[Ian Bell]

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk 6HN6P
tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA, Raytheon,
GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These tubes
look like they are NOS. The physical construction of the two types is very
similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes look
used to me. All these tubes are about a quarter of an inch shorter than the
EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the reason
for the difference in measured gm? Is it that the different types are not
in fact identical or is it that the 6AU6/A tubes are just well used of is
it something else?

Cheers

ian

[Patrick Turner]

Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk 6HN6P
tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA, Raytheon,
GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These tubes
look like they are NOS. The physical construction of the two types is very
similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes look
used to me. All these tubes are about a quarter of an inch shorter than the
EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the reason
for the difference in measured gm? Is it that the different types are not
in fact identical or is it that the 6AU6/A tubes are just well used of is
it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.

Gm stands for "general manager", and the russians manage to instal
more effective Gms into their tubes than the americans because bean
counters
cost a lot more in the US than in russia where they have only realised
they need to be employed to
keep the goodness factor low and profits high.

That any russian tube has a higher gm than its US made version means
that the grid wires
are probably very slightly closer to the cathode, so the russians seem
able to
do the business with more dexterity. Ya, those babushka women in their
factories probably
do better than mexican process workers...

Then there the wonders of the frame grid pentodes such as a E280F, with
gm at 20mA/V.
or the more common 6EJ7 with 12mA/V.

The German women in their factories could teach the russions a few
things.

We made such tubes here in Oz, Miniwatts, AWV, Radiotron, etc, mighty
fine tubes
made by splendid sheilas.

Use tubes with flair and imagination;
there is no need to be too obsessive about measurements.

Patrick Turner.




a Cheers

ian

[Ian Bell]

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These tubes
look like they are NOS. The physical construction of the two types is
very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes look
used to me. All these tubes are about a quarter of an inch shorter than
the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different types
are not in fact identical or is it that the 6AU6/A tubes are just well
used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.


I have just checked the the AVO manual again. The settings for EF94 and 6AU6
identical but the tables show the EF94 should give a gm reading of 5.2 and
for the 6AU6 it is listed as just 3.9. So it seems back in the late 30s AVO
were aware of a difference between the two types.

Gm stands for "general manager", and the russians manage to instal
more effective Gms into their tubes than the americans because bean
counters
cost a lot more in the US than in russia where they have only realised
they need to be employed to
keep the goodness factor low and profits high.

That any russian tube has a higher gm than its US made version means
that the grid wires
are probably very slightly closer to the cathode, so the russians seem
able to
do the business with more dexterity. Ya, those babushka women in their
factories probably
do better than mexican process workers...

Then there the wonders of the frame grid pentodes such as a E280F, with
gm at 20mA/V.
or the more common 6EJ7 with 12mA/V.

The German women in their factories could teach the russions a few
things.

We made such tubes here in Oz, Miniwatts, AWV, Radiotron, etc, mighty
fine tubes
made by splendid sheilas.


Interestingly the only 6AU6 that reaches 5mA/v is a miniwatt.

Use tubes with flair and imagination;
there is no need to be too obsessive about measurements.


I'm not. I just wanted to find out if I had bought a bunch of duff tubes.

Ian

[John Byrns]

In article ,
Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These tubes
look like they are NOS. The physical construction of the two types is
very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes look
used to me. All these tubes are about a quarter of an inch shorter than
the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different types
are not in fact identical or is it that the 6AU6/A tubes are just well
used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.

I have just checked the the AVO manual again. The settings for EF94 and 6AU6
identical but the tables show the EF94 should give a gm reading of 5.2 and
for the 6AU6 it is listed as just 3.9. So it seems back in the late 30s AVO
were aware of a difference between the two types.

"late 30s"? I didn't know the 6AU6 even existed at that time.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These tubes
look like they are NOS. The physical construction of the two types is
very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes look
used to me. All these tubes are about a quarter of an inch shorter than
the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different types
are not in fact identical or is it that the 6AU6/A tubes are just well
used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.


I have just checked the the AVO manual again. The settings for EF94 and 6AU6
identical but the tables show the EF94 should give a gm reading of 5.2 and
for the 6AU6 it is listed as just 3.9. So it seems back in the late 30s AVO
were aware of a difference between the two types.

My data books list EF94 as identical to 6AU6.

Just make sure Ia for the tests is the same, and gm varies considerably
with Ia.


Gm stands for "general manager", and the russians manage to instal
more effective Gms into their tubes than the americans because bean
counters
cost a lot more in the US than in russia where they have only realised
they need to be employed to
keep the goodness factor low and profits high.

That any russian tube has a higher gm than its US made version means
that the grid wires
are probably very slightly closer to the cathode, so the russians seem
able to
do the business with more dexterity. Ya, those babushka women in their
factories probably
do better than mexican process workers...

Then there the wonders of the frame grid pentodes such as a E280F, with
gm at 20mA/V.
or the more common 6EJ7 with 12mA/V.

The German women in their factories could teach the russions a few
things.

We made such tubes here in Oz, Miniwatts, AWV, Radiotron, etc, mighty
fine tubes
made by splendid sheilas.


Interestingly the only 6AU6 that reaches 5mA/v is a miniwatt.

Use tubes with flair and imagination;
there is no need to be too obsessive about measurements.


I'm not. I just wanted to find out if I had bought a bunch of duff tubes.

With old tubes, usually the noise is higher tha it should be,
and if you have a 1Meg grid biasing R, the grid will measure slightly
+ve with respect to the applied bias voltage.
So if set up for a test, with cathode bias, the grid can be slightly
above 0V potemtial.
very good tubes are very quiet, just slight hiss without sputtery LF
noises, flicker noise,
and have slightly negative grids.

Patrick Turner.



Ian

[Ian Bell]

John Byrns wrote:

In article ,
Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These
tubes look like they are NOS. The physical construction of the two
types is very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes
look used to me. All these tubes are about a quarter of an inch
shorter than the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different
types are not in fact identical or is it that the 6AU6/A tubes are
just well used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.

I have just checked the the AVO manual again. The settings for EF94 and
6AU6 identical but the tables show the EF94 should give a gm reading of
5.2 and for the 6AU6 it is listed as just 3.9. So it seems back in the
late 30s AVO were aware of a difference between the two types.

"late 30s"? I didn't know the 6AU6 even existed at that time.


I am sure you are right. ISTR that the *design* of my AVO tester dates from
1938 which is what I was thinking of. The data sheets for tube testing
could of course have been updated many times thereafter. The point is the
AVO data sheets list them with differing gm values.

Ian

[Ian Bell]

Patrick Turner wrote:



Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These
tubes look like they are NOS. The physical construction of the two
types is very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes
look used to me. All these tubes are about a quarter of an inch
shorter than the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different
types are not in fact identical or is it that the 6AU6/A tubes are
just well used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.


I have just checked the the AVO manual again. The settings for EF94 and
6AU6 identical but the tables show the EF94 should give a gm reading of
5.2 and for the 6AU6 it is listed as just 3.9. So it seems back in the
late 30s AVO were aware of a difference between the two types.

My data books list EF94 as identical to 6AU6.


Mine too.

Just make sure Ia for the tests is the same, and gm varies considerably
with Ia.


I think my particular AVO sets up grid bias voltage and plate and screen
voltages (the latter two you set on rotary) switches. You null out the
resulting Ia then flick a key which applies an ac input voltage and the
meter reads gm directly. I'll check the circuit out though to be sure.


Ian

[Patrick Turner]

Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Patrick Turner wrote:



Ian Bell wrote:

Recently on eBay I purchased some Brimar EF94 tubes, some Novosibirsk
6HN6P tubes, and some 6AU6/A tubes of various makes (Sylvania, RCA,
Raytheon, GE). I tested them all on my AVO two panel tester.

The gm of the EF94 and 6HN6P tubes measures 5mA/V or greater. These
tubes look like they are NOS. The physical construction of the two
types is very similar too.

The gm of the 6AU6/A tubes measure between 3 and 4mA/V. These tubes
look used to me. All these tubes are about a quarter of an inch
shorter than the EF94 and 6HN6P tubes.

The 6AU6A and EF94 data sheets I have are identical. So what is the
reason for the difference in measured gm? Is it that the different
types are not in fact identical or is it that the 6AU6/A tubes are
just well used of is it something else?

A 20% difference in an electronic parameter isn't really very much of
anything.


I have just checked the the AVO manual again. The settings for EF94 and
6AU6 identical but the tables show the EF94 should give a gm reading of
5.2 and for the 6AU6 it is listed as just 3.9. So it seems back in the
late 30s AVO were aware of a difference between the two types.

My data books list EF94 as identical to 6AU6.


Mine too.

Just make sure Ia for the tests is the same, and gm varies considerably
with Ia.


I think my particular AVO sets up grid bias voltage and plate and screen
voltages (the latter two you set on rotary) switches. You null out the
resulting Ia then flick a key which applies an ac input voltage and the
meter reads gm directly. I'll check the circuit out though to be sure.

Ian

I've never used a valve tester.

They were really good for servicing RF gear and TV sets,
when the main thing you needed to know was if a tube was dead of alive.

But in linear audio circuits, audio voltages are easily measured, and
the gains and overall tube behaviour.

I have a dc load that is fixed, and alter the cap coupled load to
an appropriate figure so total load is either just the 100k, dc RL
or with a cap coupled 100k which gives 50k.
The cathode is well bypassed.

At low Vout of say 10Vrms, well away from any large distortion,
and at 500Hz, gains for both RL values are measured.

For all tubes, voltage gain, A = µ x RL ( RL + Ra )

for each RL, the A is known, and Ra and µ are the same.

So from two gain equations for two RL one can find out what Ra and µ
are.

The algebra is 4th year high school.

For all tubes, µ = gm x Ra, and so gm may be calculated once µ and Ra
have been found.

This is basic reasoning you should be familiar with.

I don't rely on a tester.

Plugging in different 6AU6 should tell you all you need to know
about the tube and what gains you are likely to get.

All operations should be monitored with a CRO.

Patrick Turner.