...guys, remember that adding capacitance on the input side does modify the
loaded Q, besides reducing the tuning range of the network!
This technique of "neutralizing" part of the output reactance of the tube is
in any case frequency sensitive, being resonant, so that departing too far
from the design frequency is going to wreak havoc to it. This is exactly the
way to smoke a plate choke!
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Paul Kelley N1BUG
Sent: Thursday, February 26, 2009 7:35 PM
To: amps@contesting.com
Subject: Re: [Amps] Minimum Value of RF Plate Chokes
Hi Eddy,
Edward Swynar wrote:
> By effectively changing the circuit values in a tank circuit to
accommodate
> a less-than-perfect choke, what is the fall-out going to be to the
operating
> parameters of the tube...? After all, we are now CHANGING computed values
> for specific "Q" in the tank components, to make-up for the shortcomings
of
> the choke --- correct?
To my way of thinking, we're not changing the computed (or optimum)
values of components in the tank circuit, nor its Q, *if things are
set up and tuned correctly*. I look at it this way: Picture a tank
circuit built with the computed values. Now picture a plate choke
with less than enough reactance at the operating frequency to be
"invisible". Assuming the bypass capacitor at the cold end of the
choke has reasonably low reactance at the operating frequency,
essentially the choke looks like an inductance shunted to ground.
Now if we add a capacitor in parallel with the choke of sufficient
value to form a parallel resonant circuit with said choke, the
combination of L (choke) and C (added capacitor) presents a very
high impedance to the rest of the circuit and becomes "invisible".
Now look at the circuit. The tank input C is in parallel with the C
we added to resonate with the choke. So why not replace them with
one capacitor equal in value to the combined value of these two
capacitors? If we do this by simply increasing the value of the tank
capacitor, then when we tune the tank to resonance we are in effect
resonating the tank properly, but at the same time providing the
right C to resonate with the choke as a parallel resonant circuit,
making it essentially "invisible".
Or am I looking at this wrong?
> Also, what would happen when the final tank circuit is off-resonance
> slightly --- will RF end-up back into the power supply?
Maybe. One would hope the bypass C at the power supply end of the
choke has very low reactance and shunts most of the RF to ground. If
there is a glitch resistor in series with the HV that should at
least aid somewhat in forcing RF to ground rather than sneaking into
the power supply.
> Again, I may well be missing something critical here, but by having to go
> through all these "gymnastics" in the values of the other parts of the
> pi-tank system, it is very obvious that the presence of a "sub-standard"
> choke is hardly "invisible" in the whole scheme of things, and it seems to
> fly in the face of a properly-designed final pi-tank network, on a given
> band...
I tend to be a purist and therefore would prefer to have a choke of
high enough inductance to look invisible on its own. As a practical
matter in a multiband amp (mine covers 160-10m) while also
preventing potentially disastrous series resonances in the choke on
the higher bands, I can live with the choke needing a little
"assistance" in the form of extra pi tank input C on the lower
bands. My plate choke certainly doesn't have enough reactance at 1.8
or 3.5 MHz to be very invisible without help!, but with the cold end
of the choke bypassed well and a somewhat higher than usual glitch
resistor (I use 100 ohms) I've never had any apparent problems with
RF in the power supply (or otherwise).
However... I'm pretty much a stickler for keeping the amp optimally
tuned as I QSY. Near the amp I have a little chart with tune and
load settings for the various bands. For 160m the chart contains
optimum settings at 10 kHz intervals, and for 80m 25 kHz intervals.
With vacuum variables and turns counters, it's pretty easy to just
tweak the knobs per the chart as I move around. It has become second
nature.
73,
Paul N1BUG
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