by dcgillespie » Tue Jun 28, 2011 4:24 pm
Kevco -- If I am understanding your question correctly, I think some clarification is in order. Any tube that has 5 active elements to control the current flow at one plate (besides a heater) is called a pentode. It makes no difference if the third grid is an actual grid, as in what some call a "true" pentode, or it is a beam forming element, as used in a Beam Power tube. Both of these tube types by definition are pentodes. An EL34 and EL84 illustrate the former, while a 6L6 and 6550 illustrate the latter. However, how this third elements operates is somewhat different between these tube types.
When a true suppressor grid is present, it functions much like a control grid in reverse. That is, since the suppressor grid is most always at cathode potential, it retards the flow of any electrons that might try to flow back to the screen grid when the plate voltage is low, similar to the control grid retarding electron flow due to it's relatively more negative charge than that of the cathode.
In pentodes with beam forming elements, these elements slow the electron flow in the vicinity of the beam element, causing a space charge to form in that region. This space charge then becomes a virtual third grid that functions similar to an actual grid in repelling any lose electrons back to the plate.
A beam power tube has the additional advantage of employing aligned control and screen grids. This has the effect of letting the screen hide in the shadow of the control grid, allowing it to run cooler, and drawing less current for a given total current flow through the tube. As a result, beam power tubes are typically more efficient that pentodes employing a third grid.
In any event, both of these tube types are always considered as being a pentode, as both regulate secondary emission from the plate. Unless the screen is strapped to the plate to produce triode operation, they are always considered as operating in some form of pentode operation. If the screens are decoupled AC wise to the cathode, then they are operating in true pentode mode. If the screen is operating from a tap on the OPT primary, then it is operating in partial pentode mode (or partial triode mode depending on your point of view!), commonly call Ultra-Linear operation.
In the days before pentode tubes were available, true tetrodes were available, consisting of a cathode, two grids, and the plate. These tubes were an advancement over that of the triode, but have very quirky characteristics, as the plate voltage must never be allowed to dip below that of the screen, or otherwise, the screen will quickly be destroyed. With the advent of the pentode tube, that problem was eliminated, so that power went way up, as did gain and efficiency as well.
I hope this answers your question!
Dave