Tom suggests that reducing capacitance and increasing inductance
with the series cap suggested actually increases Q. At great risk to
my lap dog status, I decided to model the two circuits and compare
results.
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.
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.
jeff
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Randall D. DuCharme" <randyd@specent.com>; <amps@contesting.com>; "jeff
millar" <jeff@wa1hco.mv.com>
Sent: Friday, October 13, 2000 5:27 AM
Subject: Re: [AMPS] Series Tuned Tank?
> I've seen it suggested in VHF amplifier articles that this reduces
> tank Q. That isn't true.
>
> You can make the amp run on a higher frequency....maybe....but
> the tank circuit is a mess. The tube looks into a series tuned
> circuit with low impedance, lower than if you didn't have the series
> cap. Q of the *system* is actually increased over using less L and
> not having the capacitor.
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