[Amps] SB-220 bias question
xxw0qe at comcast.net
Sun Aug 30 10:47:33 PDT 2009
WHOOPS, not Analog Devices but Linear Technology.
Larry Benko wrote:
> Might I make a simple suggestion.
> Analog Devices gives away a free program called LTSpice
> (http://www.linear.com/designtools/software/ltspice.jsp) which is a
> great tool for understanding how circuits work. Just model the tube,
> build the PI or PI-L network, and measure every voltage and current for
> every level of drive. Efficiency can also be measured easily.
> This tool should be on just about everyones PC who does any design or
> who wants to understand how circuits work. LT SPice also has a Yahoo
> usergroup that is extremely helpful.
> 73, Larry, W0QE
> Bill, W6WRT wrote:
>> ORIGINAL MESSAGE:
>> On Sun, 30 Aug 2009 10:12:31 -0400, "Joe Subich, W4TV" <lists at subich.com> wrote:
>>> If you change the network by changing the plate load impedance
>>> as you do when you run 1000 watts in the high voltage position
>>> the VALUES of the capacitors must change since the value of the
>>> inductor is fixed.
>> There is your fundamental error. Simply by reducing the drive (without
>> retuning), you DO NOT change the impedance the network presents to the tube. The
>> impedance is the same and the Q is the same. Only the amount of energy fed to
>> the network changes, and of course the efficiency. Basic physics.
>> You can easily prove this for yourself. Tune up at max power, measure any RF
>> voltage or current anywhere in the output network, including the antenna, and
>> then reduce the drive. The RF voltage and/or current will decrease linearly with
>> the reduction in drive. That's why they call them "linear" amplifiers.
>> To insist that circulating current INCREASES when drive is reduced is utter
>> nonsense. Period.
>> 73, Bill W6WRT
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