The heating of the grid due to RF is the rf resistance of the grid (taking
into account skin effect) times the current squared. While what we usually
consider screen dissipation is the DC screen voltage times the DC screen
current. This is the amount of kinetic energy per second that
the electrons dissipate onto the grid when on impact. The AC resistance of
a grid is quite low.
73
Bill wa4lav
At 11:21 AM 9/24/02 -0600, John T. M. Lyles wrote:
>The question was:
>
>>Perhaps some of the gurus can enlighten me. This is something which I've
>>never seen referenced in any tube manufacturer's publications, but I've
>>always wondered if it would ever be a potential problem and possibly
>>limiting factor in VHF/UHF amplifier design.
>
>....
>
>>At VHF or UHF, these capacitances are such that the output tank circuit will
>>invariably have a high Q. This Q will result in a high RF current
>>circulating within the tank circuit with most of this circulating current
>>flowing through the tube itself. Within a tetrode, it is the screen grid
>>which forms part of this circuit.
>
>>Whereas the screen grid has certain D.C. dissipation limits as per the tube
>>manufacturers datasheet, I have never seen any mention of the maximum A.C.
>>(RF) current ratings for the grids.
>
>The short answer is:
>
>Absolutely, the limitation at high power/frequency with tubes becomes the
>screen dissipation. If the transit time effects are not a limit first.
>These days this is correct assumption
>
>No RF current ratings are given, for the reasons which Rich alluded to
>(the # parallel bars) and the fact that it would be given for only one
>cavity/circuit Q and frequency (locating the peak current somewhere in the
>screen element), whereas a J/S or Watt rating is universal, and can be
>found via analysis for whatever circuit and frequency that is used. One
>can calculate the current in the elements, of course, but also x voltage
>and get power(peak). Times duty factor to get average power (usually CW
>for ham stuff). Power dissipation infers peak current somewhere in the
>grid or screen. Also, there is probably some leveling or averaging which
>occurs due to the thermal mass of the elements and how they are mounted,
>i.e., via water cooled supports, wires, bars, etc.
>
>The reason that really big gridded tubes at VHF are made double-ended is
>to improve utilization of the limited screen grid area to make big RF
>power. Double ended circuits put the tube in the center of a half wave (or
>higher integral half waves) standing wave voltage peak, so that the E
>field interaction between elements is maxium while the magnetic (and
>current) component is located outside or near the outside of the tube
>connections. In a regular jug with handles, the field peak is in the top
>dead head space of the tube, and at the seals the RF voltage may be
>reduced, with a 1/4 (hopefully less) standing wave "up the tube"
>interaction region. If the tube gets any longer, then you pass 1/4 wave at
>the operating design freq, and waste a lot of the elements, not to mention
>putting the highest current right in the waist of the screen grid. If it
>gets bigger in diameter (a reasonable way to get higher power) then higher
>order circumferential modes become easy to excite, causing UHF or L band
>oscillations which our normal parasitic suppressors cannot fix. So the
>trick is double ending the ciruit with special tetrodes and triodes, such
>as THALES TH628 (formerly Thomson) or BURLE 7835.
>
>Going to pyrolytic graphite is another trick to allow increased
>dissipation and get higher power. Beyond the scope of this discussion, but
>quite interesting technologies seem to be in new tubes.
>73
>John
>K5PRO
>--
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
|