Thank you all for your insights in this matter. It seem like my original
thought to tweak the Q is not very good. The two dominating suggestions are:
1, add a smaller C1 to the existing and switch in/switch out
2, add a small L, that forms an LC network together with the tubes capacitance,
before the tank to lower the load resistance on higher bands
I will try solution 2 first because it is easy and fits into the RF deck.
However, I'm starting to realise that maybe a redesign of the RF deck layout is
inevitable for lots of reason, so in the longer perspective, solution 1 wil be
one that I go for.
Thanks!
73's
SM0NOR
Ulf
Sent from my iPad
On 12 dec 2010, at 05:01, "Roger (K8RI)" <sub1@rogerhalstead.com> wrote:
>
>
> 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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>>>
>>>
>>>
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