On Sat, 30 Jan 1999 13:28:27 EST Skip S Isaham <nospam4me@juno.com>
writes:
> Thank you for all the replies to my question about the grid term
>resistor
>selection.
>
>Although I haven't had time to go back to Rich's site yet, I'd like
>to
>ask the 2nd half of the question. With modern exciters higher output
>(in excess of 150-200 watts) levels,
Only a few do that reliably.
would it be considered a luxury
>to have the excess "design room input power" when choosing a value.
It makes more sense to run the exciter at its designed for ratings and
swamp the excess in the amp. Heat is almost a non-issue when the resistor
is placed in the pressurized grid compartment. A 2W carbon will run cold
at 15W key down in an NCL-2000 for instance.
>There was a mention of heat from the resistor as one consideration.
>One single HF 4cx250 grid driven tetrode amplifier I have here does
>the 1:9 ratio toroid into a higher value
>resistor, is completely stable and works well.
>
>With my TS-940's larger output,
Keep it at under 120W unless you want expensive problems with some
increasingly hard to find components.
I'm trying to choose the optimum value
>for 4CX1000 design (or similar). Would this exciter allow me
>convenience
>in selection of the termination resistance.
The minimum value allowing the maximum output!
>
>Also, would someone detail the term "triple bypass" the cold side of a
>resistor. Three separate types of capacitors, three different
>capacitors
>or three capacitors- same value in parallel?
Here is another subject sure to cause some discussion. By selecting caps
with various self resonant frequencies an/or optimum reactance/frequency
combinations it is possible to maximize/tailor frequency response. This
procedure is effective at the cold side of a plate choke and in a series
line from the input network to the cathode of a GG amp.
QST ran a Technical Correspondence piece a few years ago saying that was
so much horse dung but I failed to see any follow up and the multiple
caps method is still popular in professional journals. A GDO is a very
simple method of testing for resonances.
I still like to use parallel combinations of .01, .005 and .001MF in
160-10M amps since it covers up for less than perfect plate chokes.
73 Carl KM1H
>
>thanks
>skipp
>nospam4me@juno.com
>
>
>[snip]
>>Is there a guideline for the selection of a grid termination
>resistor,
>>or is it done by method ?
>> Hi Skipp, I guess the best answer is that it is done for
>convenience... considering the available drive power from the exciter,
>the required RF grid voltage to drive the PA tube and how much power
>you
>wish to
>dissipate in the termination resistor.
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