[Amps] Pi-L Network Question

[Carl]

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 at miamioh.edu>
To: <amps at 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 at 2.0A, with k=1.7)
> <mailto:2500V at 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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