>
>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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