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