Datasheet plate curves way above max ratings?

[RobPlatt]

I've noticed recently, a lot of tube datasheets show plate curves for values WAY above the listed maximum ratings.
I'm looking at a Svetlana (square S) datasheet (happens to be a 6N1p) that lists a max. plate voltage of 250V, 2.2W, 20mA.
The plate curves go up to 600V and 30mA, and most of the curves are in regions that are way above the max 2.2W, extending up to areas that would be making 18W. The area I actually care about is generally that tiny area in the lower left that is actually within the max limits, and there's very little detail there.

The ones from JJ, for example, are better in this regard.

Does anyone actually make useful datasheets? I'm much more used to datasheets from Motorola semiconductors that go into much more detail, listing expected variations and so on. I don't know how true this is, but I've read that our beloved 12AX7 can vary 30-50% in these values.

For those of you who are really serious about not just amp building, but actually designing and who do circuit calculations, do you measure the characteristics of the tubes yourself rather than relying on datasheets?

[CurtissRobin]

The higher voltages are probably there to illustrate the response of the tube during momentary or peak excursions. As long as those excursions are momentary the magic smoke stays inside the tube. Except for oscillators and such, few tube applications are assured of seeing steady state, constant amplitude signals.

I see nothing to gain by developing your own characteristic curves. The precision isn't especially important. They change from lot to lot anyway and even within a lot there's a wide range. Electronic devices or not, they are mechanical structures and the 30-50% variation you noted is probably as good as it gets without buying large lots and sorting/selecting tubes.

KennyO

[zaphod_phil]

The calculations in tube amp design are never done to a very high degree of accuracy, so what's shown in the data sheets is plenty good enough. It's also a good thing when you find voltages shown which are above the "official" maxima, because that's where the tubes are operated at in many guitar amps. We like to break the rules! Take for instance, EL84s being run at 350V or 6V6s at 440V....

[dotfret]

LOL I used to design things before my brain fell out, around 30 years ago.

The easy and polite way to explain running valves at high voltages is that it can be done, but you have to "starve" the input so as not to overload at the output.

I never really understood why people wanted to do it, but there are commercial designs that worked - look at the output stages of circa 1970s Accoustic amps, the HT is over 600V and the puir wee things inside have to stand it or die!

Lord, preserve us from "work functions" and deep thought ...

[CurtissRobin]

There's also the fact that they're always going to give examples of conservative design so using their info is sure to give a circuit that won't smoke.

KennyO