Gas tube regulators.
Someone asked about the dynamic impedance of gas tube regulators. I
found some RCA info which suggests about 160 ohms - that's for 105 and
150 volt regulators, both the octals and the miniatures.
Some years back, there was an amplifier design published in the RSGB
radio communications magazine using a 4CX1000, where the screen was
regulated by using 400 1N4148s in series as a shunt regulator. They were
run at about 40mA forward current, so the dynamic impedance would be
about 250 ohms, decreasing as the reverse screen current increased the
current through them. The author also claimed an advantage in the
negative temp co, so that screen volts decreased as the diodes handled
more reverse screen current. An interesting approach, and the linearity
was quite good.
AB2 operation.
As far as AB2 is concerned, study of chapter 13 (Distortion Reduction)
of Single Sideband Principles and Circuits (Pappenfus, Bruene,
Schoenike, pub.by McGraw Hill 1964) is recommended. The point is made
there that the zero signal plate current is important for minimum
distortion - "It is emphasized that even with this ideal plate-current
curve there is only one point Q (zero signal plate current) on this
curve that will yield zero distortion for Class AB operation."
It goes on (page 190):- "The optimum value for tetrodes and pentodes
depends upon the value of screen voltage as well as upon the the basic
tube characteristics. The optimum value of zero signal plate current
varies approximately as the three -halves power of the screen voltage.
Lower screen voltages are desirable for this reason, but peak plate
current may be limited, or perhaps class AB2 operation instead of class
AB1 may be necessary."
After discussion of grid current distortion and the reduction in IMD
because of the opposite direction of the curves for plate characteristic
and grid current characteristic distortion (Fig 13-5), section 13-4
states:
"Most power tubes have a more linear plate characteristic operating
Class AB2, however."
It goes on to discuss grid swamping.
Further discussing grounded grid operation, we find that it discusses
changes in input impedance with plate non-linearity, and suggest that
theoretically "A low mu triode, operated Class AB1 and driven by a
tetrode, theoretically avoids nonlinear grid loading so advantage can be
taken of nonlinear plate degeneration. The power gain of such triodes is
only about 4 and their plate efficiency is relatvely low, however."
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|