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In Reply to: RE: Wow. posted by Michael V on March 01, 2008 at 19:50:08
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
|P| |G| |K| |G| |P|
The above crude asc art represents the profile of the 12B4 and I call this the "planar model" It has a single cathode in the center and moving outward a grid and a plate on either side. Each K-G-P structure makes up a single triode and when electrically connected the two are placed in parallel. Since the tube characteristics are based on the spacing of these elements, if the G and K structures are not perfectly centered, the two triodes you parallel will have different characteristics, the fact that they are internally connected simply prevents you from seeing this differing behavior. Now to make the model a little more complex, consider what happens when the elements of the KGP structure are not perfectly parallel.
A working example of this is the EC8010 and EC8020 combo. Those two tubes share the same internals and the EC8020 has a PGKGP "planar" structure like the 12B4. The EC8010 has the same structure but with only one plate so if the 8010 is a single triode, the 8020 must be two triodes in parallel. Looking at the electrical specs of the two tubes, they both have the same mu but the EC8020 has double the Gm and 1/2 the Rp of the ec8010 just as you would expect from a paralleled triode.
dave
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