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

[Tom W8JI]

No wonder the resistors opened.
At 100mA grid current there are already 0,27W heating them 
up. Most
resistors are rated with their wattage at a 25°C 
environment.>>

Actually resistors and fuses both are very unreliable for 
life when operated anywhere near their ratings. Not only 
that, a fuse can tolerate several times the operating 
current under brief duration overloads while a resistor can 
tolerate dozens or even hundreds of times its rating.

The only way to go when you want to approach and not exceed 
a certain critical limit is an electronic overload circuit.

Grid current usually is one third of anode current in 
grounded grid linears.>>

Not really. It all depends on the tube and how the tube is 
used. An 8877 can be killed by 10-20 seconds of grid current 
time at 200mA (that's about 1/5th of anode current), and 
much less than that at higher grid currents. The damage is 
also cumulative over repeated short overloads.  Try to find 
a resistor or fuse that will fail in less than a second or 
two at 100-150mA and not fail over hours of use at 50mA. It 
can't be done.

When I was prototyping a 3CX800 amplifier I disconnected the 
grid protection because I assumed I could watch for 
overloads. Afet a days worth of what I thought were small 
tuning mistakes that I caught soon enough I ruined the 
tubes. I reconnected the electronic overload and never lost 
another tube.

73 Tom