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Re: [Amps] load resistor

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Subject: Re: [Amps] load resistor
From: "Carey Lockhart, KC5GTT" <>
Date: Sun, 22 May 2011 14:49:05 -0500
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thanks very much for that info. it is most usefull. i having tons of fun 
building the amps. sadly in my 20 years of being a ham there is still so much i 
still have to learn.

Carey, kc5gtt
  ----- Original Message ----- 
  To: ; 
  Sent: Sunday, May 22, 2011 11:10 AM
  Subject: Re: [Amps] load resistor

  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 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!

  Gerald K5GW

  In a message dated 5/22/2011 9:42:25 A.M. Central Daylight Time, 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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