W8JI wrote:
>Hi Ian,
>
>> 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.
>
>Grid dissipation is not bias voltage times grid current. It is
>the value of POSITIVE grid voltage (with respect to the
>cathode) times grid current integrated over the conducting portion of
>the grid drive cycle.
>
>I have no idea what the exact value is, but from a rough look at the
>4CX800A data it looks like 30 mA indicated on a grid meter would cook
>the grid rather quickly. It would take considerably more than 60
>volts positive voltage to drive the grid to 30mA current. So if
>the time integrated (by the meter's response) grid current is 30 mA
>we can be certain the time integrated positive grid voltage is
>VERY much higher than 60 volts!
Oops - my mistake, thanks for the correction, Tom.
I extrapolated too far from the simple fact that the 4CX800 and 400 have
the same 2W maximum grid dissipation as the 4CX250 which can be operated
in class C. Actually the V/I characteristics are quite different - at
the same instantaneous positive grid voltage, the 400 and 800 will draw
a lot more grid current than the 250.
>
>> 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.
>
>The grid can indeed be ruined by grid currents of more than a few mA
>with tubes like the 4CX1000A Ian. This is a sad fact many owners of
>30S1's discovered.
>
I'm sure they did, especially if they tried to run that particular
amplifier without ALC, or on CW with the bias switch in the 'SSB'
position (the ALC in the 30S-1 is AC-coupled and doesn't work at all on
CW).
Unlike the more modern data sheets for tetrodes intended for AB1
operation, the 4CX1000 curves do not show what happens to grid current
with positive grid voltages, but it probably does increase very quickly.
Actually the notes in the rear of the 4CX1000 data sheet say that "peak
grid currents of less than 5mA, as read on a 5mA meter" are allowed for
"peak-signal monitoring purposes". Reading between the lines, this means
that you can run up to 5mA of grid current on modulation peaks, in order
to generate an ALC signal.
The 4CX1500B data sheet (issued 20 years after the 4CX1000 data) is far
more helpful, rating the grid dissipation at "1 watt". The typical
operating conditions are for "class AB2" but with only a few hundred
microamperes of two-tone grid current. This is exactly how a real-life
"class AB1" amplifier would operate with grid-current derived ALC, and
those ratings are much more realistic and helpful than a simple "zero".
Under those operating conditions, including a *small* amount of grid
current, Eimac says that the 3rd/5th order IMD of the 4CX1500B is about
-40/-47dB.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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