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[AMPS] Re: New G2DAF amplifier information

To: <amps@contesting.com>
Subject: [AMPS] Re: New G2DAF amplifier information
From: 2@vc.net (measures)
Date: Wed, 15 Mar 2000 19:53:08 -0700
>
>Not wishing to discuss the G2DAF circuit any further, I still want to
>question your statement regarding substantial grid current causing
>distortion.  I do not think that is necessarily true.
>
>The tube used by Lars is a Philips QBL 5/3500.  If you go to his web site,
>the data sheet indicates that the tube is designed to draw substantial grid
>current in linear service.  

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? 

> Grid current alone would not cause distortion.
>Distortion would arise only when the grid current causes the plate current
>to saturate.  

This is not the case according to Eimac's constant-current curves  

>In most amplfiers using normal screen voltage, the maximum
>normal plate current would be designed to occur at zero grid current.

agreed

>Therefore, by design, grid current would cause the plate current to exceed
>the design of the tube.  

I disagree.  With lower screen potentials and grid current,  linearity 
goes to pot long before the emission limitation of the cathode is 
reached. 

> In the case of the QBL 5/3500, plate current curves
>show the tube can draw 5 amperes.  So, if the power supply could handle this
>substantial current, grid current would not saturate.

Grid current saturation is a function of the capability of the exciter.  
>
>One method to allow the flow of grid current in tubes not otherwise designed
>for grid current, would be to lower the screen voltage.  If the screen
>voltage is lower, the gain of the tube would decrease and grid current would
>not cause the plate current to flatten out.  

Screen potential varies widely and rapidly in a DAF.  

>An interesting experiment would
>be to install a tetrode amplifier with a stabilized and variable screen
>voltage.  I expect that you could pick the grid current you wanted to run
>and then adjust the screen voltage so that the desired maximum grid current
>would cause the plate current to raise to the desired level.  

Not a sound wager. 

>Less grid
>current would cause a corresponding decrease of plate current.  If the
>resulting curve is linear, the amplifier will be linear and no distortion
>will be caused by grid current.
>
I disagree.  In the grid driven config, Grid current leads to downtown 
Distortionville.  

>Read about the tests conducted by G2DAF, which are included in the article
>posted by Ian.  G2DAF stated that by adjusting the screen voltage downward,
>it is possible to eliminate the bias supply.  Perhaps there is merit in
>further research.

I measured the distortion produced by three DAFs at 22db down -  or 
worse.  There is merit in regulating the screen and running zero grid 
current, just as Eimac says.  There is no such thing as a 300mpg 
carburator or a miraculous Class C linear amplifier.  .  
>
>In a circuit and tube where zero grid current yields maximum output, your
>statement would be a true statement.  

>But there is more than one way to run
>an amplifier and they certainly merit discussion.
>
There is grounded grid, cathode driven.  Grid current does not cause 
distortion.  //   There is grid-driven.  Run grid current with grid 
driven and one makes feculence.  

later, Colin

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


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