[Amps] Grid fuses (was: Life and gain of 3-500Z)

[S. J. Blackwell]

Bill,
In my younger days, 1949-51, I built an 813 Amp modulated by TZ-40's in PP. 
The 813 used grid leak bias, no fixed bias supply. I used a VR150 to develop 
bias and placed a high quality (for the time) paper cap across the VR tube. 
My plan was to always put B+ on the amp with drive. I could then key the 
driver(off) and the VR tube would extinguish. This would leave a fixed bias 
on the grid stored in the cap, cutting off the 813 between CW dits and dahs. 
I had assumed that the 813 would eat itself if I forgot to turn on the drive 
power when the B+ was switched on. (screen supply was a dropping resistor 
from B+). Of course the plan was not fool proof, but much to my surprise , 
with the grid floating with the VR150  hung off it  the 813 never did melt 
down, even with no grid bias and the grid floating on the VR150. I allways 
assumed the electron space charge from the filament charged up the cap. I 
was just a kid then and had forgotten about that old amp. Your comments 
reminded me. Used a BC 696 with 6V6 multipliers to get to 20 meters.
   Just an old memory from the fun old days.


>My understanding is the grid accumulates electrons from either the
>space charge surrounding the cathode or the cathode-anode electron
>stream and has no way of getting rid of them. Thus the grid becomes
>negatively charged to a small extent. This negative charge repels
>additional electrons from accumulating and a balance is achieved.
>
>Willing to stand corrected if necessary.
>
>Bill, W6WRT
>_______________________________________________
73,
Sam, W5LU