I have a Lafayette tube tester that works in a very simple manner: it
connects all the tube elements, but the plate, to the cathode. It then
measures the current flowing through the diode formed by the plate on the
one hand and by all the other elements on the other hand. Very simple.

The tube tester has a potentiometer that permits to adjust the current meter
sensitivity.

What is unclear to me is why ther tube tester manual instructs to set that
potentiometer at position "30" (over a 100 scale) for almost all tubes. I
would have expected that each tube requires a different setting (like in the
I-177 tube tester), but it is not so: the magic "30" setting is valid for
almost all tubes, big or small it does not seem to matter.

Can any one please help me to understand the reason for that?

Thanks and 73

Tony I0JX
Rome, Italy

On Wed, 23 Nov 2011 22:47:33 +0100, "Tony I0JX"
wrote:

What is unclear to me is why ther tube tester manual instructs to set that
potentiometer at position "30" (over a 100 scale) for almost all tubes.

Your Lafayette is a simple emission tester, which apparently assumes
that most tubes fall in a fairly narrow range of cathode emission
values. The I-177 is a mutual conductance tester whose chart has
settings that cover the wider range of gm values found in tubes

Dale H. Cook, GR / HP Collector, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcity/radios/radtop.html

Dale H. Cook wrote:
On Wed, 23 Nov 2011 22:47:33 +0100, "Tony I0JX"
wrote:

What is unclear to me is why ther tube tester manual instructs to set that
potentiometer at position "30" (over a 100 scale) for almost all tubes.

Your Lafayette is a simple emission tester, which apparently assumes
that most tubes fall in a fairly narrow range of cathode emission
values. The I-177 is a mutual conductance tester whose chart has
settings that cover the wider range of gm values found in tubes

Right. The emission tester isn't testing the gain of the device or the
transconductance... all it is testing is how effective the cathode is
at emitting electrons.

How effective that is has to do with the surface area of the cathode, with
the temperature of the cathode, and with the composition of the cathode.
But you can be reasonably sure that most tubes of a same general technology
will have the same general emission. So if you pull some generic octal
tube or some generic miniature 9-pin tube, you can make a pretty good guess
what the emission is going to be.

Those testers are basically useless, though, since all they do is detect
one sort of tube failure, they can't detect any of the others. The ones
you used to see in supermarkets and drug stores tended to be calibrated
such that new tubes would test marginal, also, in an attempt to increase
sales...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:

Those testers are basically useless, though, since all they do is detect
one sort of tube failure, they can't detect any of the others. The ones
you used to see in supermarkets and drug stores tended to be calibrated
such that new tubes would test marginal, also, in an attempt to increase
sales...

That only worked until people started testing the new tubes before they
bought them.

Geoff.


--
Geoffrey S. Mendelson, N3OWJ/4X1GM
My high blood pressure medicine reduces my midichlorian count. :-(

"Scott Dorsey" ha scritto nel messaggio
...

Right. The emission tester isn't testing the gain of the device or the
transconductance... all it is testing is how effective the cathode is
at emitting electrons.

How effective that is has to do with the surface area of the cathode, with
the temperature of the cathode, and with the composition of the cathode.
But you can be reasonably sure that most tubes of a same general technology
will have the same general emission. So if you pull some generic octal
tube or some generic miniature 9-pin tube, you can make a pretty good guess
what the emission is going to be.

-------
I am aware that the Lafayette tube tester only indicates one parameter, i.e.
emission. As a matter of fact tubes showing almost the same emission on the
Lafayette, instead show a very different performance on a professional tube
tester (AVO).

But is still unclear to me how can a tube drawing a 0.3A filament current
(e.g. a 6AU6) show the same emission of a tube having 2.5A current (on the
Lafayette) .

73

Tony I0JX

On 11/26/2011 02:52 PM, Antonio I0JX wrote:


"Scott Dorsey" ha scritto nel messaggio
...

Right. The emission tester isn't testing the gain of the device or the
transconductance... all it is testing is how effective the cathode is
at emitting electrons.

How effective that is has to do with the surface area of the cathode, with
the temperature of the cathode, and with the composition of the cathode.
But you can be reasonably sure that most tubes of a same general technology
will have the same general emission. So if you pull some generic octal
tube or some generic miniature 9-pin tube, you can make a pretty good guess
what the emission is going to be.

-------
I am aware that the Lafayette tube tester only indicates one parameter,
i.e. emission. As a matter of fact tubes showing almost the same
emission on the Lafayette, instead show a very different performance on
a professional tube tester (AVO).

But is still unclear to me how can a tube drawing a 0.3A filament
current (e.g. a 6AU6) show the same emission of a tube having 2.5A
current (on the Lafayette) .

73

Tony I0JX
An emission tester will give a good account of the condition of many
power tubes especially rectifiers. On small signal tubes a
transconductance tester is required to get a good idea of the tubes
performance.

Hi, Tony

I looked at the schematics of the Lafayette TE-50 and TE-36B
tube checkers found on line. They are somewhat unusual in that they
have no high voltage transformer.

The usual emissions tester (Triplett, Heath, Knight Kit, Eico, etc.)
uses about a 40VAC plate voltage transformer.
The Lafayette uses full line voltage through a 470 Ohm fixed resistor
in series with the 10K adjustable "Load" pot as the plate supply.
The autotransformer in the Lafayette is used merely
to select the heater voltage.

The usual emissions tester has a "Circuit" switch that selects
the current limiting resistor, and the "Load" resistor is merely
to adjust plate current meter sensitivity.

The Lafayette has no selectable fixed load resistor: the Lafayette
"Load" pot really +is+ a load resistor - if you set it to zero
during a Quality test you may damage the tube
due to excessive current, as the manual warns.

The Lafayette circuit looks more like a constant current source,
using a higher voltage plus a higher series resistance
than the standard tube checker.
If the tube works at all, you should get a "good" meter reading.

Very odd.

73,
Ed Knobloch


On 11/23/2011 4:47 PM, Antonio I0JX wrote:
I have a Lafayette tube tester that works in a very simple manner: it
connects all the tube elements, but the plate, to the cathode. It then
measures the current flowing through the diode formed by the plate on
the one hand and by all the other elements on the other hand. Very simple.

The tube tester has a potentiometer that permits to adjust the current
meter sensitivity.

What is unclear to me is why ther tube tester manual instructs to set
that potentiometer at position "30" (over a 100 scale) for almost all
tubes. I would have expected that each tube requires a different setting
(like in the I-177 tube tester), but it is not so: the magic "30"
setting is valid for almost all tubes, big or small it does not seem to
matter.

Can any one please help me to understand the reason for that?

Thanks and 73

Tony I0JX
Rome, Italy

Edward Knobloch wrote:

The Lafayette has no selectable fixed load resistor: the Lafayette
"Load" pot really +is+ a load resistor - if you set it to zero
during a Quality test you may damage the tube
due to excessive current, as the manual warns.

The Lafayette circuit looks more like a constant current source,
using a higher voltage plus a higher series resistance
than the standard tube checker.
If the tube works at all, you should get a "good" meter reading.

Not really. It was designed as a cheap tube tester for home use or the amateur
TV/radio repair person.

If you consider that most failures of vaccuum tubes are caused by the heaters
then a simple tapped transformer would do for the average guy fixing your
TV in your home, or his collection of TV's or radios. Probably 6.3 volts
alone would do for TV's and maybe a handful of other voltages for radios e.g.
5, 6.3, 12.6, 35, 50 and 70.

Any conductance out of the tube would be good enough to declare it "working".

Beyond that would require some skill in diagnosing failures, or a good tube
tester which was beyond the means of an amateur "fixer" or junior tech, so the
set would have to be hauled off to the shop for repair by someone with real
skill and understanding.

In plain English, you could make a good living in the 1950's and 1960's
carrying around a cheater cord, a similar tube tester and a suitcase full
of replacement tubes.

You could make an arrangment with the local TV repairman with a real shop
and real skills to fix the ones you could not and give you a reduced price
if you brought it in, or a finders fee if he had to go out to it.

I made a fair amount of pocket money in the late 1960's just opening sets
and looking for lit heaters. The ones that did not light up or were not warm
went to the drugstore for their tube tester.

It's hard to imagine, but many people still had tube TV's and radios well
into the 1980s. It's impossible to tell when US TV watchers got rid of the
last of them, as they continued to work with over the air broadcasts
until the digital switch, and the BBC ran a 405 line system until 1985, but
I'm sure there were plenty of 625 line tube TV's around afterward.

Note that I am not talking about collectors, special purpose (e.g. ham/swl)
radios etc. I'm talking about your average consumer who at least into
the 1980's bought things because the old one wore out, not because the
90 day warranty expired. :-)

Geoff.



--
Geoffrey S. Mendelson, N3OWJ/4X1GM
My high blood pressure medicine reduces my midichlorian count. :-(

On 12/01/2011 01:54 AM, Geoffrey S. Mendelson wrote:
Edward Knobloch wrote:

The Lafayette has no selectable fixed load resistor: the Lafayette
"Load" pot really +is+ a load resistor - if you set it to zero
during a Quality test you may damage the tube
due to excessive current, as the manual warns.

The Lafayette circuit looks more like a constant current source,
using a higher voltage plus a higher series resistance
than the standard tube checker.
If the tube works at all, you should get a "good" meter reading.

Not really. It was designed as a cheap tube tester for home use or the amateur
TV/radio repair person.

If you consider that most failures of vaccuum tubes are caused by the heaters
then a simple tapped transformer would do for the average guy fixing your
TV in your home, or his collection of TV's or radios. Probably 6.3 volts
alone would do for TV's and maybe a handful of other voltages for radios e.g.
5, 6.3, 12.6, 35, 50 and 70.

Any conductance out of the tube would be good enough to declare it "working".

Beyond that would require some skill in diagnosing failures, or a good tube
tester which was beyond the means of an amateur "fixer" or junior tech, so the
set would have to be hauled off to the shop for repair by someone with real
skill and understanding.

In plain English, you could make a good living in the 1950's and 1960's
carrying around a cheater cord, a similar tube tester and a suitcase full
of replacement tubes.

You could make an arrangment with the local TV repairman with a real shop
and real skills to fix the ones you could not and give you a reduced price
if you brought it in, or a finders fee if he had to go out to it.

I made a fair amount of pocket money in the late 1960's just opening sets
and looking for lit heaters. The ones that did not light up or were not warm
went to the drugstore for their tube tester.

It's hard to imagine, but many people still had tube TV's and radios well
into the 1980s. It's impossible to tell when US TV watchers got rid of the
last of them, as they continued to work with over the air broadcasts
until the digital switch, and the BBC ran a 405 line system until 1985, but
I'm sure there were plenty of 625 line tube TV's around afterward.

Note that I am not talking about collectors, special purpose (e.g. ham/swl)
radios etc. I'm talking about your average consumer who at least into
the 1980's bought things because the old one wore out, not because the
90 day warranty expired. :-)

Geoff.



Those "drug store" tube testers had a gazillion different sockets
(mostly octal IIRC) to make them simpler for the average Joe to use.
Rather than having a bank of switches to connect the 4 test leads
(heater, heater, cathode, plate) to the various elements the tester just
had different sockets for each possible base connection. For a tube
that had multiple elements (IE: 6SN7GT) the tester either just put them
all in parallel or used TWO sockets and tested each half independently
They were emission testers and probably calibrated on the conservative
side to sell tubes. Still if you knew how to read them you could get a
good idea of the actual condition of the bottle. If you held down the
test button while the tube was warming up you got some more information.
If the tube suddenly jumped in emission or took a very long time to
gradually reach a good reading (or very much higher than good), that
could be a bad sign. Tapping the tube while watching the meter could
show microphonics or shorts. The testers also had short and gas
indicators, but I didn't put too much faith in them.

On 12/01/2011 01:54 AM, Geoffrey S. Mendelson wrote:
Edward Knobloch wrote:

The Lafayette has no selectable fixed load resistor: the Lafayette
"Load" pot really +is+ a load resistor - if you set it to zero
during a Quality test you may damage the tube
due to excessive current, as the manual warns.

The Lafayette circuit looks more like a constant current source,
using a higher voltage plus a higher series resistance
than the standard tube checker.
If the tube works at all, you should get a "good" meter reading.

Not really. It was designed as a cheap tube tester for home use or the amateur
TV/radio repair person.

If you consider that most failures of vaccuum tubes are caused by the heaters
then a simple tapped transformer would do for the average guy fixing your
TV in your home, or his collection of TV's or radios. Probably 6.3 volts
alone would do for TV's and maybe a handful of other voltages for radios e.g.
5, 6.3, 12.6, 35, 50 and 70.

Any conductance out of the tube would be good enough to declare it "working".

Beyond that would require some skill in diagnosing failures, or a good tube
tester which was beyond the means of an amateur "fixer" or junior tech, so the
set would have to be hauled off to the shop for repair by someone with real
skill and understanding.

In plain English, you could make a good living in the 1950's and 1960's
carrying around a cheater cord, a similar tube tester and a suitcase full
of replacement tubes.

You could make an arrangment with the local TV repairman with a real shop
and real skills to fix the ones you could not and give you a reduced price
if you brought it in, or a finders fee if he had to go out to it.

I made a fair amount of pocket money in the late 1960's just opening sets
and looking for lit heaters. The ones that did not light up or were not warm
went to the drugstore for their tube tester.

It's hard to imagine, but many people still had tube TV's and radios well
into the 1980s. It's impossible to tell when US TV watchers got rid of the
last of them, as they continued to work with over the air broadcasts
until the digital switch, and the BBC ran a 405 line system until 1985, but
I'm sure there were plenty of 625 line tube TV's around afterward.

Note that I am not talking about collectors, special purpose (e.g. ham/swl)
radios etc. I'm talking about your average consumer who at least into
the 1980's bought things because the old one wore out, not because the
90 day warranty expired. :-)

Geoff.



FWIW I have a black and white tube set, a portable, with maybe a 17in
screen. It still works. Fact is, if I set that TV on channel 3 and
hooked it to my digital TV box, I could see over- the-air shows just
fine- and that would be pretty ironic, I suppose. I may try that later
next week, just for the heck of it. I rarely turn that set on, but
something triggered my irony detector. LOL!

David