Not wishing to discuss the G2DAF circuit any further, I still want to
question your statement regarding substantial grid current causing
distortion. I do not think that is necessarily true.
The tube used by Lars is a Philips QBL 5/3500. If you go to his web site,
the data sheet indicates that the tube is designed to draw substantial grid
current in linear service. Grid current alone would not cause distortion.
Distortion would arise only when the grid current causes the plate current
to saturate. In most amplfiers using normal screen voltage, the maximum
normal plate current would be designed to occur at zero grid current.
Therefore, by design, grid current would cause the plate current to exceed
the design of the tube. In the case of the QBL 5/3500, plate current curves
show the tube can draw 5 amperes. So, if the power supply could handle this
substantial current, grid current would not saturate.
One method to allow the flow of grid current in tubes not otherwise designed
for grid current, would be to lower the screen voltage. If the screen
voltage is lower, the gain of the tube would decrease and grid current would
not cause the plate current to flatten out. An interesting experiment would
be to install a tetrode amplifier with a stabilized and variable screen
voltage. I expect that you could pick the grid current you wanted to run
and then adjust the screen voltage so that the desired maximum grid current
would cause the plate current to raise to the desired level. Less grid
current would cause a corresponding decrease of plate current. If the
resulting curve is linear, the amplifier will be linear and no distortion
will be caused by grid current.
Read about the tests conducted by G2DAF, which are included in the article
posted by Ian. G2DAF stated that by adjusting the screen voltage downward,
it is possible to eliminate the bias supply. Perhaps there is merit in
further research.
In a circuit and tube where zero grid current yields maximum output, your
statement would be a true statement. But there is more than one way to run
an amplifier and they certainly merit discussion.
An amplifier running less screen voltage than specified by the manufacturer
could have undesireable characteristics and I would not recommend doing so
on the air without adequate testing under all conditions.
Colin K7FM
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