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[Amps] RF Grid Current

To: <amps@contesting.com>
Subject: [Amps] RF Grid Current
From: g4ftc@hotmail.com (David G4FTC)
Date: Wed, 25 Sep 2002 11:01:13 +0000



>From: 2 <2@vc.net>

>For example, the 8938 is rated at 500MHz/4000VDC.
>since the anode-grid C is 10pF, (32-ohms@500MHz) and the anode potential
>is c. 2600V.rms, the RF circulating current through the grid to ground is
>c. 82A.rms.  Sure, 82A sounds looneytunes/bonkers, but is it ?    The
>8938 has over 100 parallel gold-plated grid bars, so the current in each
>conductor is under 800mA.

No - the 82 amps isn't bonkers - similar calculations to that above made me 
curious as to current rating of the tube's internal structures. Even the 
ubiquitous 4CX250B will see some 20 amps of circulating current at 500MHz. 
Obviously the tube is designed to handle such currents with a suitable 
margin otherwise it would rapidly fail in service, and I suppose the tube 
manufacturers don't list the values as it is part of the internal design of 
the tube which the user doesn't really need to know as long as he operates 
the tube within it's design ratings. If you wish to operate the tube in some 
unusual mode then it's a case of "Please contact the manufacturer for 
specific advice".

On the few times that I've opened tubes, the grid wires have always seemed 
as being extremely fine and delicate. I've also had experience of the 
internal tube structures being destroyed by mechanical mishandling on more 
than one occasion. I myself once by accident dropping a tube from about 3ft 
onto a carpeted floor and this was enough to write it off! A painful lesson.

But the one parameter which would be useful to equipment designers is the 
tube's self neutralising frequency and I would have thought that this would 
be quoted in the tube's datasheet. The only place where I've seen it listed 
is in Eimac's "Care and Feeding" book.

Many thanks to all those who responded to my post for their advice.


David G4FTC






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