[ARC5] Plate impedance...

[Brian Clarke]

Yes, Robert,

That is the static resistance, as a DC device. Ken asked for the impedance.

With the plate curves it is possible to calculate the dynamic impedance, ie, the rate of change of current with respect to change of plate voltage at a fixed grid voltage - that is the one we use when calculating, say, the input impedance of an audio output transformer or of an RF tank circuit. Then we need to know the plate current excursion, because plate curves are not linear. 150 mA is just the operating point - is that for one 1625 or both? The problem with calculating plate impedance for pentodes and tetrodes is that the plate current curves are so flat that you get very high values. See Langford-Smith, F [1960] Radio designer's handbook, Iliffe, London, p16.

Frequency does enter into the dynamic impedance because of the various inter-electrode capacitances, including those in the base and wiring. At RF, you almost need the device in front of you to do the calcs.

73 de Brian, VK2GCE.
  ----- Original Message ----- 
  From: WA5CAB at cs.com 
  To: kgordon2006 at verizon.net ; ARC5 at mailman.qth.net 
  Sent: Monday, December 15, 2008 3:02 PM
  Subject: Re: [ARC5] Plate impedance...


  550/0.150 = 3666.67 ohms.  Call it 3.5K.  The operating frequency doesn't enter into it.

  In a message dated 12/14/2008 9:02:22 PM Central Standard Time, kgordon2006 at verizon.net writes: 

    Anyone know what the plate impedance is of a pair of 1625 
    at, say, 7.0 Mhz, a plate voltage of 550 VDC, and a plate 
    current of about 150 mA?

    Ken Gordon W7EKB


  Robert Downs - Houston
  wa5cab dot com (Web Store)
  MVPA 9480 


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