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Old 8th April 2008, 02:35 PM   #1
GordonW is offline GordonW  United States
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Default 6JN6 screen-drive (g2 driven) plate curves?

Anyone got a link to, or would be willing to post, a plate curve for a 6JN6, in screen/g2 drive mode (ie, g1 and g3 set to zero volts, showing varying g2)?

I've been looking around- there's supposed to be GE datasheets floating around that have this, but for the life of me, I can't find it. I want to use it as a model for my 6JB6s (which should be functionally identical, with the exception of different bases)...

Thanks!

Regards,
Gordon.
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Old 8th April 2008, 04:39 PM   #2
ilimzn is offline ilimzn  Croatia
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What seems to be the problem? It's right here:

http://www.mif.pg.gda.pl/homepages/f...123/6/6JN6.pdf

Page 3...
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Old 8th April 2008, 05:56 PM   #3
GordonW is offline GordonW  United States
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See, I KNEW it was SUPPOSED to be easy to find.

But, I looked in several normal places, and for whatever reason, couldn't find it.

Thanks in any case!!

Regards,
Gordon.
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Old 8th April 2008, 07:33 PM   #4
GordonW is offline GordonW  United States
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OK. Need a reality check...

Using the curves above, I'm getting some calculations that are a bit "optimistic-seeming", compared to what I've seen/heard from others here.

I drew a load-line for 2500 ohms primary impedance (5000 ohm CT PP transformer), and 550 volts B+ on the tube plate (I "extended" the graph out to where 550V would be, and drew the line from 550V/0ma to 0V/220ma, corresponding to 2500 ohms).

From this, it looks like I can EASILY get FULL DRIVE from 100 volts on the g2 grid... I was working with the assumption that I'd need at least 150 volts, and was working to try and get 3dB headroom over that. Looks like, if this is correct, I've actually got 6 dB headroom, if I only need 100V!

Anyone see anything wrong with this analysis?

Nothing wrong with more voltage headroom... but if it's true, I certainly can breathe a bit easier about my alignments!!

Regards,
Gordon.
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Old 9th April 2008, 07:04 AM   #5
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Default "bipolar mode"

Hello Gordon,
in this mode the tube is similar to a "bad" bipolar transistor.
Thus you will need some kind of negative feedback to get it linear.
If you take current feedback you'll need more voltage at g2.

Kind regards,
Darius
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Old 9th April 2008, 07:57 PM   #6
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oe, I've found that with a low source impedance driving the screens, the 6JN6 is actually quite linear. Look at the curves- remarkably even spacing, like the best pentode you've ever seen. Where the feedback is needed is to drop the source impedance presented to the output transformer and speaker.
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Old 10th April 2008, 04:14 PM   #7
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Default #6

Hello SY,
I am not so happy with the spacing.
At 100mA plate current the transconductance is 2,4mA/V
and at 250mA it is3,2mA/V. The "hfe" change isn't lower.
Both current and voltage drive at g2 are not providing linear amplification.
Thus you'll need some kind of negative feedback.

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Darius
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Old 11th April 2008, 01:02 AM   #8
GordonW is offline GordonW  United States
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The thing about the increasing transconductance with output current, is that it will, to an extent, act to "counteract" sag... power supply, output transformer signal compression, etc.

That could actually make the feedback's job EASIER...

In practice... these type tubes in screen-drive have been sometimes measured at LOWER open-loop distortion numbers than many normal pentodes, and lower than many triodes... the proof is in the pudding...

Regards,
Gordon.
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Old 11th April 2008, 02:04 AM   #9
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oe, remember that for a normal plate load, you're talking about the region where the tube is maxing out. And since it's push-pull, the transconductance curvature largely cancels out. I'm a few thousand miles from home at the moment, but when I get back, I'll pull out my lab notebook and post my measurements. The midband distortion at 10W out was something like 1.5%, almost all third order, but I'll give you exact numbers. This was with a 600V B+ and 6k plate to plate, if memory serves.

I would recommend to anyone who can dig up the Bascom King review of the Berning EA-2100 in Audio to do so; he showed some very nice low current measurements of the 6JN6 in screen drive.
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Old 11th April 2008, 07:09 AM   #10
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Default no PP and please ...

Quote:
Originally #9 posted by SY
... The midband distortion at 10W out was something like 1.5%, almost all third order, but I'll give you exact numbers. ...
Hello SY,
this result sounds really bad to me.
Try out solid state devices.
Sorry, but this is definitely not the way
I want to build up a tube amp.

Kind regards,
Darius
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Old 11th April 2008, 07:27 AM   #11
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Quote:
Originally #8 posted by GordonW
... In practice... these type tubes in screen-drive have been sometimes measured at LOWER open-loop distortion numbers than many normal pentodes, and lower than many triodes... the proof is in the pudding...
This is a very "clear" statement.

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Old 11th April 2008, 08:28 AM   #12
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I don't know if you've actually looked at datasheets or done bench measurements for common output tubes in conventional circuits, but that's pretty darn good open-loop performance for an output stage, and quite a bit better than any conventional pentodes. I'm puzzled by your statements.

Now if you want to compare to an emitter follower complementary output stage, we need to talk about cathode followers, not grounded-cathode. And my numbers are for a class B stage- let's see bipolars do THAT without a lot of high order harmonics.
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Old 11th April 2008, 09:03 AM   #13
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Default #12 pentode in bipolar mode vs bipolar transistor

Hi SY,
if you run a pentode in the bipolar mode,
you have to compare it with a bipolar transistor in an equivalent circuit.
This is grounded emitter of course. The current transfer curve of an e.g.
BU4525 (hfe 10...15) is much more linear. If you prefer voltage driving
you must place a resistor in series to the base.
At this high driving voltage you'll get excellent results from the
bipolar transistor. It makes no sense to replace the bipolar transistor
by a tube in the bipolar mode, because the bipolar transistor provides
you much better results.

An ideal bipolar transistor has no voltage change at its base,
no hfe change, and horizontal output curves starting at 0 Volt.
A real bipolar transistor is close to this, your pentode (in the
bipolar mode) is far away from this ideal.

Kind regards,
Darius
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Old 11th April 2008, 10:29 AM   #14
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What do you mean by "bipolar mode" in the context of a tube circuit? It's a term I'm unfamiliar with, so I have trouble understanding your point; screen drive is not linear with current, nor is it supposed to be. And it would help if you could cite actual linearity measurements at similar efficiencies for a similar (i.e., push pull) topology. Danke schoen!
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Old 11th April 2008, 11:59 AM   #15
GordonW is offline GordonW  United States
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Sy, IIRC, the distortion you cited- as being almost all third order... there were VERY LITTLE higher harmonics, right?

This is DRAMATICALLY different from most solid-state devices... which frequently, have CONTINUING significant energy well out into the harmonic series.

It's MUCH easier to get rid of 1.5% third harmonic, than it is to get rid of .5% (or less) FIFTH, SEVENTH or NINTH open-loop harmonics, in many cases, IME...

Also, the lower-numbered the harmonics, the more the human ear-brain system "ignores" them, as well... this has been increasingly documented by researchers. And, is why much of modern amp design research (by those who build higher-end audio equipment) is concerned LESS with lower aggregate distortion numbers now, and more concerned with GETTING RID of the HIGHER harmonics...

As for comparison of the 6JN6 and such in screen drive to conventional pentodes- well-regarded pentode and beam-power tubes such as 6V6, EL34, 6L6, 6550/KT88 and such, FREQUENTLY spec out open-loop distortion ratings, under OPTIMUM LOADING (ie, the RIGHT anode load for the tube) of 3%, even 5%, before feedback. It's RARE to see even a TRIODE with less than 1% distortion... and usually at MUCH LOWER power levels. I don't think I've ever seen a triode amp spec'd with OPEN LOOP distortion under 1%, at over 50 watts output... if it exists, I would like to see a link to such, as that would be quite remarkable...

For example of the above, here's a link to the datasheet for the famous 6550... used in MANY really high-performance amps. Note that the SMALLEST distortion spec'd in any alignment is like 2.5%...

http://www.nj7p.org/Tube4.php?tube=6550

Also, datasheet for the commonly-used EL34/6BQ5. Note also, no distortion spec'd under 2.5% here, either:

http://www.nj7p.org/Tube4.php?tube=6bq5

6L6GC- lowest distortion number here is 2%. And despite the lower numbers, MANY would still claim (probably mistakenly, IMHO) that the 6550 is a better tube...

http://www.nj7p.org/Tube4.php?tube=6L6GC


Regards,
Gordon.
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Old 11th April 2008, 02:15 PM   #16
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Default OT?

Quote:
Originally #14 posted by SY
What do you mean by "bipolar mode" in the context of a tube circuit? It's a term I'm unfamiliar with, so I have trouble understanding your point; screen drive is not linear with current, nor is it supposed to be. And it would help if you could cite actual linearity measurements at similar efficiencies for a similar (i.e., push pull) topology. Danke schoen!

Hi SY,
if you want to explain a bipolar transistor you can say it is
similar to a pentode driven at g2 but the voltage at the
base is much smaller.
An ideal pentode has a fixt ratio between g2 and anode current.
This ratio characterises a pentode and is called hfe at bipolar
transistors. A pentode and a bipolar trinsistor is a good one,
if this ratio remains constant.
The bipolar mode is sometimes called enhanced triode mode.
There is nothing triode like but a strong similarity to a bipolar
transistor. Thus I call it bipolar mode.
I thaught we are talking about a device here (6JN6 g2 driven,
bipolar mode). So why do you come up with harmonics,
distortion of different amlifier topologies? Do you want to
discuss about a device or a topology? Sorry but I am not
interested in a topology and harmonics discussion here.

Kind regards,
Darius
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Old 11th April 2008, 02:50 PM   #17
ilimzn is offline ilimzn  Croatia
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Default Re: OT?

Quote:
Originally posted by oldeurope
Hi SY... So why do you come up with harmonics,
distortion of different amlifier topologies? Do you want to discuss about a device or a topology? Sorry but I am not interested in a topology and harmonics discussion here.
And since you are not the person who posted the original question (did you read the original question?) why should it matter to us all contributing answers to the original question, what your interests are?

That being said, the discussion was about choosing a good load line for a 6JN6 in G2 drive mode, push-pull. It seems to me that SY's answer precisely fits that discussion. Besides, a component has no linearity or whatever unless it's working in a circuit, which is a yet another word for topology. Take note tof what the title of this thread is, so why are we discussing bipolar transistors?
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Old 11th April 2008, 03:44 PM   #18
GordonW is offline GordonW  United States
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Default Re: OT?

Quote:
Originally posted by oldeurope


So why do you come up with harmonics,
distortion of different amlifier topologies? Do you want to
discuss about a device or a topology? Sorry but I am not
interested in a topology and harmonics discussion here.

Kind regards,
Darius
Because without careful analysis of the harmonic STRUCTURE of an output stage, there is NO real way to qualify the actual "fidelity" of an amplifier, AS HEARD BY THE HUMAN EAR. As experiments in hearing continue (testing the acuity or noticeability of harmonics, especially higher-order harmonics), there is more and more data supporting the claim that HIGHER order harmonics make up the majority of the problems associated with "bad sound" from otherwise "good measuring" amps.

A bipolar transistor or MOSFET amplifier may have very low "total" distortion (oftentimes in the thousands of a percent range), but compared to typical modern tube amp designs, the transistor amp will frequently have MORE upper harmonic energy. While the tube (especially screen-driven) will have significant third-harmonic output, it will have VERY LITTLE harmonic content above 7th harmonic, compared to anything else.

It's not just about total distortion... it's about WHAT KIND of structure the distortion takes. Since tube amps generally need less GLOBAL feedback, they tend to not "re-map" harmonics up to higher order numbers... which can be a BIG THING, for the overall sound of the amp!

Regards,
Gordon.
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Old 11th April 2008, 04:17 PM   #19
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Looking at the 6JN6 in G2 drive curves, it appears to me that two opposing effects are occuring, which might be optimiseable.

At high plate current/low plate voltage, the screen current goes up dramatically, thus causing the "hfe" between screen and plate current to droop seriously. On the other hand, voltage drive would be causing the usual 3/2 power expansive growth of gm, giving the increased curve spacings seen in the charts. So we have compression in one case and expansion in the other.

It would seem that maybe some optimum drive impedance (combined with some optimum load Z) must exist for minimum distortion in g2. Which would play one effect off against the other. Also, since the g2 grid is further away from the cathode, we should not be seeing the usual grid wire proximity effect (island effect, inselbildung) compared to normal g1 drive. So we might possibly end up with a more linear device this way. Just a thought.
------------------------------------

Another approach to "fixing" the "hfe" would be to put a Mosfet cascode above the g2 driven pentode and use a zener (maybe 50V) and pullup resistor to control its gate a few volts above the screen. This would cause the pentode's plate to always follow a few volts above the screen, giving a constant ratio for screen current. (ie, constant hfe or Beta) (I see this same nice effect when using g2 feedback from a Mosfet follower driven by the plate with a CCS pullup on the plate.) G2 Current drive should then be quite linear (well, versus current out anyway).

Don
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Old 11th April 2008, 04:29 PM   #20
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Quote:
Originally posted by GordonW
I drew a load-line for 2500 ohms primary impedance (5000 ohm CT PP transformer), and 550 volts B+ on the tube plate (I "extended" the graph out to where 550V would be, and drew the line from 550V/0ma to 0V/220ma, corresponding to 2500 ohms).

From this, it looks like I can EASILY get FULL DRIVE from 100 volts on the g2 grid... I was working with the assumption that I'd need at least 150 volts, and was working to try and get 3dB headroom over that. Looks like, if this is correct, I've actually got 6 dB headroom, if I only need 100V!

Anyone see anything wrong with this analysis?
Since (I assume) that this amp is mostly class B, the loadline will be closer to Z/4, or 1250 ohms, putting the Vp=0 at around 440mA. Adjust negative Vg1 so that the 'knee' of the max Vg2 voltage you will have at signal will intersect the loadline (probably near Vp=50V). I believe this will give the best linearity.
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Old 11th April 2008, 05:11 PM   #21
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Thank you very much Don,
this #19 is the best post in this thread.
You are making thoughts how to optimize the device
instead of this harmonics distortion off topics.
I have another idea to increase linearity in the bipolar mode.
Kind regards,
Darius
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Old 11th April 2008, 07:36 PM   #22
GordonW is offline GordonW  United States
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Quote:
Originally posted by Boris_The_Blade


Since (I assume) that this amp is mostly class B, the loadline will be closer to Z/4, or 1250 ohms, putting the Vp=0 at around 440mA. Adjust negative Vg1 so that the 'knee' of the max Vg2 voltage you will have at signal will intersect the loadline (probably near Vp=50V). I believe this will give the best linearity.
Yep, that sounds right, on second thought. It's too easy to forget the effect of the OTHER side on a PP transformer.

I was just conversing with someone else, who had suggested a very slightly negative voltage for g1... and I was thinking of ways to do that. One idea that came to mind, was to "voltage-divide" it... by a resistor from my B- rail (or a dropped-voltage B- rail, to make the resistor ratios a bit more "even") and a resistor from the cathode. That way, the g1 could "track" the cathode a bit, while still biasing the thing downward a bit too. Might help gain go up a tad, as opposed to just tying the g1 to a negative static voltage?

Oooh... an idea... what about putting an LED between g1 and cathode, and a resistor between g1 and the negative rail, to hold g1 at a CONSTANT negative bias voltage relative to the cathode? That should be able to be done at NEGLIGIBLE currents, I would think...

Regards,
Gordon.
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Old 11th April 2008, 10:54 PM   #23
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Gordon, exactly right, the third dominates, as it should. I still don't get the analogy to bipolars since the Vbe is almost constant and Vg2k is anything but constant.

edit: I tried biasing the grid slightly negative in my 6LF6 screen drive amp and wasn't impressed with the results- it seemed to reduce the mu which was already low.
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Old 11th April 2008, 11:31 PM   #24
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Hey Sy,

Did you talk to tubelab about the negative g1 voltage? I hope he chimes in here.
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Old 12th April 2008, 12:09 AM   #25
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No, I haven't. George and I seem to have followed some similar paths independently and come to pretty similar conclusions (especially about MOSFET drivers), so I'll be interested to see what he's got to say, especially if he went at it more systematically than I did.
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