On 12/11/2010 10:35 PM, Jim W7RY wrote:
> Please look at the way I handled a low minimum value cap in my 3CX800 amp. I
> used a small cap for 10, 15 and 20 meters. Then switched in a larger cap for
> 40, 80 and 160.
Although they only used a 2 section cap, Hallicrafters did this in the
HT33B amp. On 75 they operated a "knife switch" with a cam on the band
switch shaft.
> Pictures @
>
> http://s900.photobucket.com/albums/ac206/w7ry/3CX800%20Amplifier/
>
Nice job of construction.
Did you use the toroid only on 160?
73
Roger (K8RI)
> 73
> Jim W7RY
>
> --------------------------------------------------
> From: "Ulf (SM0NOR)"<ulf@sm0nor.com>
> Sent: Saturday, December 11, 2010 10:26 AM
> To:<Amps@contesting.com>
> Subject: [Amps] PI-L Thoughts!
>
>> Hi all,
>>
>> I'm looking for some advice regarding the tank.
>>
>> I'm about 2/3 on the way to complete my first amplifier build and I'm
>> having some thoughts about how to get C1 to low enough to load on 10
>> meter.
>>
>> To no surprise the 25-300 pF capacitor measured a min of 40 pF when
>> mounted in the RF deck. This will give me trouble on the higher bands. I
>> know this is discussed a lot in the literature, but I'd like your thoughts
>> on one specific solution.
>>
>> The obvious solutions are to either get another C1 with a lower min
>> capacitance or to get a vacuum variable. Maybe I could do the first
>> solution, but that would probably make it necessary to rebuild the
>> switching section with another deck to be able to switch in different
>> sections of a split stator cap. The vacuum solution is really easy, but I
>> have not really budgeted for that.
>>
>> This makes me think of a third solution mentioned in the handbooks but not
>> really discussed in depth; and that is to tweak the Q to make it work. And
>> I would like to have the opinions of all you experienced builders out
>> there!
>>
>> Here are my calculations (based on one of the handbooks)
>>
>> C1 C2 L1 Q
>>
>> 160 330 1400 30.4 11
>> 80 174 716 14.1 12
>> 40 96 388 7.2 12
>> 20 43 182 3.9 11
>> 15 45 161 1.7 16
>> 12 43 151 0.7 18
>> 10 43 143 0.6 20
>>
>> For simplicity I did not add the values for L2 in this post.
>>
>> Ok, what do you think of this? Is it Ok with a Q of 20 on 10 meter? I'm
>> using 5 mm copper tubing in the 10-20 coil. Or would it be better with one
>> of the other solutions? Tweaking the Q is by far the most simple solution
>> given the components I already have.
>>
>> Thanks for any advice!
>>
>> By the way..... the build is based on a GU74B and I'm aiming for a 1KW
>> PEP.
>>
>>
>> 73's
>>
>> Ulf (SM0NOR)
>> ulf@sm0nor.com
>>
>>
>>
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