> Both designs assume a single 813 at 2250V, 145 ma plate current,
> 10675 ohms plate load resistance. I plotted complex Zin for the two
> circuits as seen by the tube versus frequency for fixed component
> values adjusted for 29.5 MHz
>
> 1. Conventional Pi-Net
> Q of 35 (minimum that works with the tube at 29.5 MHz),
> Ctune = 18 pf (14 pf for the tube and 4 pf strays)
> L = 1.7 uH
> Cload = 240 pf
> BW = 200 kHz (where Rp drops to 10000 ohms)
>
> 2. Series tuned Pi-Net
> Q = ??? (Not the same as pi-net formula)
> Ctune = 50 pf (in series between plate and inductor)
> L = 2.28 uH
> Cload = 240 pf
> BW = 145 kHz (where Rp drops to 10000 ohms)
>
> Indeed the Q is higher for the series tuned tank as indicated by a
> 25% reduction in bandwidth. I'll make some measurement with
> my virtual grid dipper to confirm the analysis shortly.
If you look in VHF publications, you'll see the opposite is claimed.
The claim is made that Q decreases, but the exact opposite
happens.
> The comment about a series tuned circuit and tube seeing a lower
> impedance still puzzles me. That effect doesn't show up on the
> the Zin vs frequency plots. Except for a narrowing of the peak,
> indicating higher Q, the real and imaginary plots look identical.
> But, so far the model only looked at frequencies around 29 MHz.
Look at the impedance at the L that would normally be the input to
the tank, and compare it to the impedance at the anode.
All you've done is add inductance that wasn't needed, and
cancelled it with series C. It can't do anything but increase system
Q. Even the extra stray C added by the capacitor makes the
problem worse.
The lowest Q system is an inductor straight off the tube.
73, Tom W8JI
w8ji@contesting.com
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