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Re: Tube coils for beginners 2


On Sun, 26 Jan 1997 22:25:14 -0700 Tesla List <tesla-at-poodle.pupman-dot-com>
writes:
>Subscriber: dwt-at-efn-dot-org Sun Jan 26 22:17:53 1997
>Date: Sun, 26 Jan 1997 18:00:07 -0800
>From: David Trimmell <dwt-at-efn-dot-org>
>To: tesla-at-pupman-dot-com
>Subject: Re: Tube coils for beginers 2
>
>Mark,
>
>> 
>>    David a 4cx250 will work, but DON'T take the usual route with 
>this
>> tube and tie the screen and control grids
>> together to simulate a triode, if you try to run it this
>> way you stand a VERY GOOD chance on distroying the tube.
>> 
>>  This tube must be run as a tetrode with voltage on the
>> screen, you could run the screen and control grids tied
>> together via a capacitor for a common RF connection but
>> not common for AC/DC potential because the screen under
>> these conditions will draw excessive current as its
>> Mu factor is much higher than the control grid.
>> 
>>  The 813, 803, 4-1000, 4-125 thru 4-500 tubes can be run
>> in the triode connection safely.
>> 
>>  Good triode tubes are: 811A, 572B, 3-400, 3-500, 3-1000,
>> 833 etc.
>> 
>>                         Mark Graalman (WB8JKR) TCBA#1399
>
>When you say "you could run the screen and control grids tied together
>via a capacitor for a common RF connection" what would be a proper way
>to calculate that value of capacitance? I may start out using these
>4cx250's as they will be free, but still have little desire of killing
>them to soon! 
>
>Thanks,
>
>David Trimmell <dwt-at-efn-dot-org>
>

   Hi Dave,

 I'm sorry, I guess I should have been more clear. The
capacitance I mention is to simply insure that the control and screen
grids are at about the same RF potential, the capacitor can be any value
that has a
reasonably low reactance at the frequency the coil the
coil will run at and for most a value of .005 to .01 ufd
would be fine. A voltage rating of about 1 to 2 KV would
be ok as well.
  I would select a screen voltage of about 250 volts 
limited to about 35 to 40 ma. A dropping resistor could
supply this voltage from the plate supply.
  This tube only has a 2 watt control grid rating so be
careful during tune up that you keep the grid feedback and coupling level
to the secondary in check as to not
overdrive the grid on the tube.


                     Mark Graalman