> The Xc of the load cap does not change by simply reducing the
> drive.... how could it? That's nonsense. For a given
> frequency, the Xc of a cap is constant regardless of voltage,
> current or anything else.
Again, you did not read what I wrote ...
If you change the network by changing the plate load impedance
as you do when you run 1000 watts in the high voltage position
the VALUES of the capacitors must change since the value of the
inductor is fixed.
For example at 14 MHz:
3000 Ohms (Q=12) 6000 Ohms (Q=24)
C (tune) 42 pF 41 pF
L 3.36 uH 3.36 uH
C (load) 231 pF 393 pF
3000 Ohms is approximately 2800 V @ 800 mA (1500 W PEP out)
6000 Ohms is approximately 2800 V @ 360 mA (1000 W DC in)
In order to use the SAME output at the lower power, the tube
operating conditions must be changed to approximately 2000 V
@ 500 mA. Otherwise the circulating currents, losses, and
stress on the components increase significantly.
> I don't know where you got the idea that circulating current
> goes up with lowered drive, but it doesn't.
Obviously you need to do some more studying of RF networks.
Circulating current increases with network Q - that's the
primary reason for the higher losses (I^2*R losses in the
coil). If you use the same inductor in a 6000 Ohm to 50 Ohm
pi network than you used in a 3000 Ohm to 50 Ohm pi network,
the network Q and the circulating current will go up
> -----Original Message-----
> From: Bill, W6WRT [mailto:dezrat1242@yahoo.com]
> Sent: Sunday, August 30, 2009 2:43 AM
> To: lists@subich.com
> Subject: Re: [Amps] SB-220 bias question
>
>
> ORIGINAL MESSAGE:
>
> On Sun, 30 Aug 2009 00:27:43 -0400, "Joe Subich, W4TV"
> <lists@subich.com> wrote:
>
> >The voltage doesn't go up because the Xc of the loading capacitor
> >goes down. Look at the component values for a 2500 to 50 Ohm
> >pi network with a Q of 10 and compare them to a 5000 to 50 Ohm
> >pi network with a Q of 20. You'll see that the inductor values
> >are essentially the same but the capacitor values roughly double.
> >The circulating current goes up but the voltage at the output
> >node remains the same as it must for the same power level
> >at 50 Ohms.
>
> REPLY:
>
> The Xc of the load cap does not change by simply reducing the
> drive.... how could it? That's nonsense. For a given
> frequency, the Xc of a cap is constant regardless of voltage,
> current or anything else.
>
> I don't know where you got the idea that circulating current
> goes up with lowered drive, but it doesn't. If it did, Ohm's
> law requires that the voltages present go up too and they
> don't. You can prove this yourself with a voltmeter anywhere
> in the tank circuit.
>
> 73, Bill W6WRT
>
>
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