> Peter Voelpel wrote:
>> I imagine the two holer was built to run the ham limit.
>> If you inspect the Pi values and calculate back, you will
>> see for what power
>> level they are constructed.
Why would that be? The range of Q is one of the least
critical parameters so long as it is a minium of something
significantly greater than the square root of the loadline
impedance over the load impedance. I can use the same tank
component values at 10,000 watts as 1,000 watts with very
little change in performance so long as they are large enogh
in size and meet the minimum Q requirement.
>> The drive requirement will be lower with two tubes for
>> the same output
It won't be significantly lower in grounded grid.
In a grounded grid PA that is not saturated the gain is
largely determined by the ratio of driving to load line
impedance. Placing two tubes in parallel changes both a
nearly equal amount. As a matter of fact, this is a huge
worry in amplifiers using multiple tubes. Say you have three
8874's in parallel. If a single tube fails the drive power
to the other two tubes simply increases, and the net output
is very close to the same amount. This can quickly lead to
loss of another tube from excessive drive.
There is some gain increase, but it certainly is not
anywhere near a 1:1 improvement.
A 4-1000A is a good 1000-1200 watt output tube when in
grounded grid, but it is generally gain starved in a good
stable circuit. The reason is the driving impedance of a
4-1000A is high, around 70-90 ohms. The high driving
impedance reduces the gain compared to other tubes like
3-1000Z's or 3-500Z's operating at the same anode voltage.
It generally takes 140-150 watts to make 1500 watts on a
4-1000A in cathode driven service even when the anode is at
4 or 5 kV.
Of course power meters are all over the place and
regeneration from poor layout can increase gain, so we can
expect to see higher claimed gains but all in all the
4-1000A is not the "big tube" we fondly recall. Like the
Rhombic or V Beam or a 66 Mustang GT Hi-po that only runs
15's in the 1/4 mile, they gained that reputation when we
just didn't know what a good thing really was.
73 Tom
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