Andrew Williamson wrote:
>I don't know much about amps or tubes but I'm learning a LOT from the
>discussions on this reflector. That's why I subscribed. So, here's a
>question which I hope isn't too far off topic.
>
It's absolutely spot-on topic.
>As a user of an ETO91b which uses a pair of 4CX800A/GU-74b's can anyone
>tell me what the grid current is when the red 'grid max' light
>illuminates. Is it in the 3-10mA range that is 'nutsville'. I'd hate
>to damage the tubes as I believe they are quite delicate in the grid
>current area, even if they are cheaper than 3CX800's.
It depends what you call "delicate". The 4CX800 has a rated maximum grid
dissipation of 2W, which means that at the normal bias voltage of -60V
you could run as much as 30mA grid current without damage.
You wouldn't want to do that, for linearity reasons. The tubes are meant
to operate in class AB1, which is why the ALC operates at the first
trace of grid current and the amplifier trips to standby at 3mA. Also
the 91b has a low-impedance grid bias supply, so small amounts of grid
current do not spell IMD "nutsville".
What about the 4CX350A, 4CX1000 and other tubes with a rated grid
dissipation of "zero"? Think about that: it cannot be literally true.
The grid must have some power dissipation capability. 1mA of grid
current adds only 60 milliwatts of heat load to the grid (which is right
up close to a cathode that dissipates 1000 times more heat energy). I'm
not saying that anyone should aim to run grid current, but the "Absolute
Maximum" power dissipation rating of "zero" should not be taken
literally.
It's also notable that "zero" is only seen in data sheets that are 30
years old now. It was probably meant to educate people about those new
tubes that were not intended to run grid current, but it was over-
dramatic and created a "disaster" mentality among users. With later
tetrodes intended to be operated in class AB1, Eimac and Svetlana have
started to give more realistic numbers.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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