[Amps] Pi-L Network Question

[Vic Rosenthal]

My experience might not be helpful, but for what it's worth:

I've used the G3SEK spreadsheet for two amplifiers, both with pi 
networks (not pi-L). The first was a class-C amplifier with a pair of 
813's. I found that the calculated value for the loading capacitor on 
160 was inadequate, but what seemed to be wrong was the calculated 
inductance. I ended up increasing the inductance to get proper loading 
and good efficiency on that band. I used K=2.0 for class C.

The other amplifier was a 4CX1000A. Here the issue was that the 
impedance shown by the load line was much lower than what would be 
produced by a K of 1.5 - 1.7 as suggested for class AB (it was closer to 
a K of 2). I went with the load line value and the amplifier seems to 
load properly with reasonable efficiency.

On 3/11/2014 8:44 AM, Jim Garland wrote:
> 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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-- 
Vic