Ian Iveson wrote:
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.

I was just suggesting they wanted to show them in the best possible kight.

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.


The 6AU6 is a bit odd - it is labelled as an RF pentode - but it seems
to have been used a lot in triode mode in low level audio applications.
There are examples in RDH4 and there's even a short paper on using the
screen as the anode in triode mode to get better screening of the heaters.

My RF knowledge is sadly lacking so I have no idea if the anode currents
in the data sheet are in fact normal for RF applications. It is quite
possible I am barking up entirely the wrong tree.

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.

Sort of. It is hard to tell how skewed the grid lines are from the data
sheet but at present Ia is just under 4mA.

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


That's the thing. In a preamp, two tube factors contribute to noise - gm
and Ia so it is quite typical to run the first stage at a very low
current - say 1mA or less - and the curves are unreadable in that region.

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.

Precisely what I an talking about.

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

That's the best data sheet I have found so far for the triode curves
except perhaps for a Mazda one in French. I have used a graphics program
to blow up the region of interest and I have printed it out on an A4
sheet. Looks quite good even at 1mA.


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.


I am not sure why you would want to operate it near cut off. AIUI sharp
cut off is simply a result of an evenly wound grid - and that's how all
grids were to start with as it was the obvious and easy way to make
them. As the grid is evenly wound the grid field is uniform so there is
just one -ve voltage at which the grid stops ALL the electrons reaching
the anode. The other factor is that the closer the turns on the grid the
higher the gm. I understand the first need was for a variable gain stage
for AGC in radio and someone thought of varying the grid turns spacing
to vary gm. As the grid gets more -ve only the most closely packed grid
turns stop electrons reaching the anode at first, so gm falls, then as
it gets more -ve, more widely spaced turns stop electron flow and so on
until eventually full blown cut off is reached. This is a remote cut off
tube.

HTH

ian