[Amps] Pi-L Network Question

Tue Mar 11 12:59:31 EDT 2014

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 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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>
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-- 
Vic