Triode mode question

[catalin gramada]

Hello. I wonder if matter if a pentode is used in triode mode without screen grid connected somewhere ? I did not found any infos regard what did a difference in triode mode with the screen grid connected to the plate vs screen grid floated, not connected at all. What differences could be ? Any thoughts please ? Thanks.

[Chuck H]

I don't have the technical electronics answer for this, but AFAIK the screen grid in a pentode is spaced to focus electrons at the plate. Without it the plate is not physicaly close enough to the control grid (EDIT: Technically the spacing would be relative to the cathode and the control grid is the "valve" as it were). The typical "triode" connection for a pentode tube essentially makes the screen and plate one thing so that proximity to the control grid (or cathode) is correct. Also AFAIK a pentode won't operate with the screen grid disconnected. Some guitar amplifiers disconnect the screens as a "standby" method.

[catalin gramada]

I don't have the technical electronics answer for this, but AFAIK the screen grid in a pentode is spaced to focus electrons at the plate. Without it the plate is not physicaly close enough to the control grid. The typical "triode" connection for a pentode tube essentially makes the screen and plate one thing so that proximity to the control grid is correct. Also AFAIK a pentode won't operate with the screen grid disconnected. Some guitar amplifiers disconnect the screens as a "standby" method.

Thanks. Allright, so in a pentode/tetrode the plate and cathode are spaced so away to do a useful electrons path without the screen grid interposed in between...?

[Helmholtz]

The (negatively charged) electrons that pass the grid are attracted and accelerated by the nearest positive electrode.
With a pentode/tetrode the nearest positive electrode is the screen.
Simplifying one might say the plate hides behind the screen (is screened) so the electrons can't "feel" the plate potential.

When a screen is floating, it might even get charged negative by landing electrons.
A negative screen will repel further electrons and thus stop the plate current.