[AMPS] Re: grid current.

[measures]

>
>Rich said:
>
>"Eimac makes two tetrodes that are designed to draw grid current in linear
>service  --the 4CX1500B/8660 and the 4CX600J   Typical grid current is
>less than 0.5 mA in either.  //  Is the data sheet for the QBL5/3500 a
>JPEG photocopy?  Do you know the url?  How much grid current does
>Phillips recommend for linear service?  What do they say about using
>unregulated screen potential?"
>
>
>The URL is  www.pocab.se/amp/g2daf.htm

I see no data. sheet on the QBL5/3500.  The theme song from "The Good, 
The Bad, and The Ugly" was a nice touch.  
>
>Look at the data sheet yourself so that I do not misquote it.  

Yeah, right. 

>It is larger
>than .5 ma.  

 How much grid current does Phillips recommend for linear service?  

>The data sheet says nothing about unregulated screen potential.
>That is not at issue in my note.  My assumption was a "stabilized and
>variable" screen supply.

ok 
>
>The G2DAF circuit is not at issue here.  It is the operation of a linear
>amplifier with less than the "normal" screen voltage.  One problem in some
>tubes would be that grid dissipation would increase.  

Another problem is that, on constant-current curves,  linearity is 
essentially lost when the zero volts grid potential line is crossed. 

> In some tetrodes or
>pentodes, this would exceed specified maximum permitted.  But, in many
>surplus tubes, there is plenty of grid dissipation to spare - which is why
>the 813 works well as a grounded grid amplifier.

This discussion is not about grounded-grid / cathode driven.  It is about 
grid-driven.  
>
>The matter was brought up, Rich, for the process of learning.  

One can not learn anything useful about grid-driven amplifiers by 
learning about cathode-driven amplifiers.

The Misdirection Card:

> I do not find
>anything significant in your response to cause me to not want further
>information.  It seems you have closed your mind to learn.  I am reminded of
>the old P-47 warbird.  When it first went into production, it had a 3 bladed
>prop and was a dog.  Soon, thereafter, someone tried a 4 bladed prop.  -
>which was just the thing needed to transfer all that horsepower.  This
>single change made a substantial improvement in not only speed but the
>ability to turn, resulting in a kill ratio of 18-1.  I suppose there were a
>number of engineers out there who scoffed at the idea because it came with a
>3 bladed prop.  But, sometimes things are learned by trying them.  I expect
>you would have been arguing against even trying a 4 bladed prop. because
>there is no 300 mpg carb.
>
>Suppose, years ago,  some "ham" had suggested to you that he could drive the
>cathode instead of the grid.   I expect you would have belittled him - since
>the tube was originally never designed to be cathode driven.
>
 Not a good bet.  One of the first amps I built was a 3CX100A5 in 
cathode-driven config. for the 23cm band.   

>When the cathode is driven and the grid draws substantial current, 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 did not know that
>tubes were so smart.

They aren't.  RF-NFB reduces distortion in cathode-driven service because 
the input is in series with the output.  . 

>Does this mean that when I go to the local parts store and purchase a grid
>current meter that I need to specify whether it is to be installed in a
>grounded cathode or driven cathode because the grid current is different?

zzz
>
later, Colin

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


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