Plate characteristics at low anode currents
 

Ian Iveson wrote:
Don Pearce 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.

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.

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.

Ian

Plate characteristics at low anode currents
 

On Dec 2, 8:02 pm, Ian Thompson-Bell wrote:
Ian Iveson wrote:
Don Pearce 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.

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.

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.

Ian
- Show quoted text -

hey-Hey!!!,
The large plate current curves can be scaled( that is calculate
different values for the Y-axis). I can't find the ref. that gives the
instructions...but when I do, you'll be the second one to know it.
cheers,
Douglas

Plate characteristics at low anode currents
 

Multi-grid wrote:
On Dec 2, 8:02 pm, Ian Thompson-Bell wrote:
Ian Iveson wrote:
Don Pearce 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.
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.

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.

Ian
- Show quoted text -

hey-Hey!!!,
The large plate current curves can be scaled( that is calculate
different values for the Y-axis). I can't find the ref. that gives the
instructions...but when I do, you'll be the second one to know it.
cheers,
Douglas

Excellent. Looking forward to it.

Cheers

ian

Plate characteristics at low anode currents
 

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

Plate characteristics at low anode currents
 

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

Plate characteristics at low anode currents
 

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!

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.

Plate characteristics at low anode currents
 

On Dec 2, 8:51 am, 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

Don't know if this does you any good:

http://hereford.ampr.org/Tube4.php?tube=6au6

But it is a start.

Peter Wieck
Wyncote, PA

Plate characteristics at low anode currents
 

Ian Thompson-Bell wrote

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.

I vaguely remember from previous discussions that screen
current is unusually high as a proportion of cathode
current.

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.

At which point ra will be a bit higher than 10k by the
looks.

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.

But not typical to use a 6AU6. Curves for an ECC83, for
example, are scaled appropriately for your application.
Compare with those for an ECC82, where scale is similar to
what you have for the 6AU6. That's why I assumed you where
talking about the driver end of the pre-amp.

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.

OK, that's similar to last time I asked what sharp cut-off
valves are for, and why. Consequently I know what they are
and how they work, but I still don't know what they are for,
or why...although the variable gain application is useful to
know, as I have been wondering if I can do my
expanding-feedback headphone amp with valves, instead of
this chip I got for a wah-wah pedal. Actually, comparing
with common small triodes eg ecc82, I can't see how or where
the sharpness manifests itself in the anode characteristics
for triode operation.

On the face of it, one might think that cut-off
characteristics only matter when you are likely to be
operating close to cut-off, but then perhaps that's when a
remote cut-off valve would be better because clipping would
be softer.

John probably knows all this stuff.

Are you sure you will have enough headroom at such low
current? Are you using global nfb?

cheers, Ian

Plate characteristics at low anode currents
 

Peter Wieck wrote:
On Dec 2, 8:51 am, 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

Don't know if this does you any good:

http://hereford.ampr.org/Tube4.php?tube=6au6

But it is a start.


If nothing else, Peter, it shows someone has created a graph at lower
plate currents - pity it is for pentode connection as I am using triode
connection.

Cheers

ian

Plate characteristics at low anode currents
 

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