The problem with a grounded grid or any RF amplifier that has a non-linear
input impedance ie non-resistive is that the exciter is not loaded with the
same impedance during the entire RF cycle. This introduces harmonic
generation (which is not a serious problem because it will be filtered out
later anyway) and intermodulation distortion. If the instantaneous V-I
curve of this impedance instead of being a straight line (resistive) can be
described by a polynomial and that polynomial contains odd order
components ,it will produce odd harmonics and IMD.
A low pass filter has no energy storage. The idea is that a band pass
filter with a capacitor on the output of it connected across
the non-linear cathode circuit (or grid in some cases). The capacitor in
conjunction with the inductor make an AC energy storage system. The amount
of energy stored in the system is related to the Q. A low pass filter has
a Q of less than 1 (actually it is Q= center frequency/
bandwidth=1/2). The reason for the capacitor being the at the output is
because it is a short term constant voltage device. It and the inductance,
tries to maintain a constant sinusoidal AC voltage between the cathode and
ground during several RF cycles. If the capacitive reactance is 1/3 of the
cathode's minimum instantaneous impedance it maintains a very good
sinusoidal wave form for a single RF cycle (what is needed) and does fairly
good for 3 cycles. This is why high Q is not necessary. The exciter sees
a constant impedance over each RF cycle also.
If the output component of the filter connected to the cathode is an
inductor (Inductor output L or T or Pi-L) it will try to maintain a
constant sinusoidal RF current thru the RF cycle. This will make the IMD
even worse.
Well, there is my simple explanation.
73
Bill wa4lav
At 08:00 AM 5/4/2003 -0500, Robert & Linda McGraw K4TAX wrote:
>I'd agree with the question, "how does the problem manifest itself?". I use
>a Paragon to drive the Centurion (which does have low pass filters for each
>band on the input). I'd suppose there is enough stored energy to provide
>counter EMF when the tube input Z is on positive peaks.
>
>Perhaps I don't see it as this only relates to amps that have only a passive
>input.
>
>73
>Bob, K4TAX
>
>----- Original Message -----
>From: "Michael Tope" <W4EF@dellroy.com>
>To: "2" <2@vc.net>; "carl seyersdahl" <carlseye@tampabay.rr.com>; "AMPS"
><amps@contesting.com>
>Sent: Saturday, May 03, 2003 6:20 PM
>Subject: Re: [Amps] G-G Amplifiers With No Tuned Input.
>
>
> > How does the problem manifest itself, Rich. I have used the
> > Alpha 78 quite a bit with a Kenwood TS-940, Ten-Tec
> > Paragon, and Kenwood TS-950SDX. All of these rigs are
> > solid state, but don't seem to have any trouble driving the
> > Alpha 78.
> >
> > BTW, the Paragon uses relay switched low-pass filters (5
> > pole I believe) to filter the output of their PA. I suspect
> > Kenwood does similarly. These filters may be providing
> > the needed energy storage to average out the non-linear
> > impedance presented by the unfiltered cathode input on the
> > Alpha 78.
> >
> > 73 de Mike, W4EF.......................................
> >
> > >> Amplifiers such as the MLA-2500, SB-230, 374, 78, and 76PA have no
>tuned
> > >> input. This is not a problem when such a g-g amplifier is driven by a
> > >> tube-output radio such as the TS-830S. The reason is that although the
> > >> input Z of a cathode-driven triode fluctuates from roughly 1/3 of the
> > >> average driving Z on negative peaks to a virtual open circuit on
>positive
> > >> peaks, the Pi-network tuned circuit in a tube-output radio acts as a
> > >> flywheel to smooth out the Z fluctuation at the cathode/s.
> >
> >
> >
> > _______________________________________________
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> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
>
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