Carey, you can confirm resonance with the plate tuning capacitor. If you
see a peak in output power while adjusting the control, there is resonance.
A slight increase in C will lower the frequency of resonance and the power
will drop. A slight decrease in C will raise the frequency of resonance and
the power will also drop.
The plate loading can be confirmed in a similar manner. Maximum power
output happens with optimum load C. When the load C is adjusted up or down in
value and the plate tune is adjusted for resonance, the power output will
drop relative to the optimum load C adjustment.
As load C is increased in value, the plate load impedance is lowered and
there will be a resulting increase in plate current.
Strip line designs are not noted for having highest efficiency. The primary
cause of this is unequal rf currents flowing between the strip line and
various parts of the tube anode connection. The strip line is basically
connected to about 1/3 of the tube anode and the other 2/3 of the anode has
very
little current flow.
There are losses in all parts of the plate circuit, including the anode
cooler where it connects to the strip line. If most of the current flows in
1/3 of the area, two things happen: 1> the loss resistance is 3X and 2> the
rf current in this area is also 3X.
Loss being related to current squared times resistance, then the total loss
becomes 3X3X3 or 27 times greater than a truly balanced anode to resonator
rf current flow. What could be a low loss of say 10 watts, quickly becomes
a 270 watt loss and this shows up as lower power output and efficiency.
The designs using parallel resonators, such as the W6PO design, will have
far better rf current balance and associated power output and efficiency. A
cylindrical coaxial resonator design is the best of all but few 2m designs
have been published. A coaxial resonator for 144 MHz is not small!
There are a number of 2m GS35B amplifiers documented at
_www.nd2x.net/base-1.html_ (http://www.nd2x.net/base-1.html) if you haven't
been there.
Operating conditions are shown and you can compare your results.
Others have mentioned the measurement accuracy of both the dc metering and
rf output power. These factors can cover quite a range of performance
discrepancy. It may well be that your amplifier is working as well as it ever
can!
73,
Gerald K5GW
In a message dated 5/22/2011 9:42:25 A.M. Central Daylight Time,
kc5gtt@gmail.com writes:
Hey guys,
before i fire my amp back up i think i need to check the stripline
resonance. i have 4100 volts on the plate of the gs35. what formula should i
use
to come with a resistor value?
or am i going about it wrong. it was working before but not like it
should. i had very poor efficency. which we have more than covered HI. any
suggestions? at that voltage 1500 out with 100 watts drive was a reach. i feel
i
should be able to get that with much less drive. i just dont know where to
start
Carey, kc5gtt
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