Re: [Amps] Pi-L Network Question

["Carl" <km1h@jeremy.mv.com>]

All TL-922's are that way since the JA's did not have the low end of the 
band when built. I started working them up around 1970 if I remember in the 
80's.
Carl
KM1H


----- Original Message ----- 
From: "Vic Rosenthal" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Tuesday, March 11, 2014 12:59 PM
Subject: Re: [Amps] Pi-L Network Question


> The TL922 I have would not load properly at 1810 kHz without adding 
> capacity. Although now it works, it is less efficient than on other bands 
> and it probably should have more inductance. I couldn't find an easy way 
> to do this neatly, so I left it alone. I suspect it was designed for 1.9 
> mHz.
> It is a Japanese model -- you can set it up for 200-240 V but not 120, and 
> it came with 10 meters installed. I wonder if they are different in any 
> other way?
> On 3/11/2014 9:33 AM, Carl wrote:
> > Add a carbon pot at the anode end and feed an antenna analyzer into the
> > output connector. Find out what the difference is between calculated and
> > tested and do the math to find the K factor used.
> >
> > Ive often found even commercial amps to be shy on the 160 and 80M load C
> > especially when a lot of fixed C is used and the measured C is well on
> > the low side of what is printed.
> >
> > One amp that is shy on 160M C and L is the Clipperton L and its variants.
> >
> > The SB-200 is also low on C at the low end of 80 with a typical antenna.
> >
> > For my own amps I modify and make the pi network load the various160 and
> > 80M antennas since some have a high VSWR at places and I refuse to waste
> > money on an external tuner. Ive been doing that since the 60's.
> >
> > Carl
> > KM1H
> >
> > ----- Original Message ----- From: "Jim Garland" <4cx250b@miamioh.edu>
> > To: <amps@contesting.com>
> > Sent: Tuesday, March 11, 2014 11:44 AM
> > Subject: [Amps] Pi-L Network Question
> >
> >
> > > Hi all,  I've built a tank circuit using the on-line Pi-L network
> > > calculators by VE3OZZ and also G3SEK. These are both based on the
> > > equations
> > > published in an article in August 1983 QST (by W5FD). Althought the 
> > > G3SEK
> > > calculator is somewhat more sophisticated (it corrects for tube and 
> > > stray
> > > inductances and capacitances), both calculators give about the same
> > > answers
> > > for 80m and 160m. I'm finding that the predicted values for C1 and L1 
> > > are
> > > very close to what I need to tune the amplifier, but the predicted
> > > values of
> > > C2 are far lower than what is required to load the amplifier properly.
> > > I'm
> > > wondering if there could be an error in the W5FD formulas, and if anyone
> > > else has experienced the same problem? (If there is an error, it 
> > > probably
> > > wouldn't be noticed on the higher frequency bands, because the load cap
> > > would most likely have enough tuning range to compensate for the error.)
> > >
> > >
> > >
> > > I've computed the Pi-L network values over a range of plate
> > > impedances. (My
> > > amp uses bandswitched L1 and L2, so those values don't change.) What I
> > > find
> > > is that as the plate impedance increases (e.g., tuning the amp at a 
> > > lower
> > > power level), the equations predict that Q goes from 10 to 18, C1 
> > > doesn't
> > > change, C2 increases only about 5 percent. In other words, according
> > > to the
> > > on-line calculators, tuning to a lower power requires a minor tweaking 
> > > of
> > > the load capacitance, but that's all
> > >
> > >
> > >
> > > At 3.5 MHz, for my amp, the equations predict a load capacitance of
> > > 1057 pF
> > > and on 1.8 MHz, a load capacitance of 2057 pF.  I'm finding that, in
> > > practice, those predicted values are more than 1000pF too low.
> > >
> > >
> > >
> > > Here are some details of the actual tank circuit:
> > >
> > > The design plate impedance is nominally 720 ohms (2500V@2.0A, with 
> > > k=1.7)
> > > <mailto:2500V@2.0A,%20with%20k=1.7)> , Q=10, and I've computed network
> > > values for a range of plate impedances from 720 to 1440 homs. The actual
> > > tank circuit is:
> > >
> > > 80m:  L1=8.4 uH,  L2=3.8 uH
> > >
> > > 160m: L1=16.3uH, L2=7.4uH
> > >
> > >
> > >
> > > I'm using two paralleled 1000pF doorknobs for a plate blocking
> > > capacitance.
> > > The plate choke is 225 uH, bypassed at the base by 7700 pF. The safety 
> > > RF
> > > choke is 470uH, with an 18 ohm DC resistance.  The tune and load caps 
> > > are
> > > 30-240pF and 33-1000pF air variables padded with doorknobs, as required.
> > >
> > >
> > >
> > > The tank seems to tune smoothly, with no heating or quirkiness. THe only
> > > problem is that I need much more C2 capacitance than the formulas
> > > predict.
> > > At this point, I'm at a loss to explain the discrepancy, other than
> > > wondering if there's an error in the formulas somewhere.
> > >
> > >
> > >
> > > 73,
> > >
> > > Jim W8ZR
> > >
> > > _______________________________________________
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> > > Amps@contesting.com
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> > >
> > >
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> Vic
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