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
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