double the quiescent voltage" to get the tentative line, which was (as with the 6S33S) about 1.2Kohm. However after getting 35% I wondered if it were worth calculating for another line!

MrCurwen: after -100V the curves go in -20V increments rather than -10, so I think they're actually pretty similar in separation.

Artosalo: You're probably right - I've never measured the distortion on my amplifiers, but then I never designed them and just followed a diagram. Now I've an oscilloscope I'd rather like to do some tests!

[MrCurwen]

So they do. My bad. =D

[Kaidanovsky]

It had me confused me too - especially as I was looking at them through semi-translucent baking paper as I'm out of trace! I don't think they want people putting a load of bias on there.

[Kaidanovsky]

I was looking at these valves originally because they look rather exciting (and come on, why have your music made by little light bulbs if they don't look a bit out of the ordinary?) and thinking about push-pull. If my distortion figures are accurate (indeed DF96, some people dislike that idea! I think a wardrobe malfunction might be on its way, even if we allow for non-ideal speaker loading) then surely P-P is the only way to go?

Another question is are the distortion figures only tenable at high output power levels? If the volume were low or the speakers sensitive; would they be correspondingly decreased to the point of not mattering, or do they have a proportional relationship to output power?

[DF96]

Roughly speaking, for a single-ended amplifier, 2nd order distortion is proportional to volume, 3rd order goes like volume² etc. Under some circumstances, and at higher output levels, it can get more complicated.