Another (possible) consideration: any indirectly heated valve - especially where there is a close grid→cathode spacing - is liable to accumulation of cathode-coating material. Frame-grid types would qualify.
With a potentially emissive grid, with a normal Ua (say, 200V) and large negative Ug (say -200V), one may worry about enough grid-current to fuse a 10µm grid-wire. Maybe?
With a potentially emissive grid, with a normal Ua (say, 200V) and large negative Ug (say -200V), one may worry about enough grid-current to fuse a 10µm grid-wire. Maybe?
In this article from a Philips Technical Review from 1956 (starts on page 181; the relevant part for this discussion starts halfway on the right hand side of page 185; I couldn't upload only this article because of it's size) the authors describe that a negative control grid will be hit by positive ions.
They don't describe the effects of not obeying the maximum value(s) for cathode to grid voltage but I would think that the more negative the control grid goes, the faster the ions will hit the control grid. Maybe the limit (also?) has to do with this effect?
https://worldradiohistory.com/Archive-Company-Publications/Philips-Technical-Review/50s/Philips-Technical-Review-1956.pdf
They don't describe the effects of not obeying the maximum value(s) for cathode to grid voltage but I would think that the more negative the control grid goes, the faster the ions will hit the control grid. Maybe the limit (also?) has to do with this effect?
https://worldradiohistory.com/Archive-Company-Publications/Philips-Technical-Review/50s/Philips-Technical-Review-1956.pdf
The control grid is hot, its right close up to the cathode surface, and if the tube has significant grid current it will be even hotter - so perhaps the voltage limit is to prevent bad things happening due to thermionic emission from a hot grid during negative signal swings?
Or it could simply be a field-emission limit due to the tiny separation from the cathode. (This is I think the same as flashover mentioned above) Its likely to be quite conservative rating to allow for manufacturing defects that create sharp points where field can concentrate.
Just a thought or two.
Or it could simply be a field-emission limit due to the tiny separation from the cathode. (This is I think the same as flashover mentioned above) Its likely to be quite conservative rating to allow for manufacturing defects that create sharp points where field can concentrate.
Just a thought or two.
Your use of 'signal swings' made me wonder if the value for the maximum control grid to cathode voltage is the value without swing. If so, than with swing you could go more negative than the stated value, comparable with Va max. for power tubes in datasheets (driven, the anode voltage swings way higher than Va max.).
Addition: There would be not much use to go more negative than the maximum value because the tube is already deep in cut-off, but maybe it can happen in some (dc-coupled) circuits.
I tried to find "Rating systems for electronic tubes and valves and analogous semiconductor devices" (IEC Publication 134 from 1961, in today's code: IEC 60134:1961). In Philips tube manuals they refer to that norm when explaining the meaning of tube ratings. I couldn't find it for free though.
Addition: There would be not much use to go more negative than the maximum value because the tube is already deep in cut-off, but maybe it can happen in some (dc-coupled) circuits.
I tried to find "Rating systems for electronic tubes and valves and analogous semiconductor devices" (IEC Publication 134 from 1961, in today's code: IEC 60134:1961). In Philips tube manuals they refer to that norm when explaining the meaning of tube ratings. I couldn't find it for free though.
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Corona exists in space...because it is not a vacuum, but gaseous near vacuum of Helium, Deuterium, and Tritium (and probably other things I dont know about)
Presumably, these can be ionised, just like any other gas.
But corona does not exist in a vacuum.
(Unless there are fluid atoms in that vacuum to ionise...making it not a vacuum)
Presumably, these can be ionised, just like any other gas.
But corona does not exist in a vacuum.
(Unless there are fluid atoms in that vacuum to ionise...making it not a vacuum)
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