Karl-Arne's right. I didn't look at the schematic and photos until now. The
only time we ever paralelled two and left them permenantly connected was for a
mono-band amp. We used to use two air variables, one for the load C and another
paralelled with it in full mesh to get enough C. Sice the voltage is much lower
at load C, you can get by with a smaller air variable as long as it meets the
voltage rating across the load. You could probably get buy with one of Oren
Elliots APL series caps here. For doing a multi-band amp, you need to switch in
the caps only when they're needed for a certain band. Also, NPO is what you
should use here, or air variable / vacuum, and at that maybe paralell two or
more of the door knob types to get the value needed to increase the RF current
rating. That is if you can't find a large enough NPO doorknob cap to do it by
itself. If paralelling, you might get by with several NPO disc types if they
are the thick high current types. I've seen those used in the QRO amps if I
recall.
The tank coil conductor size varies with the frequency of course over the skin
effect. In about all of the ARRL handbooks, there is a table about this showing
the size used for each frequency and by the power rating. Over coil heating is
why QRO uses tubing for the entire coil. The end towards 10 meters will get the
hottest of course.
By the way, that was a good job for a first time builder! As you build more,
they get easier and easier.
Best,
Will
*********** REPLY SEPARATOR ***********
On 5/11/06 at 6:54 PM Karl-Arne Markström wrote:
>If you don't disconnect the parallel fixed padder across the C2 in the
>Pi-network on 14 MHz,
>you may end up in serious heating trouble, as well as using a very
>unsuitable operating point for the PA tube.
>
>The pi-network calculations for a tube this size calls for
>a C2 in the order of 200 - 250 pF on 14 MHz for a loaded Q of 12.
>
>600 pF results in a loaded Q in the range of 25 - 30, and the coil losses
>will be in the order of
>5 - 10 times higher than normal due to the fact that tank circuit losses
>are
>proportional to the square of the circulating currents. They are in turn
>proportional to loaded Q.
>
>A doubled loaded Q means 4 times higher coil losses.
>
>73/
>
>Karl-Arne
>SM0AOM
>
>
>
>
>----- Original Message -----
>From: "Partain, Chuck" <Chuck_Partain@maxtor.com>
>To: <amps@contesting.com>
>Sent: Thursday, May 11, 2006 1:54 AM
>Subject: [Amps] tank coil heating
>
>
>> I'll go peruse the archives again but I'm going to post this here anyway.
>> I noticed while on 20m tonight running about a kw that the tank coil was
>getting
>> HOT, one of the wires connected to it (the 20m section) was turning a
>nice shade
>> of darker! the coil is warm.
>>
>> also When I pulled the cover (I was looking to see where all the hot
>smell was comming from)
>> I noticed that a doorknob cap I have installed next to the load vacuum
>cap was also warm!
>> i'm using that cap to get me on 80m its in parallel with the 1000pf
>vacuum cap
>>
>> everyone I talk to with it said they notice NO noticible difference when
>its on or off only
>> a few s units higher! (no audible distortion)
>>
>> the cap I am using is labelled n4700 591m 30kv 706h and has a c with
>an m inside of it.
>>
>> are these suitable for what I am doing? hell, its 30kv! why warm? what
>should I be looking for?
>> the amp seems to load up pretty good into the vertical with an swr of
>less than 1.3 to 1
>>
>> thanks for any help
>>
>> chuck, ka1mwp
>>
>> the schematic is at
>>
>> http://home.comcast.net/~ka1mwp
>>
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>> --
>> No virus found in this incoming message.
>> Checked by AVG Free Edition.
>> Version: 7.1.392 / Virus Database: 268.5.5/335 - Release Date: 2006-05-09
>>
>>
>
>
>--
>No virus found in this outgoing message.
>Checked by AVG Free Edition.
>Version: 7.1.392 / Virus Database: 268.5.6/337 - Release Date: 2006-05-11
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|