While the below is an ideal situation it certainly isnt mandatory in many
cases.
Take the older amps with just a .01 coupling cap for the input that are
regularly in use with a SS exciter. The various Dentrons, Hunters, Gonsets,
etc are common examples and a lot of Alphas used only a broadband toroid and
various resistive terminations on them to run GG 8874's and 3CX800's quite
well.
I cant speak to most modern rigs but I can use the built in tuners in my
TS-940 or TS-950SD to drive any of the customer amps I service as part of my
business to rated power, even on 10M. The narrow pi-network and tuning range
on 160 of the TS-830 6146B finals limits the drive somewhat but works just
fine on other bands. Some years ago I used a Drake T4XC to do the same.
Ive also tried various MFJ and other tuners between a SS rig with the
internal tuner off and get a good match on all bands, and again rated
output.
IMD levels were down a bit compared to a similar tubed amp with a built in
tuned input but nowhere near the 5-10dB of running a SS rig directly into a
.01 to the cathode without the benefit of an internal or external tuner.
All these compromises take minimal or no extra power compared to using the
TS-830 as the benchmark and many were tested when a certain self appointed
expert on everything was pontificating on various forums a few to many years
ago.
Carl
KM1H
----- Original Message -----
From: "Gary Schafer" <garyschafer@largeriver.net>
To: "'Manfred Mornhinweg'" <manfred@ludens.cl>; <amps@contesting.com>
Sent: Friday, July 25, 2014 10:57 PM
Subject: Re: [Amps] SB-200 input.
It is always best to have the GG amplifiers tuned input circuit as close
to
the cathode as possible.
There needs to be capacitance directly from the cathode to ground. A pi
network input is ideal and easiest to deal with. A parallel tuned input
circuit can also be used as the tune capacitor is directly from the
cathode
to ground but it also needs to be very close to the cathode.
The rule of thumb as I recall, there needs to be about 13 pf per meter of
capacitance from cathode to ground. The 10 meter band would require about
130 pf of capacitance at the cathode side of a pi network.
Without this capacitance directly at the cathode the efficiency also
suffers
as there is not a direct path to ground for the harmonic currents in the
cathode.
A tube type exciter with pi network output is better than nothing but is
not
ideal as the output capacitor of the pi network is too far away from the
cathode of the driven amplifier.
Also using an external antenna tuner such as is found used with most
transistor type radios is usually a T type network. This doesn't provide
the
required capacitance to ground for the cathode unless the amplifier has
its
own built in pi network input matching system.
73
Gary K4FMX
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Friday, July 25, 2014 11:54 AM
To: amps@contesting.com
Subject: Re: [Amps] SB-200 input.
Hi all,
adding to Carl's comment:
> Tuned inputs provide the flywheel effect that makes the tubes easier
to
> drive and improves IMD 5-10dB.
That's indeed to biggest reason to use a tuned input. The impedance of a
class
AB grounded grid tube's cathode varies dramatically throughout the RF
cycle.
During a good part of the cycle, the tube is cut off entirely, and the
cathode
impedance is essentially infinite. During the rest of the cycle, the
tube
conducts, and the cathode has an impedance that changes depending on the
instantaneous current. The curves of triodes are pretty crooked, and
this
reflects on the cathode. Having a big flywheel there, meaning a
relatively high
Q network of some sort, helps a lot in keeping the waveform clean, and
in
providing a huge drive current during that part of the cycle when the
cathode
needs it, while accumulating energy from the driver not needed by the
cathode
when it's in cut-off.
The point is that with any grounded grid amplifier, you need a
reasonably high Q
network between the driver's output tube or transistors, and the
amplifer's
cathode. If the driver radio has a tuned output network, such as the PI
tank of
a tube rig, that's really quite enough, and a WELL DESIGNED broadband
drive
matching circuit can work OK. Same thing if you have a solid state radio
with an
antenna tuner inserted between the radio and the amplifier. In that case
the
tuner provides the required Q and flywheel effect - or at least we can
hope the
Q is high enough! But if you have a bare bones solid state radio, whose
only
flywheel effect comes from a 5 pole lowpass filter having a Q of unity,
you will
see trouble with a broadband cathode driving circuit. In that case, a
tuned
drive circuit works wonders.
We can also see it this way: When you load a radio with a correctly
tuned
antenna, or with a plain resistor (dummy load), it sees a constant load
all
through the RF cycle, and that load is ideally 50 ohm, making an SWR of
1:1. But
if you connect it to the cathode of a grounded grid tube, it sees a
terribly
varying impedance, with the SWR being far above 1:1 most of the time,
and at
infinity for a part of the RF cycle. No amount of broadband matching can
fix
that! Any matching transformer can only change an impedance by a fixed
ratio, it
cannot match to a wildly varying impedance. But if you insert a tuned
circuit,
which acts as a flywheel, this tuned circuit evens out the varying
impedance of
the cathode, and presents the average impedance to the driving radio,
smoothly
and cleanly. This impedance may still be wrong, but at least it's
constant
throughout the RF cycle, and that allows matching it to a nice 1:1 SWR
with
either a broadband transformer or a resonant network. Usually, when you
go to
the trouble to install such a resonant circuit, it's very easy to tap it
for 50
ohm, or to elaborate it into a PI section having 50 ohm input impedance.
So
that's what's usually done, rather than combining a broadband
transformer with a
set of simple tuned circuits.
Please note that there is yet another problem: The tuned cathode
matching
circuit can very well even out the cathode impedance over the RF cycle,
but it
cannot do anything when the average impedance over the RF cycle varies
according
to the amplitude of the signal! So, even with a tuned matching network,
the
input SWR can change according to the exact instantaneous drive level.
That is,
an amplifier might present a 1:1 SWR to the radio at 50 watts CW drive
level,
but at 100W or at 20W the SWR will be higher. During SSB transmission,
the SWR
will be varying all the time, according to the signal's envelope. Some
radios
cope better with this than others. If an amplifier needs far less drive
power
than the radio can deliver, it's a good idea to insert an attenuator, so
that
the radio runs at full power, and the attenuator dampens the SWR changes
of the amp.
But the best way to avoid all this trouble is to avoid grounded grid
amplifiers!
If you use a grid-driven amplifier in class AB1, the tubes almost don't
load the
driver at all. A dummy load inside the amplifier loads the driver
smoothly and
cleanly. No tuned drive circuits are required, no drive distortion
happens, and
many tubes can be driven without needing any impedance transformation,
and just
a low drive power at 50 ohm. The disadvantage, of course, is that the
tubes have
to be tetrodes, at least, with an additional screen power supply.
Triodes aren't
linear enough to use them in this way.
When using MOSFETs rather than tubes, the situation in this regard is
way better
than with grounded grid tubes , but not as good as with grid-driven
tubes in
class AB1. By using proper circuit design, with enough negative feedback
and
resistive gate swamping, normally the performance is pretty good without
needing
any tuned input circuit. But in extreme cases, when milking a marginal
MOSFET
for all the gain it can provide, leaving no room for gate swamping nor
negative
feedback, a tuned input circuit can be required!
Many of you of course knew most or all of this, which is all pretty old
technology, but perhaps somebody learned something new to him. And I had
fun
writing it...
Manfred
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Visit my hobby homepage!
http://ludens.cl
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