On Jul 20, 2006, at 9:00 AM, Tom W8JI wrote:
> 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.
How many ampere disconnect transistor do you use in your electronic
grid protection circuit, Tom?
>
> 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
The 8877's grid dissipation rating is 20w on some Eimac spec sheets
and 25w on other Eimac spec sheets. . Typical cathode-grid driving
potential is 70v-pk or 50V-rms. How does 50v @ 200mA damage the grid?
> ...
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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