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cascodes [message #9109]

Mon, 25 July 2005 18:44

Thrint
Messages: 43
Registered: May 2009

Baron

From a few cascode-able valves like 6BQ7 and 6DJ8, the schematic shows a pair of triodes stacked on top of each other and the upper grids attached to a voltage divider reference.

The composite valve behaves like a pentode. So what happens if you sub the top triode for a pentode? Use the same voltage divider for the control grid, dropping R and bypass to its cathode for g2 and what does it do now?

How to model this composite? I know how to gneerate the composite's plate curves for any pair of triodes, but the triode/pentode combo looks a bit more complex...
thanks,
Douglas

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Penscodes? [message #9112 is a reply to message #9109]

Tue, 26 July 2005 04:39

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Altough I didn`t try it, IMO - pentode can be implemented (little problem with g2 voltage, but some people use pentode for "upper" tube in mu-follower, or like active load). But, IME - gain of pentode depends very little of "upper" tube. A~gm1*Rl, where gm1 is gm of the "lower" tube.
Composite model for "penscode" - I have no idea, maybe SPICE it first and see what PC says...

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Re: Penscodes? [message #9115 is a reply to message #9112]

Tue, 26 July 2005 06:38

Damir
Messages: 1005
Registered: May 2009

Illuminati (2nd Degree)

Little correction, above sentence is:
IME - gain of cascode depends very little of "upper" tube.

BTW - Doug, besides "penscode" - do you work on something?

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Re: Penscodes? [message #9135 is a reply to message #9115]

Sun, 31 July 2005 17:26

Thrint
Messages: 43
Registered: May 2009

Baron

Hey-Hey!!!,
I have a triode on the bottom, and have installed a pentode on top. Actually, I just turned the upper triode back into a pentode with a dropping R to its g2, and a cap between g2 and cathode.

I have a feeling I have turned the composite circuit into somthing with good AC impedance, or horizontal plate lines. Even better than the traditional triode-on-a-triode cascode.

The upper element of the cascode does do one important thing, it determines where the horizontal, pentode-ish plate characteristics start, relative to the Ec2 voltage. I suspect that the pentode offers a lower voltage start, and more horizontal when they happen, set of plate characteristics.

And I have a day job which gets in the way of serious experimentation.
regards,
Douglas

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