[R-390] grid current

[Steve Hobensack]

You could have a gassy tube.

Steve   N8YE


> Date: Mon, 8 Jun 2009 13:05:54 -0500
> From: "Barry" <n4buq at knology.net>
> Subject: [R-390] OT(just a bit): From whence cometh the grid current?
> To: <r-390 at mailman.qth.net>
> Message-ID: <000601c9e863$c44c9790$6401a8c0 at ingrnet.com>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
> 
> I have an HP-606A that I'm checking over and have found the voltage on the 
> screen grid of one of the 6AW8A amplifiers in the power supply is off (low) 
> a bit.  The voltage is supplied to the screen grid via a voltage divider 
> from B+ that consists of a 1.2M and a 100K resistor from B+ (500V) to 
> ground.  With a 500V supply and nothing else connected, this theoritically 
> gives 38V at the junction of these two resistors.
> 
> If I unplug the tube and measure the voltage at the screen grid pin, I get 
> close to 38V; however, when I plug in the tube, it drops to about 32V.
> 
> My question is where is the grid current coming from?  Is it coming from 
> ground up through the 100K or from B+ down through the 1.2M?  I assume it 
> draws it from ground because as little as 1mA would drop a significant 
> amount of voltage through that 1.2M resistor.
> 
> Sorry if this is a really dumb question but I just don't seem to understand 
> how the current flows in this "simple" little circuit.  I assume that since 
> I'm measuring the voltage from ground to the screen and plugging in the tube 
> drops the voltage, then the current is flowing through the 100K resistor; 
> however, if I measure the before-and-after voltage from B+ to the screen 
> grid, would I see a difference there too?
> 
> Thanks,
> Barry - N4BUQ 
> 
> 
> 
> ------------------------------
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