6JN6 screen-drive (g2 driven) plate curves? - diyAudio
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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!

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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

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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!!

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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
SY is offline SY  United States
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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
SY is offline SY  United States
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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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