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.