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Old 4th April 2008, 03:08 PM   #1
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Default cascode vs asymmetrical cathode coupled

Variability of RIAA eq with tube characteristics?
split from post #16
Quote:
Originally posted by Sheldon


Which triode characteristic does the cascode kill? The one triode characteristic I'm aware of, that the cascode arrangement kills, is the Miller capacitance. I don't think the plate curves don't all of a sudden become pentode like.

Sheldon
Hi Sheldon,
the influence of the plate voltage in the plate current
for example is lost.
Yes, the plate curves become pentode like.
The bottom tube sets the plate current of a cascode
arrangement, in a pentode the g2 current sets the
plate current.
Similar to the base current setting the collector current in a
bipolar transistor.

Kind regards,
Darius
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Old 4th April 2008, 04:16 PM   #2
Sheldon is offline Sheldon  United States
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Ok, I'll buy that part. However, I think that the distortion profile will not be pentode like, in that the grid curves will not bunch up going from 0 to negative. So maybe it's something in between. Are there any examples of cascode curves published?

In my preamp (a version of the Bench amp), the second gain stage is a triode. I would expect that this would dominate the distortion profile, as the cascode should be quite linear.

Sheldon
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Old 4th April 2008, 04:29 PM   #3
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Quote:
Originally posted by Sheldon
Are there any examples of cascode curves published?

Yup, some datasheets for tubes designed to be run that way actually have curves. Look at 6BK7 and 6BQ7, for example...
Attached Images
File Type: gif cascode.gif (42.8 KB, 827 views)
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Old 4th April 2008, 05:05 PM   #4
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Default Cascode similar to pentode

Quote:
Originally posted by pmillett



Yup, some datasheets for tubes designed to be run that way actually have curves. Look at 6BK7 and 6BQ7, for example...

Hi pmillett
your attachement shows exactly what I expected from a
cascode arrangement, thanks.
Kind regards,
Darius
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Old 4th April 2008, 05:35 PM   #5
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Default #2

Quote:
Originally #2 posted by Sheldon
... However, I think that the distortion profile will not be pentode like, in that the grid curves will not bunch up going from 0 to negative. So maybe it's something in between. Are there any examples of cascode curves published?

In my preamp (a version of the Bench amp), the second gain stage is a triode. I would expect that this would dominate the distortion profile, as the cascode should be quite linear.

Sheldon
Hi Sheldon,
I am sure it will be pentode like because in a cascode
the lower triode works in short circuit and not with gain.

The output tube should dominate the distrotion profile.
But this does not mean that the preveous stages have
no influence in it.
I want an all triode line.

Kind regards,
Darius
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Old 4th April 2008, 05:46 PM   #6
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Hello pmillett,
can I use your attachenent from post #3
http://www.diyaudio.com/forums/attac...amp=1207322982
in my blog, please?
http://coupling-triode.blogspot.com/

Kind regards,
Darius
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Old 4th April 2008, 06:16 PM   #7
Sheldon is offline Sheldon  United States
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Sehr interresant!

Below 150V the plate curves are triode like, above that, pentode like. Well Darius, maybe we can eat our cake and have it too.

Sheldon

edit: I see that you were busy posting before I responded to Pete's neat little graph. Nice site Darius. I'll have to stare at it a while before I comprehend some fraction of it. For sure, corrected some assumptions I held and learned some new stuff today about cascodes.
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Old 4th April 2008, 07:44 PM   #8
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Quote:
Originally posted by oldeurope
Hello pmillett,
can I use your attachenent from post #3
http://www.diyaudio.com/forums/attac...amp=1207322982
in my blog, please?
http://coupling-triode.blogspot.com/

Kind regards,
Darius
Yes, of course, no problem...

Pete
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Old 5th April 2008, 07:58 AM   #9
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Default #7 #8

Quote:
Originally # 7 posted by Sheldon
Sehr interresant!

Below 150V the plate curves are triode like, above that, pentode like. ...
Hello Sheldon,
interesting discussion, thanks.
Pelase note that the anode of the cascode can not provide
any anode current at voltages below the cathode voltage
of the upper section. In Pentode curves the g2 voltage is fixt,
in this graph it comes from a high R voltage divider.
The curves below say 150V are mainly given by these resistors.
Thus there is nothing triode like in this cascode arrangement.

#8 Thank you very much, Pete.

Kind regards,
Darius
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Old 5th April 2008, 04:43 PM   #10
Sheldon is offline Sheldon  United States
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Default Re: #7 #8

Quote:
Originally posted by oldeurope


Hello Sheldon,
interesting discussion, thanks.
Pelase note that the anode of the cascode can not provide
any anode current at voltages below the cathode voltage
of the upper section. In Pentode curves the g2 voltage is fixt,
in this graph it comes from a high R voltage divider.
The curves below say 150V are mainly given by these resistors.
Thus there is nothing triode like in this cascode arrangement.

Darius
I think I understand what you are saying. But let me make sure. The region below about 150V definitely looks triode like (the influence of plate voltage on current). The upper grid is set at 125V according to the figure. Are you saying that the high value resistors are functioning as a type of grid leak biasing, and that is causing the effect of plate voltage on current? Does it follow that in this region this configuration is not actually behaving as a cascode?

Sheldon
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