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

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

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

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

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

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

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

"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.

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.

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