I am sure most know this, but if you use a scope to measure RF voltage,
be sure to compare the voltage rating of your scope probes to the
maximum voltage that you expect. Note that the rating goes down with
increasing frequency.
I learned this the hard way.
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 16/12/2019 20:03, Jim Garland wrote:
Good advice, Steve. Markku should also check the calibration of his RF
wattmeter. In my experience, these often become quite inaccurate at the
low end of the HF spectrum, and always in the direction of reduced
sensitivity. It might turn out he doesn't have an efficiency problem,
after all!
(When I need to measure RF power accurately, I measure the p-p RF
voltage across a dummy load with a well-calibrated high frequency
oscilloscope (in my case, a Tek 2465B)
73,
Jim W8ZR
On 12/16/2019 9:40 AM, Steve wrote:
From what you describe I think the anode load resistance at the tube
is too low. That is not necessarily linked to the Q of the matching
network.
There are calculators online which will help you calculate the best
load impedance and then the matching network values
http://www.ifwtech.co.uk/g3sek/pi-l-net.zip
is one of the popular ones.
To find out what load you have now, with the amplifier powered off,
look into the antenna end of the matching network with an antenna
analyser and put different value resistors from anode to ground until
you see a low VSWR reading.
When you have calculated a new value to use, put that resistor in
place and change the matching network values until you can adjust for
low VSWR.
Steve G8GSQ
All
I have a GU-78 amplifier based on the Russian R-140 military station.
It turns out that the efficiency is low on 160 m and I suspect that
the tank circuit Q is low on 160 m.
However I am not sure if this is the cause. The tank circuit Q is
not high, that I can tell from the capacitor values.
Another symptom of non-optimal loading is that the screen current is
negative unless I really push the drive to very high output and very
close to having grid current.
I have tried to look for information about low Q tank circuit
problems but no luck so far.
Markku
OH2RA/OG2A/WW1C
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