Ian Thompson-Bell wrote

Surely if it were truly interesting it would be published
already?

That's what I would have thought. However, I expect
manufacturers wanted to show their products in the best
possible light even in those days, so bigger voltages,
bigger currents and their consequent better mu and gm
figures would find their way onto the data sheets.

Seems a bit over-cynical to me. Each manufacturer made many
valves so there was a need to distinguish between them.
Perhaps we are more inclined now to use valves outside of
the niche applications for which they originally competed. I
note the 6AU6 was intended for radio. Knowing nothing about
radio, I can't guess why low current in triode mode is not
given as a typical condition.

You might expect the published curves to cater for the
published typical operating conditions.

Ian, consider the output impedance of your stage, and
make sure your loadline is for actual AC operating
conditions, and not just whatever load resistor you are
using. It may become obvious that you need more current.


ra is about 10K and the anode resistor is 39K so the
output impedance is less than 10K. It feeds a load of
470K.

OK. I assumed you were in the region where grid lines are
bunched and skewed clockwise, right at the bottom of the
graph, where ra rises considerably. Also suspected you may
have been using it as a driver into a lower load.

But, if the valve is a 6AU6, 5mA gets you out of that
region, and your load allows a decent voltage swing

Anode characteristics charts I find are on a scale that puts
5mA well up the left axis for pentode operation, but only
about one fifth of the way up for triode connection.
Similarly, typical operating points extend down to 5mA for
pentode, but not for triode, where about 12mA is suggested.
So in each case, the graphs are scaled around the suggested
operating points.

All the same, resolution is reasonable for triode 5mA. Clear
enough to confirm your 10k using a ruler on-screen:

http://www.mif.pg.gda.pl/homepages/f...93/6/6AU6A.pdf

To my eye, the triode looks quite lumpy at low currents.

One thing I wonder, which relates to all of this, is why a
sharp cut-off valve should be typically operated nowhere
near cut-off? I could do with a lesson on what sharp cut-off
valves were for and why.

cheers, Ian