Hi Rich:
I thought you were against the G2DAF amplifier. Now you go and publish data
that supports it!
The first part of your statement is incorrect. "Some say that screen
potential can wander about with no serious consequences". This is your
conclusion of what someone has said, which only means that you have not been
listening. I have heard no one say that. In any amplifier, the screen must
be at a "perfect" voltage for linearity to occur. The thrust of the G2DAf
is that the "perfect" voltage can change with drive.
In order for the D2DAF amplifier to be linear, the gain of the tube must
change with drive. The data from your curves you supplied supports that.
In your example, you left the grid voltage constant, when in fact it is
changing. According to my hypothesis, the inefficiency of the voltage
doubler (or quadrupler) is essential to linearity. As the grid voltage
increases, the screen voltage increases, which causes the screen current to
increase, which causes the relative screen voltage to sag because of
increased current demand. In certain tubes, this effect will counteract the
change in gain of the tube. This is also the reason why it will not work
with some tubes.
Using a 6L6 beam power tube on my bench, I increased grid voltage and then
changed screen voltage to yield the appropriate gain for the amplifier to be
linear. In effect, I reverse enginered the G2DAF. I then had curves for the
6L6 showing what screen voltage was necessary for the appropriate grid
voltage - to yield an output that was in exact proportion to grid voltage.
Linearity required that the screen voltage "sag". It is then that I
realized that the G2DAF amplifier could be linear if and only if the voltage
doubler decreased relative output as screen current rises. It does that.
Either by accident or by the well thought out engineering of G2DAF, it
satisfies the requirement of linearity - at least under constant drive. The
dispute is left to whether the screen can follow the grid voltage fast
enough to remain linear under all voice conditions.
I cannot confirm that it will. I do not know, yet. However, Lars has
tested his under all of the conditions that other grounded grid amplifiers
get a clean bill of health and it passed. I have heard that G2DAF was a
meticulous engineer and his associates who heard it told me it sounded fine.
Your examples of a few bad amplifiers built and operated under unknown
conditions does not convince me that the G2DAF amplifier does not and cannot
work.
I am glad to see you are finally coming around. Perhaps the 8171 would be a
good candidate for the G2DAF circuit. Send me one up to evaluate and I will
build one using it. Let me know your favorite frequency and I will come up
unannounced 100 khz away..
Colin K7FM
Colin K7FM
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