[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 
increasing frequency.

I learned this the hard way.

Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
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)
> 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

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