[Amps] PI-L Thoughts!
w7ry at inbox.com
Sun Dec 12 13:56:25 PST 2010
The toroid is the "L" coil. the open coil is for 10, 15, 20 and 40. The
ceramic vertical coil is 80 and 160. All the tank components and the band
switch are from a Ten-Tec 425 Titan amplifier. I ordered them directly from
Ten-Tec many years ago.
I use this amplifier regularly for RTTY contesting at 1500 watts out on all
bands. No extra cooling.... Just as you see it.
The medium sized transformer with the black laminations mounted directly
next to the HV power supply board is a 240 to 120 volt isolation
With this setup, I just need a 3 wire 240 volt power cord (2ea 120V legs and
a safety ground). This transformer provides the 120 VAC for the blower,
G3SEK triode board, and the filament transformer with this transformer which
I ordered from Mouser.
Not to open this can of worms again, BUT I ELECT not to use the chassis as a
NEUTRAL for the 120 volt loads.
From: "Roger (K8RI)" <sub1 at rogerhalstead.com>
Sent: Saturday, December 11, 2010 8:01 PM
To: <amps at contesting.com>
Subject: Re: [Amps] PI-L Thoughts!
> 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
>> 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 @
> Nice job of construction.
> Did you use the toroid only on 160?
> Roger (K8RI)
>> 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
>>> 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
>>> have not really budgeted for that.
>>> This makes me think of a third solution mentioned in the handbooks but
>>> really discussed in depth; and that is to tweak the Q to make it work.
>>> I would like to have the opinions of all you experienced builders out
>>> 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
>>> of the other solutions? Tweaking the Q is by far the most simple
>>> 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
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>>> Amps at contesting.com
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