I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot, contaminated
grid will emit a few electrons, some being picked up by the cathode, more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"Robert Baer" wrote in message
...
Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)

Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)

On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams"
wrote:

I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot, contaminated
grid will emit a few electrons, some being picked up by the cathode, more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim

What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Are there secondary emission effects at the grid?

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

John

"John Larkin" wrote in message
...
What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Should be something like that. Although I wonder, since negative current
[magnitude] should rise somewhat with decreasing grid voltage as it becomes
a better cathode. Of course, it may also be that, as a cathode, it is
already saturated (no substantial space charge), where current wouldn't
vary much with voltage.

I wonder what conditions these measurements were taken in -- probably
static, in which case I wonder if the plate temperature, and therefore
everything inside the plate, was at thermal equilibrium when these
measurements were taken. If so, then towards cutoff, the triode will be
running a goodly bit cooler and the grid won't emit much. It follows that
high plate dissipation and grid voltage just under the virtual cathode
potential (about -1V) should produce the maximum leakage.

Are there secondary emission effects at the grid?

I don't think so. I suspect the electric field doesn't increase much until
the electrons are past the grid (i.e. not enough leaks through the grid
spaces), so in the grid region, there aren't any electrons with more energy
than the grid's work function. A hot, contaminated grid might have some
secondary emission though.

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

I would love to see a 300B in a TO-247 package. Well, no, I wouldn't love
to *see* one, but it would be quite fun to use.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

Robert Baer wrote:
John Larkin wrote:

On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams"
wrote:


I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot,
contaminated
grid will emit a few electrons, some being picked up by the cathode,
more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim


What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Are there secondary emission effects at the grid?

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

John

Yesssss!
Ages ago, when i saw an article about a semiconductor designed as an
electron emitter, i wondered when that would be used as a low power
cathode - cut energy requirements in half (roughly)!
**
Thanks for the comments - may be helpful when i sit down to think
about them.

Part of the problem in miniaturizing tubes is that it's really hard to
sustain a good enough vacuum, due to having all that surface area per
unit volume. That's the limiting factor in micro field emitters, for
instance.

Organic LEDs use a layer of (I joke not) metallic calcium as an electron
source.

Cheers,

Phil Hobbs

Tim Williams wrote:

I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot, contaminated
grid will emit a few electrons, some being picked up by the cathode, more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms

"Robert Baer" wrote in message
...

Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)





1) Positive lead of the DVOM is at the grid and negative lead is at
adjustable Vc; values shown aer what was read on the meter.
Note the neck of the giraffe tilts negative to the left.
2) The schematic clearly shows that the meter resistance is 1Meg.

flipper wrote:
On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:


Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.


and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?
See the top curve where i have calculated the grid voltage WRT cathode.

John Larkin wrote:

On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams"
wrote:


I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot, contaminated
grid will emit a few electrons, some being picked up by the cathode, more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim


What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Are there secondary emission effects at the grid?

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

John

Yesssss!
Ages ago, when i saw an article about a semiconductor designed as an
electron emitter, i wondered when that would be used as a low power
cathode - cut energy requirements in half (roughly)!
**
Thanks for the comments - may be helpful when i sit down to think
about them.

flipper wrote:

On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:


Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.


and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?
OK, you found me out.
What i have is a GR1862B Megohmmeter which uses a 12AU7 as a
differential detector in a semi-bridge configuration (don't ask why - go
to GR on that Q).
I am getting tired of having tubes slowly degrading so that i cannot
adjust the infinity at max setting (pot setting slowly crawls from near
center to always one end).
I was hoping that i could characterize the 2 triodes and use the data
as a guide - maybe even selecting a tube that would start on one end so
as tholast longer in this application.

flipper wrote:
On Sun, 04 Nov 2007 15:42:09 -0800, Robert Baer
wrote:


flipper wrote:


On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:



Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.



and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?


OK, you found me out.


I apologize if that was supposed to be a secret.


What i have is a GR1862B Megohmmeter which uses a 12AU7 as a
differential detector in a semi-bridge configuration (don't ask why - go
to GR on that Q).
I am getting tired of having tubes slowly degrading so that i cannot
adjust the infinity at max setting (pot setting slowly crawls from near
center to always one end).
I was hoping that i could characterize the 2 triodes and use the data
as a guide - maybe even selecting a tube that would start on one end so
as tholast longer in this application.


Well, tubes do age and I'm not familiar with a GR1862B so I can't help
much on that score but I'd suggest you test your measuring setup by
checking anode current with and without the grid probe attached to see
if it remains constant.
I have been looking at the new graph via Hexcel and am not happy with
it; it appears that it lies.
The meters are hard-wired in series with the grids; if the current is
small then the grid voltage is not much different than the setting
(100mV per 100nA).
Using those el-cheapo DVOMs in this app is ideal in that
"recommended" grid resistance is 1meg and the DCV input is 1meg,
allowing a 200nA FS reading.

On Sun, 04 Nov 2007 15:35:36 -0800, Robert Baer
wrote:

flipper wrote:
On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:


Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.


and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?
See the top curve where i have calculated the grid voltage WRT cathode.

Your calculations appear to be inconsistent with the info given. For
example 141 nA through 1 megohm generates 0.141 volts, not 14.1 volts.
The whole table looks like it was calculated based on a 100 megohm
grid resistor, not 1 megohm as stated elsewhere.

Robert Baer wrote:
Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)

I found my error; wrong neck (multiplier) for the giraffe.
Basically, in the range of interest; grid current does not affect the
set grid voltage.

On Sun, 04 Nov 2007 18:24:16 -0500, Phil Hobbs
wrote:

Robert Baer wrote:
John Larkin wrote:

On Sun, 4 Nov 2007 04:24:07 -0600, "Tim Williams"
wrote:


I think you have that backwards, grid current should be unquestionably
positive for any voltage above the virtual cathode potential (~ -0.8V
here), rising to miliamperes for positive voltages.

Grid contamination would vary widely with age and use. A hot,
contaminated
grid will emit a few electrons, some being picked up by the cathode,
more
by the plate.

What's the maximum value of negative grid current you've seen across
samples?

BTW, what is the resistance of your meter in the modes used?

Tim


What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Are there secondary emission effects at the grid?

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

John

Yesssss!
Ages ago, when i saw an article about a semiconductor designed as an
electron emitter, i wondered when that would be used as a low power
cathode - cut energy requirements in half (roughly)!
**
Thanks for the comments - may be helpful when i sit down to think
about them.

Part of the problem in miniaturizing tubes is that it's really hard to
sustain a good enough vacuum, due to having all that surface area per
unit volume. That's the limiting factor in micro field emitters, for
instance.

They usually have to work in UHV to prevent ion damage and stuff.
Nanotube bundles work OK, for a while, until the ends get flayed off;
they *are* noisy. Nanotips wear out fast, too. There was a diode that
just flung electrons out the junction into space, but I don't know
what's become of that.


Organic LEDs use a layer of (I joke not) metallic calcium as an electron
source.

I hope the humidity seals are good.

John

BFoelsch wrote:
On Sun, 04 Nov 2007 15:35:36 -0800, Robert Baer
wrote:


flipper wrote:

On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:



Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.



and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?

See the top curve where i have calculated the grid voltage WRT cathode.


Your calculations appear to be inconsistent with the info given. For
example 141 nA through 1 megohm generates 0.141 volts, not 14.1 volts.
The whole table looks like it was calculated based on a 100 megohm
grid resistor, not 1 megohm as stated elsewhere.
Check; has been corrected - see previous posting and most recent.

Robert Baer wrote:

Some 12Au7 tubes have "unbalanced" triodes when viewed this way, and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)

OK; measured cathode-to-plate voltage in the GR Megohmmeter; is 38.7V.
Best i could do was to change my supply to 42.9V as choice of zeners
was rather limited.
Also measured grid-to-cathode voltage where the source was stiff WRT
a 10Meg DVM (Check: -2.138, Disch: -1.859, 100Kmeg: -1.860V).
Then i pulled the (almost unbalanced) tube from the Megohmmeter and
made measurements and saw a few surprises.
First, and most delightful, was that "peak" was almost totally gone.
Second, the grid current was a lot larger at -0.5V relative to 100V
plate.
Not too surprising was that the grid current was substantially lower
at "reasonable" biases; a lot less that i would have guessed.
The Red curve is for the grid in the hi-z arm, where the unknown
resistor goes.
Seems that there is very little difference in the grid currents,
which tends to imply that the "meter balance" pot setting may be rather
sensitive.
No adjustment pots are shown in the simplified schematic.
Suffice to say the FP controls "Check" and "set infinity" are between
the cathodes, and the thru-panel "meter balance" (unlabelled) adjustment
seems to vary the voltage on the reference and unknown resistors.
**
Now i do not know what the heck to look for in the tube data...