[AMPS] Re: Variable Screen Voltage!

[measures]

>
>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.

How do you keep the screen/cathode voltage constant if there is a 
substantial resistance in the cathode?  AB1 amplifiers are not well 
suited for NFB.  
>
>And, you are right regarding the stabilized voltages.  Once the grid draws
>current, a stabilized grid voltage is required - 

When the grid draws current, the exciter is no longer looking into a 
suitable match.  

>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.  

With 1000v for the screen, it would.  With E-screen=185v, the peak 
cathode current would be 8% of normal and P-out would be c.100w.  .  

> Since the manufacturer does not
>provide data for this operation, should it not be done

For those who know the 3/2 power law, the manufacturer provides all the 
data needed. 

>.  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?
>
It works.  The fly in the ointment is that it makes more feculence than 
class AB1.  
>
cheers, Colin

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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