>
>Rich says:
>
>>My guess is that he is not all that familar with constant current curves.
>
>Constant current curves are not necessary to design amplifiers.
Surely. I like to see where the linearity of the constant current curves
takes a turn for the worse, and to stay clear of there.
They are
>only another way to present the transfer characteristic of the active
>device. The RCA transmitting tube handbook shows how design from usual Ip/Vp
>curves (with Vg as a parameter). I submit Rich, that Pappenfus et al really
>do know about the subject - and did so 40 years ago.
>
> >>Not necessarily so. It depend on the relative ratio of grid swamping
> to non
> >>linear grid current loading.
>
> >Tossing off the effect of rapidly decreasing grid resistance as
>grid current flows does not wash. .
>
>So the input impedance gets so low?
Sometimes. Changes in load Z is not good news for modern radios.
> If I have a 12.5 ohm swamping resistor,
>and the grid current is 100mA at 50volts pk, I have 4 amps peak in the
>swamping resistor, and 100mA in the grid. Hardly a noticeable change.
>
agreed. . However, there are other grids that present a fairly low R
when driven to 50v positive.
>
> >> Additionally, you appear to assume that the
> >>input impedance of grounded cathode AB1 stage is constant and high.
>
>> [chortle]
>>
>Does this suggest that you don't assume that?
>
As long as there are those who try to operate grid-driven tubes with grid
current, the input R of the grid can not be assumed to be anything.
With zero grid-current. the grid resistance can be assumed to be high.
>> >> .This is not necessarily so - because of Miller effect.
>> >> If the driver has adequte NFB to reduce its output impedance, the effects
>> >>are minimised.
>> >
> >I know of no commercial radio that works well with other than a
>50-ohm load.
>
>How about KWM2, 32S1, TR4, T4X, SB101, FT101, FT102, TS850 etc.............
>all of which can manage a 2:1 SWR?
>
history
>If you were to say that 'In general, in amateur service, operation of a
>grounded cathode stage with grid current is undesirable because of
>intermodulation difficulties', I'd agree.
ok
>But your sweeping generalisation
>that 'grid current causes splatter' is not always so, and there is a vast
>amount of professional literature to prove that point.
Eimac makes two tetrodes that are specifically designed to operate in
grid driven service. with grid current. The 8660/4CX1500B is the largest
of the two. According to
Typical Operation, 0.06mA of two-tone grid current is apparently ok with
2900v on the anode. . However, the constant current curves indicate
that the 8660/4CX1500B would be more linear with zero grid current.
Any guesses at to what would happen if a 8660/4CX1500B were used in a DAF
circuit. - with say 20mA of grid current instead of 0.06mA of grid
current?
- However with ordinary tetrodes,, grid current is clearly to be avoided
unless one is either trying to clear away a wide swath during a contest,
or operating on CB Channel 6. .
>
cheers, Peter
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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