>From Peter:
> Grid current alone would not cause distortion. Distortion would arise only
>when the grid current causes the plate current to saturate
Or if the regulation of the RF drive voltage was poor, or if the grid bias
shifted when grid current was drawn.
>how does the plate know it is cathode driven and therefore ok, yet distort
if
> the grid current comes while the cathode is grounded.
I think the answer to that is that distortion is reduced in gg because of
negative feedback. However, I haven't done the sums on how much feedback
there is.
It is interesting to read in Eimac's 'care and feeding', as well as
Pappenfus, that some grid current actually reduces distortion. It is also a
fact that spectrum analyser testing showed that my 4-250As produced better
IMD in AB2 than in AB1 - BUT they do have a shunt regulated bias supply, a
grid swamping resistor of 200ohms and a regulated screen supply. (And some
RF NFB, too - but that was there in AB1)
I will buy that grid current is bad if the circuit isn't designed for it.
Thanks Peter:
I agree. Grounded -grid has negative feedback, which reduces distortion.
However, the same negative feedback could be introduced into a grid driven,
grounded cathode design. In fact, simply installing a resistor in the
cathode of a grid driven circuit introduces negative feedback.
And, you are right regarding the stabilized voltages. Once the grid draws
current, a stabilized grid voltage is required - assuming one is using a
grid voltage. If the grid is operating at dc ground, then that is not a
requirement.
Now, let us complicate things. Assume a ham has a 3-1000 amplifier with a
defective tube - which runs 3,000 volts on the plate. It is a conventional
grounded grid configuration. He looks for a replacement tube but finds they
are $800 new and $300 used. Suppose he has a box of 4-1000 tubes in his
cluttered hamshack and thinks he will try one of those in grounded grid.
However, at 3,000 volts his 100 watt driver will only produce about 850
watts. Modifying the power supply is difficult. So, he comes up with the
idea of raising the screen voltage above ground to increase the sensitivity
to allow the 4-1000 to work at 3,000 volts. Now, assume he installs a
variable but regulated screen voltage on the screen grid and adjusts the
screen voltage so that the resting current of the 4-1000 is the same as the
3-1000. Let us say the regulated screen voltage is 185 volts. So, he
builds a small regulated supply to fit into the original cabinet that
produces 185 volts. Now, let us assume that the amplifier now drives with
100 watts to a full 1,500 watts output.
Any comments about whether this would work. Since the manufacturer does not
provide data for this operation, should it not be done. Is this the 300 mpg
carb that Rich says is impossible? Since the screen was varied at one
point, is it incapable of working? Since adjusting the screen voltage while
leaving the grid at zero voltage was suggested by G2DAF in his original
article, does that mean it cannot work?
Any constructive thoughts?
Colin K7FM
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