[Amps] Low efficiency on 160 m

[Steve]

 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