[R-390] grid current

[Barry]

I can check that.  I tried swapping it with another with the same results; however, both are most likely the same age used under much the same conditions so both could be gassy.

Thanks,
Barry - N4BUQ
  ----- Original Message ----- 
  From: Steve Hobensack 
  To: r390 reflector 
  Cc: n4buq at knology.net 
  Sent: Tuesday, June 09, 2009 6:09 PM
  Subject: RE: grid current


  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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