[Amps] Pi-L Network Question

Tue Mar 11 13:23:27 EDT 2014

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 at gmail.com>
To: <amps at 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 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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>>
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>
> -- 
> Vic
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