Ref: Triode vs Tetrode/Cathode Drive
If you compare data sheets of a couple of modern popular tubes, 8877 triode
and GS23B tetrode, with cathode drive you find both tubes have a rated
transconductance of 55,000mho at rated plate current. One would expect similar
gains
from similar ratings.
Experience with the triode, 8877, at 144mhz shows about 15db gain at 2700w
output. Experience with the GS23B at 432mhz shows about 14.5 db gain at 2500w
output. The 432mhz amplifier has about 7% cavity loss at this frequency so a
good part of the small gain difference can be accounted for here.
To acheive this level of output power with the GS23B tetrode, full rated
screen voltage must be used. When the screen voltage was lowered, the plate
current was lowered by a similar amount. The power input and power output was
reduced. To achieve proper idling plate current with the lowered screen
voltage, the
grid bias had to be reduced by a similar ratio. When this was done, the
driving power had to be reduced to keep the grid current within ratings. The
gain
of the amplifier was about the same with lowered screen voltage but power
output was reduced.
There may be some credance to Rich's comment about screen voltage having no
effect on tube gain but it certainly has a major effect on plate current and
power output.
Additionally, every tube type has it's own characteristics. In cathode driven
service a high mu tetrode like the GS23B will certainly have more gain than a
lower mu tetrode like the 4-400A. The same happens with triodes, comparing
the 8877 with a 3-1000Z, for example.
On the subject of "Super Drive", defined by Rich as having a gain of 4 or 5,
why in the world would anyone be satisfied with such sorry performance? It is
hard to understand why this kind of circuit would be used and judging from the
lack of information doing a Google search, it probably is not used.
I don't think there is any disagreement about the gain of the tetrodes being
higher in grid driven vs. cathode driven service. The input impedance of many
modern cathode drive tubes is on the order of 20 to 50 ohms and the input
impedance of the grid driven design is determined primarily by the termination
resistance used. It takes a lot more drive power to develop a given drive
voltage
swing when the impedance is considerably lower.
The real reasons to use a tetrode instead of a triode in cathode drive
service are based on the need for added input/output isolation at vhf/uhf and
the
availability of good tubes at a decent price. If I had a tetrode on hand for an
h.f. amplifier project and no triode on hand, guess which tube would be used?
Tnx/73,
Gerald/K5GW
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