> >The max. allowable R of the
> >supply is found in the technical specifications for each tube. For
> >example, on page 4, column 1. of the specs. for the 4CX15,000A, the
> >maximum R is stated to be 100k ohms. In your opinion, Ian, is a
> >regulated supply, or an unregulated supply, most likely to have an output
> >R of 100k?
> >
> I do not interpret that as a recommendation to deliberately choose a
> poorly regulated grid supply. IMO it's merely a recommendation not to
> allow the grid to "float" and charge up electrostatically. All it means
> is that you cannot use a high-value grid resistor for "contact-
> potential" biasing, as you can with small receiving tubes.
That's correct Ian. That's why Eimac strongly disagreed with
suggestions to use "grid fuses" unless they are accompanied with
proper shunt resistances.
> >>Similarly, please can you quote what and where Eimac says that the
> >>correct method to configure a class AB1 amplifier is to adjust the grid
> >>bias for zero grid current,
> >
> >You've got to be putting me on, Ian. AB1 means zero grid current. .
> >
> Yes, but there's another way to achieve that, namely to adjust the
> drive, either manually or by ALC.
Eimac usually give a grid and screen voltage that provides operation
in an optimum area of voltage ratios. While you can "fudge" around a
bit, Rich's carte blanche suggestion of picking voltages to adjust
drive might very well move the tube into problem operating areas.
For example, if I had a 4CX1000A with a 50 ohm grid resistor and used
Rich's method with a FT1000, the tube would be operated far outside
optimum bias and screen voltages for normal quiescent current. The
peak RF grid voltage would be 140 volts, requiring 140 volts of bias.
Absolute maximum screen voltage is 400 volts, and at 400 volts the
tube would be cut off.
If I understand Rich's method, we either must exceed the rated
screen voltage or operate class C (zero grid current class C, which
is possible).
> > However, the practice makes sense
> >to me. To explain: There are two means of setting ZSAC: by adjusting
> >grid V, and by adjusting screen V. Grid V adjustment is constrained by
> >the requirement that zero grid current be maintained---which leaves
> >choosing to adjust the screen V. Does this make sense to you?
>
> As the Scots say, "I hear you" - which means that I appreciate what
> you're saying, but don't agree that it's the only "right" way to do it.
It also might easily be the wrong thing to do. Maybe Rich will show
us how to use his method with a 4CX1000A, with a 50 ohm grid
resistor, and 200 watts PEP excitation.
I think the statement is far too general to be true, although it is
a good "fine tuning " method if a minor tweaking is needed.
73, Tom W8JI
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