Plate characteristics at low anode currents
 

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

Plate characteristics at low anode currents
 

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

--
Pearce Consulting
http://www.pearce.uk.com

Plate characteristics at low anode currents
 

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

Plate characteristics at low anode currents
 

On Sun, 02 Dec 2007 15:07:54 +0000, 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.

Wouldn't mind a look if you can put it somewhere convenient, although
from interest only, as it is unlikely I will do any design work with
it.

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.

Have you tried using log axes? They might make things more usable over
a wider range.

d
--
Pearce Consulting
http://www.pearce.uk.com

Plate characteristics at low anode currents
 

Don Pearce wrote:
On Sun, 02 Dec 2007 15:07:54 +0000, 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.

Wouldn't mind a look if you can put it somewhere convenient, although
from interest only, as it is unlikely I will do any design work with
it.


I have uploaded it here http://www.ianbell.ukfsn.org/data/

as an XL spreadsheet file.

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.

Have you tried using log axes? They might make things more usable over
a wider range.

d

I think what I might to as an interim fix is simply blow up the data
sheets on the PC and capture the area of interest and then print that
full size on an A4 sheet.

Cheers

ian

Plate characteristics at low anode currents
 

On Sun, 02 Dec 2007 15:49:08 +0000, Ian Thompson-Bell
wrote:

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.

Wouldn't mind a look if you can put it somewhere convenient, although
from interest only, as it is unlikely I will do any design work with
it.


I have uploaded it here http://www.ianbell.ukfsn.org/data/

as an XL spreadsheet file.

Interesting. Just tried sorting on the three columns, and while Ra and
Mu have a degree of correlation to each other, Gm appears to have none
at all.

d

--
Pearce Consulting
http://www.pearce.uk.com

Plate characteristics at low anode currents
 

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?

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.

Ian

Plate characteristics at low anode currents
 

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

After a search lasting only an hour and a quarter (my filing system
must be better than I thought) I have in my hand a sheet of Ia-Eb-Eg
curves for a 6SL7-GT. It is clearly an enlargement from another
document. Unfortunately I can't tell whether from another sheet, a
book or a computer file, except that the file isn't presently on my
computer, so the source is a mystery. However, it is precisely the
sort of thing you want. It looks like a normal set of transfer curves,
with two exceptions: that on the left the plate current scale runs
from 0 to 2A and on the right from 0 to 8mA, and that there are two
sets of curves on the sheet, one hard lines, one dotted lines. There
are thus such curves available, or at least in one case, the 6SL7-GT.
If it is the tube you're actually working on, I should be happy to
photograph the sheet I have and post it on my netsite for you to
download, print out and use as graph paper.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

Plate characteristics at low anode currents
 

Andre Jute wrote:

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

After a search lasting only an hour and a quarter (my filing system
must be better than I thought) I have in my hand a sheet of Ia-Eb-Eg
curves for a 6SL7-GT.

Thank you for going to so much effort.

It is clearly an enlargement from another
document. Unfortunately I can't tell whether from another sheet, a
book or a computer file, except that the file isn't presently on my
computer, so the source is a mystery. However, it is precisely the
sort of thing you want. It looks like a normal set of transfer curves,
with two exceptions: that on the left the plate current scale runs
from 0 to 2A and on the right from 0 to 8mA, and that there are two
sets of curves on the sheet, one hard lines, one dotted lines. There
are thus such curves available, or at least in one case, the 6SL7-GT.

That sounds just like what I need.

If it is the tube you're actually working on, I should be happy to
photograph the sheet I have and post it on my netsite for you to
download, print out and use as graph paper.


Unfortunately that is not the tube I am using. I am using a 6AU6 in
triode configuration.

Cheers

Ian

Plate characteristics at low anode currents
 

Don Pearce wrote:
On Sun, 02 Dec 2007 15:49:08 +0000, Ian Thompson-Bell
wrote:

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.

Wouldn't mind a look if you can put it somewhere convenient, although
from interest only, as it is unlikely I will do any design work with
it.

I have uploaded it here http://www.ianbell.ukfsn.org/data/

as an XL spreadsheet file.

Interesting. Just tried sorting on the three columns, and while Ra and
Mu have a degree of correlation to each other, Gm appears to have none
at all.

d


I should have mentioned that the second group of three columns of ra, mu
and gm, are averages for the group of tubes up to and including the line
on which the figures occur. The reason for this is that each group is by
the same manufacturer. The manufacturers of each group, from top to
bottom a

Brimar (EF94) boxed set of 5
Novosibirsk (4)
GE (5)
RCA (5)
Sylvania (5)
Tungsol (3)
Motorola (2)
The last three are a Miniwatt, a Raytheon and a Philco.

Note these are not randomly selected tubes. First I tested a large
number on my AVO tester to eliminate the duff and feeble ones. Then I
built a rig to measure gm, ra and mu from which the above results were
obtained. Later, by shorting the grid on the same rig I measured the
output noise of the same tubes. The Brimar and Novosibirsk were
consistently the quietest and 4 out 5 of the GE ones were as good. The
rest were at least 50% noisier and some were as much as 5 times as noisy.

Cheers

Ian

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

Plate characteristics at low anode currents
 

Ian Iveson wrote:
Ian Thompson-Bell wrote
snip

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.

Ah but, strangely enough the 6AU6 IS/WAS used in audio preamps. It is
used today in a top of the range Sony condenser mic, see:

http://www.gigasonic.com/Sony-C800GPAC.html


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.


OK. I too found the ECC83 curves are scaled appropriately and it is
basically an AF tube. The ECC82, though often used in audio application,
is basically and RF tube and as you say has curved scaled more like the
6AU6. I think that is the explanation why the curves I want don't exist.

Cheers

Ian

Plate characteristics at low anode currents
 

On Dec 3, 2:01 pm, 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!

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.

Nah, all those measurements and plotting besides is a six-Tylenol job.
Ian can do this very simply in a single shot, and get a curve, or many
curves if he wants to be fancy, complete with grid markings to plot
on. All he needs is a filament tranny and a socket and a distribution
strip screwed to a piece of scrap ply, a single resistor in the
cathode circuit, his scope and an amp with a working power supply to
tap off, plus a variac to get the right voltage if it is different
from what the amp PS supplies. The B+ goes in to the anode with the OV
attached to the grounded side of the fil. The negative grid bias goes
into the grid in series with the fil. This is most easily done by
attaching the negative end of whatever makes the bias (batteries are
good; I used a Lego trainset power supply which is a small AC/DC
variac) to the grid and the positive end to ungrounded side of the
filament. Scope horizontal or X also to the grid, scope vertical or Y
between the resistor (100 ohm is good) and the cathode, the other end
of the resistor to 0V. Connect grid screen and anode and bingo, that
is all the connections; it takes less time to dig out the parts and
screw and solder it up than to describe. The scope will show an
elongated J lying at an angle and that is the transfer curve for the
negative bias you feed, with the anode current up the left scale and
the anode voltage along the bottom scale. Photograph, feed photo to
computer or print out, draw loadline over scope's own gridlines, and
Bob's your uncle. To make many curves on the same sheet, put camera on
tripod for stability and position, change electrical parameters, take
multiple exposures (I took singles and just overlaid them in the
computer; lost faster than fartarsing around with tricky multiple part-
exposures). Don't forget to label each negative bias curve.

Whole procedure is a hell of a lot faster than fiddling with a DMM and
the uncertainties of how it loads your circuit, or pulling out a VTVM
and setting it up.

I don't know if Ian has said how he took his earlier measurements, but
I'd be surprised if he hasn't already worked out this junkbox curve
tracer for himself.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

Plate characteristics at low anode currents
 

On Dec 3, 10:02 pm, Andre Jute wrote:
On Dec 3, 2:01 pm, 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!

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.

Nah, all those measurements and plotting besides is a six-Tylenol job.
Ian can do this very simply in a single shot, and get a curve, or many
curves if he wants to be fancy, complete with grid markings to plot
on. All he needs is a filament tranny and a socket and a distribution
strip screwed to a piece of scrap ply, a single resistor in the
cathode circuit, his scope and an amp with a working power supply to
tap off, plus a variac to get the right voltage if it is different
from what the amp PS supplies. The B+ goes in to the anode with the OV
attached to the grounded side of the fil. The negative grid bias goes
into the grid in series with the fil. This is most easily done by
attaching the negative end of whatever makes the bias (batteries are
good; I used a Lego trainset power supply which is a small AC/DC
variac) to the grid and the positive end to ungrounded side of the
filament. Scope horizontal or X also to the grid, scope vertical or Y
between the resistor (100 ohm is good) and the cathode, the other end
of the resistor to 0V. Connect grid screen and anode and bingo, that
is all the connections; it takes less time to dig out the parts and
screw and solder it up than to describe. The scope will show an
elongated J lying at an angle and that is the transfer curve for the
negative bias you feed, with the anode current up the left scale and
the anode voltage along the bottom scale.

Er, you have to rewire to get anode voltage on the scope horizontal or
X; as described you get grid voltage on the X.

Photograph, feed photo to
computer or print out, draw loadline over scope's own gridlines, and
Bob's your uncle. To make many curves on the same sheet, put camera on
tripod for stability and position, change electrical parameters, take
multiple exposures (I took singles and just overlaid them in the
computer; lost faster than fartarsing around with tricky multiple part-
exposures). Don't forget to label each negative bias curve.

The labelling requirement is a good reason to tape some of your wife's
pattern-cutting paper or baking tray paper (both are translucent
enough) over the oscilloscope screen and take a tracing as well as a
photograph. Common projection film and Koki pens also work nicely.

Whole procedure is a hell of a lot faster than fiddling with a DMM and
the uncertainties of how it loads your circuit, or pulling out a VTVM
and setting it up.

I don't know if Ian has said how he took his earlier measurements, but
I'd be surprised if he hasn't already worked out this junkbox curve
tracer for himself.

Andre Jute
Visit Jute on Amps athttp://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

Plate characteristics at low anode currents in triode mode
 

"Ian Thompson-Bell" wrote in message
...
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

Hi Ian:

If you give me the range of plate currents and voltages, I can plot you a
set of curves on my tracer and email you a digital camera photo of same. For
example, if you tell me that the maximum y-axis current you want is 5mA and
the x-axis voltage (Vak) is to be 300 volts, I can plot you a set of 11
curves, including Vgk=0V. I can set the grid volts per step to anything
you'd like within reason.

Just let me know the values you want and I'll give it a try. Please also
let me know:

1) Do you want the screen grid connected through 100 ohms (or other value)
to the plate, or just a direct connection?
2) I can connect the suppressor grid anywhere you want. Either to the plate
or to cathode are the obvious choices. Where would you like it or do you
want curves for both connections?

All the best : Doug Bannard, P.Eng. )

Plate characteristics at low anode currents in triode mode
 

Doug Bannard wrote:
"Ian Thompson-Bell" wrote in message
...
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

Hi Ian:

If you give me the range of plate currents and voltages, I can plot you a
set of curves on my tracer and email you a digital camera photo of same. For
example, if you tell me that the maximum y-axis current you want is 5mA and
the x-axis voltage (Vak) is to be 300 volts, I can plot you a set of 11
curves, including Vgk=0V. I can set the grid volts per step to anything
you'd like within reason.

Just let me know the values you want and I'll give it a try. Please also
let me know:

1) Do you want the screen grid connected through 100 ohms (or other value)
to the plate, or just a direct connection?
2) I can connect the suppressor grid anywhere you want. Either to the plate
or to cathode are the obvious choices. Where would you like it or do you
want curves for both connections?

All the best : Doug Bannard, P.Eng. )



Thanks Doug, that is a really kind offer. I am mainly interested in
plate currents up to 5mA and plate volts up to 250V. I am considering
two alternative triode connections of the 6AU6.

1. G3 and G2 tied to plate (no series resistors)
2. G3 and plate tied to ground with G2 used as a plate.

Curves for both configurations would be nice.

Many Thanks

Ian

Plate characteristics at low anode currents
 

Ian Thompson-Bell wrote:

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

The 6AU6 was expected to work at 10mA ia and 200V Ea in RF circuits of
many kinds.
Typical app is a limiter in an FM radio IF strip.
But in this app they spend their lives being grossly overdriven with
signal
to cause then to clip hard while the grid voltage applied is quite -ve
and the tube average dc is
very low.

Triode op was mainly for audio apps, and
you just never see them se up in triode at Ia = 10mA.

µ is nearly constant, but Ra and gm vary widely with Ia.

I rely on what I observe.

Patrick Turner.

Plate characteristics at low anode currents in triode mode
 

"Ian Thompson-Bell" wrote in message
...
Doug Bannard wrote:
"Ian Thompson-Bell" wrote in message
...
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

Hi Ian:

If you give me the range of plate currents and voltages, I can plot you a
set of curves on my tracer and email you a digital camera photo of same.
For example, if you tell me that the maximum y-axis current you want is
5mA and the x-axis voltage (Vak) is to be 300 volts, I can plot you a set
of 11 curves, including Vgk=0V. I can set the grid volts per step to
anything you'd like within reason.

Just let me know the values you want and I'll give it a try. Please also
let me know:

1) Do you want the screen grid connected through 100 ohms (or other
value) to the plate, or just a direct connection?
2) I can connect the suppressor grid anywhere you want. Either to the
plate or to cathode are the obvious choices. Where would you like it or
do you want curves for both connections?

All the best : Doug Bannard, P.Eng. )

Thanks Doug, that is a really kind offer. I am mainly interested in plate
currents up to 5mA and plate volts up to 250V. I am considering two
alternative triode connections of the 6AU6.

1. G3 and G2 tied to plate (no series resistors)
2. G3 and plate tied to ground with G2 used as a plate.

Curves for both configurations would be nice.

Many Thanks

Ian

Hi Ian:

I have run the curves and have sent scans of them in JPEG format to you
separately. To ensure that I am sending you meaningful scans, I have tested
two 6AU6s on my New London 901 Transconductance Analyzer, an instrument
which can test almost all receiving tubes under actual operating conditions,
with each element voltage regulated and adjustable.

One of the 6AU6s meets all of the requirements of a new tube as per the
standard pentode test conditions in the General Electric Tube Manual
(Essential Characteristics). These test conditions for pentode connection
are as follows:

For Vak = 250 volts, Vg2k = 150 volts, Cathode resistor of 68 ohms and G3
connected to cathode:

Ip (Ia) = 10.6 mA, Ig2 = 4.3 mA and gm = 5200 uS

The second 6AU6 has reached its end of life and under the same applied
voltages shows:

Ip(Ia) = 5mA, Ig2 = 1mA and gm = 2000 uS

I have sent you scans of each tube for grid steps of -0.5 volts /step
and -1.0 volts per step, for the standard triode connection (G2 and
G3 strapped to plate) and for the triode connection where G2 is used
as the anode with G3 and plate strapped to cathode. This gives you
8 scans in total.

I hope that these are of some help. Please feel free to share them with
anyone else if you wish.

All the best : Doug Bannard

Plate characteristics at low anode currents in triode mode
 

"Ian Thompson-Bell" wrote in message
...
Doug Bannard wrote:
"Ian Thompson-Bell" wrote in message
...
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

Hi Ian:

If you give me the range of plate currents and voltages, I can plot you a
set of curves on my tracer and email you a digital camera photo of same.
For example, if you tell me that the maximum y-axis current you want is
5mA and the x-axis voltage (Vak) is to be 300 volts, I can plot you a set
of 11 curves, including Vgk=0V. I can set the grid volts per step to
anything you'd like within reason.

Just let me know the values you want and I'll give it a try. Please also
let me know:

1) Do you want the screen grid connected through 100 ohms (or other
value) to the plate, or just a direct connection?
2) I can connect the suppressor grid anywhere you want. Either to the
plate or to cathode are the obvious choices. Where would you like it or
do you want curves for both connections?

All the best : Doug Bannard, P.Eng. )

Thanks Doug, that is a really kind offer. I am mainly interested in plate
currents up to 5mA and plate volts up to 250V. I am considering two
alternative triode connections of the 6AU6.

1. G3 and G2 tied to plate (no series resistors)
2. G3 and plate tied to ground with G2 used as a plate.

Curves for both configurations would be nice.

Many Thanks

Ian

Ian:

Please send me your email address to . When I tried your
ruffrecords email address, it didn't work for me.

All the best : Doug

Plate characteristics at low anode currents
 

"Ian Iveson" wrote in message
. uk...

I could do with a lesson on what sharp cut-off valves were for and why.

FM limiters, TV sync seperators, pulse amplifiers, blocking oscillators
etc.?

David.

Plate characteristics at low anode currents
 

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.


Probably because that were other cheaper tubes you could use for that,
in products in production back in the day.