[Amps] PI-L Thoughts!
w7ry at inbox.com
Sun Dec 12 13:39:19 PST 2010
The tank coils and the band switch is from Ten-Tec. They used this in the
Titan 425. The tune-load knobs and Jackson Brothers drives are from them
The 4th switch deck is to control the band switching relays on the WD7S
tuned input board. I don't remember how I switched the large tune cap into
the circuit but if you look at a Titan 425 schematic, you will see.
You will also notice a large string of orange caps mounted on a piece of
perf board and connected to the lager tune cap. This is the 160 meter tune
padder. These are 33 PF 6KV caps from Mouser. The caps are all in parallel
Thanks for all the kind words!
From: "Ulf (SM0NOR)" <ulf at sm0nor.com>
Sent: Sunday, December 12, 2010 12:49 AM
To: "Jim W7RY" <w7ry at inbox.com>
Subject: Re: [Amps] PI-L Thoughts!
> Hi Jim,
> Really nice work!
> How did you switch in the larger cap? I see you have a three deck switch.
> One would go to the input network, one to the Pi and and to the L.
> Carefully scrutinising your pictures it looks like you actually added a
> fourth deck. In that case I understand how you did it.
> Sent from my iPad
> On 12 dec 2010, at 04:35, "Jim W7RY" <w7ry at inbox.com> 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.
>> Pictures @
>> Jim W7RY
>> From: "Ulf (SM0NOR)" <ulf at sm0nor.com>
>> Sent: Saturday, December 11, 2010 10:26 AM
>> To: <Amps at 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
>>> 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
>>> Ulf (SM0NOR)
>>> ulf at sm0nor.com
>>> Amps mailing list
>>> Amps at contesting.com
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