Re: [TenTec] Centurion 40 m. oscillation

["Rob Atkinson, K5UJ" <k5uj@hotmail.com>]

Sounds like the Orion may be sensitive to common mode RF and/or ten tec's 
fixed tuned input circuits in the centurion.  If an 8' jumper is needed you 
would think they'd say that in the manual.  The real solution is variable 
tuned input circuits as offered by brand x.  Maybe it costs more to include 
that.   I'd try putting a swr analyzer on the input to the centurion, keying 
it, and sweeping 40 meters.  you might be surprised at what you find.   I 
found that on some bands such as 160 m. the input swr is hardly flat.  It 
can be 1.5:1 or as high as 1.9.  Your typical s.s. exciter doesn't like 
that.   I have gotten around that problem by using rigs with built-in 
autotuners.   Varying the length of exciter-amp jumper is an okay solution 
if you spend your entire ham existence operating on only one band.   For an 
excellent treatise on this subject read the following excerpt on g-g tuned 
inputs by YT1VP from his amp page:

Tuned Input Circuits for Class AB2 Cathode-Driven [grounded-grid]

Even though grounded-grid amplifier circuits look simple, they are not. The 
grounded-grid amplifier's tuned input circuit is in series with and out of 
phase with the anode current pulses. The RF cathode current's approx. half 
sine wave pulses are the sum of the anode and grid currents. Since the 
driver is connected to the other end of the tuned input, some of the RF 
cathode current finds its way back to the driver. Consequently the driver 
interacts with the amplifier. The Q of the amplifier's tuned input affects 
this interaction.

Modern solid-state output MF/HF transceivers use a broadband push-pull RF 
output stage. In order to meet FCC requirements, Butterworth and/or 
Chebyshev pass band filters are used to suppress spurious emissions. Such 
filters introduce inductive reactance or capacitive reactance within their 
pass bands. In other words, the output impedance of a modern transceiver is 
seldom 50 ±j0 ohms. When driving a tuned input in a grounded-grid amplifier, 
filter reactance interacts with the input reactance in the tuned input. The 
length of the coax between the driver and the tuned input affects the 
interaction.

When tube manufacturers state the cathode driving impedance in grounded-grid 
operation, they are talking about an average value. The instantaneous 
driving impedance fluctuates wildly during the sine wave input signal. 
During most of the positive half of the input cycle, the grounded-grid looks 
negative with respect to the cathode--so the flow of current is cut-off. 
Since virtually no current flows, the driving impedance is extremely high.

During the negative swing in the input cycle, the grounded-grid is 
relatively positive. A positive grid accelerates electrons away from the 
cathode, producing high anode-current and grid-current. Due to the large 
flow of current, the input-impedance is low during the negative half of the 
input cycle.

Consider a pair of 3-500Zs. When the driving voltage is peaking at negative 
117v, the anode-current is at its peak, and the instantaneous anode-voltage 
is at its lowest point--about +250v. At this instant, the total, peak 
cathode-current is 3.4a. Thus, the instantaneous cathode driving impedance 
is 117v/3.4a = 34.5 ohm--and the peak driving power = 117v x 3.4a = 397W.

In other words, the instantaneous driving impedance swing is from 
near-infinite all the way down to 34.5 ohms. The instantaneous drive power 
requirement varies from 0w at the positive peak to 397w at the negative peak 
of the input sine wave. Thus, the input pi-network's job is to act as a 
flywheel/energy storage system and a matching transformer. That's why a 
simple broadband transformer can not adequately do the job of matching the 
driver impedance to the cathode impedance in a grounded-grid amplifier.

The Q of a tuned circuit is like the mass of a flywheel. More Q makes for a 
better flywheel--which does a better job of averaging the wild swings in 
input-Z--thereby producing a lower input-SWR. The trade-off is that more Q 
means less bandwidth. With a high Q, the input SWR may be near-perfect at 
the center of the band, but unacceptable at the band edges. Thus, a 
compromise is in order. Eimac® typically recommends using a pi input network 
Q of 2 for Class AB2 grounded-grid operation. To arrive at a Q of 2, the 
reactance [X] of the input capacitor, C1, is minus j50 ohm÷2=minus j25 ohm. 
Using C=1÷[25(2f)], approximately 220pF of input capacitance is needed for a 
Q of 2 on the 10m band. In actual practice, however, 220pF may be far from 
the value that produces a satisfactory SWR with a particular model 
transceiver and a particular length of coax. It may be possible to find a 
length of coax that would ameliorate this problem on 10m--but there are 
eight other bands to contend with below 30MHz. Since band switching 
different lengths of coax is hardly practicable, it would be useful if the 
input capacitors were adjustable in a grounded-grid amplifier's tuned input 
circuits. Adjustable coils are also useful.

the full item is at http://www.qsl.net/yt1vp/Introduction.htm     and is 
excellent reading.

rob/k5uj

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