Patrick Turner wrote:

Ian Thompson-Bell wrote:
Don Pearce wrote:
On Sun, 02 Dec 2007 13:51:44 +0000, Ian Thompson-Bell
wrote:

I have being playing around with plate characteristics graphs, looking
at how basic parameters (gm,rp and mu) vary with operating point. The
most frustrating thing about this is that the published curves all
extend to rather large plate currents and voltages. For my preamp I am
working at plate currents of less than 5mA and the curves here are
rather cramped. Is there any source of curves that are expanded for the
lower plate current region?

Cheers

Ian
It's a long, boring Sunday afternoon. Perfect time to do a set of
measurements like this and post the results somewhere. If this is an
area of valve performance that is poorly documented it would be
interesting to know what kind of variation there is between samples.

d

Been there, done that, wrote the spreadsheet.

Some months ago I ran 32 different 6AU6 tubes in triode mode and
measured gm, rp and mu at an Ia of 3.8mA and Vp of 100V. I have the
results in a spreadsheet if you are interested.

I don't really want to repeat that work every time I decide to use a
different tube. What I don't understand is why the manufacturers data
extends so far away from normal operating points such that the data at
the operating point is of poor resolution - but I guess most them aren't
around to ask. The best data for the 6AU6 in triode mode I have lists Ia
up to over 30mA and Vb to over 400V. OK so you load line is likely to
hit the Ia axis at twice your quiescent, but if that is say 5mA and Vb
is 250V then you are trying to read info off the graph in the bottom
left hand eighth of the graph.

Hving said that, I have just found a similar graph for a 12AT7 in which
Ia only goes up to 5mA - much more sensible - so maybe its just this
particular tube.

Ian

There are very few data sheets which are inadequate as you suggest.

Search and yee shall find!

I have searched extensively and so far I have found 8 different data
sheets for the 6AU6 all of which show relatively high plate currents in
triode connection.


Otherwise, simply make your own data sheets.

Ra, µ, and gm can all be easily measured using 2 different load values
and for say 20Vrms output on the higher value load of say 68k for 6AU6.

As long as THD 4% then your results will be fairly accurate enough.

µ = gm x Ra for all tubes and gain, A, = µ x RL / ( RL + Ra ) for all
tubes.
Use an accurate DVM at 400Hz sine wave, simple.

By simple high school algebra, you can do the rest.

Patrick Turner.

Indeed, we discussed this some months ago and the results referred to
above are from the tests I did following our discussions (plate current
just under 4mA). For this particular tube it is no problem for me to
tweak the test rig along the lines you suggest to obtain values for
lower plate currents and I may well do just that.

Maybe it is just this tube which, in audio circuits, is operated far
from the region used in its intended RF applications so the (old)
published data sheets do not show the region I am interested in in any
detail. I just hope the next tube I use has good data in the region I use.

Cheers

Ian