[Amps] 3CX800 - a science experiment

[Joe]

So,
Would running the AMP Up-Side-Down possibly un "SAG" the offending part 
and the tube work again maybe?

Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 6/1/2019 10:23 AM, Ron Youvan wrote:
>    Steve N2IC London wrote:
>
>> As I have previously mentioned, I have two Alpha 3CX800A7LG's that 
>> appear to have grid-cathode shorts that develop within a few minutes 
>> of applying filament voltage. After seeing VE5RA's posting about 
>> shorts clearing themselves after 6 minutes, I did some experiments 
>> with the two tubes. 
>  /* snip */
>> After a 20 minute warmup, I have these voltages:
>>
>> + filament to grid 9.5
>> - filament to grid 0.5
>>
>> + filament to cathode 8.9
>> - filament to cathode -2.9
>>
>> The grid ring is extremely hot.
>>
>> What's going on in the tube ?
>
>   I know I answered this before, but I can't locate it in my reply's to
> amps.  (must-a gone to another reflector ???)
>
>   Modern transmitting tubes, mostly of the coaxial style have a
> cylindrical heater (cage or mesh construction) or cathode (pipe)
> surrounded by a cage type control grid which is surrounded by a
> cylindrical cage type tetrode screen grid or by a (triode) cylindrical
> anode. (plate)
>
>   In this construction the electrons travel to the plate in "vertical
> sheets" in line with the "slots" in the control grid, the "slots" in the
> screen grid (if any) are aligned with the electron sheets.
> Keeping the electrons in sheets has several benefits.
> Electrons moving in a vacuum are repulsed by each other, only a strong
> magnetic or electrostatic field can keep them moving in a organized
> manor instead of re-ordering into a cloud.
>
>   With extended use (15,000 to 30,000 hours of normal use) these control
> grid wires will slump (inwardly and outwardly at their bottoms) until
> eventually some touch the cathode or filament creating that problem.
>
>   If oriented horizontally they would sage in the center in a very 
> short time.
>
>   If these tubes are operated in excess of their specification for
> control grid dissipation the overheated control grid will sag
> prematurely causing this problem at an early age.