[Amps] Low efficiency on 160 m
Victor Rosenthal 4X6GP
k2vco.vic at gmail.com
Mon Dec 16 14:26:07 EST 2019
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
I learned this the hard way.
CWops no. 5
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)
> 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
>> There are calculators online which will help you calculate the best
>> load impedance and then the matching network values
>> 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
>>> 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.
More information about the Amps