[AMPS] Re: New G2DAF amplifier information

[measures]

>
>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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