You can estimate from published curves.
Zeroth approximation:
Low triode works into a short circuit (vertical load line). Anode current merely goes through the upper triode - infinite Ra so curves are actually horizontal lines.
First approximation:
Low triode works into a load of 1/gm, so almost vertical load line. Upper triode sees lower triode as cathode degeneration resistor of Ra, so upper anode impedance is (mu+2)Ra. Curves are pentode-like.
Second approximation:
Remember that gm depends on current. This is when it gets messy.
Zeroth approximation:
Low triode works into a short circuit (vertical load line). Anode current merely goes through the upper triode - infinite Ra so curves are actually horizontal lines.
First approximation:
Low triode works into a load of 1/gm, so almost vertical load line. Upper triode sees lower triode as cathode degeneration resistor of Ra, so upper anode impedance is (mu+2)Ra. Curves are pentode-like.
Second approximation:
Remember that gm depends on current. This is when it gets messy.
Not sure if it helps you any but the datasheets for some of my favorite "cheap alternates" have cascode curves... like the 6BK7 and 6BQ7:
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BK7B.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BQ7A.pdf
Pete
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BK7B.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BQ7A.pdf
Pete
Not sure if it helps you any but the datasheets for some of my favorite "cheap alternates" have cascode curves... like the 6BK7 and 6BQ7:
Great, that is helpful. Thanks.
Is this what you need?
http://www.ianbell.ukfsn.org/pics/e88cc-phillips.pdf
Last page but one.
Cheers
Ian
http://www.ianbell.ukfsn.org/pics/e88cc-phillips.pdf
Last page but one.
Cheers
Ian
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