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Title: Re Triode RF Amplifier Biasing Post by: ashart on September 21, 2009, 02:15:58 AM Gentlemen, here’s the problem:
1. Class C Telegraphy typical operation tables for an 810 triode at Eb=2000v, show Eg=-160 and Ig=0.040 for CW. The Telephony tables at Eb=2000, show Eg=-350 and Ig=0.070. Ignoring any fixed bias for the moment, those values would require grid resistors of 4K for CW and 5K for phone. 2. Yet, in no QST or Handbook design, have I ever seen an included means of changing a grid resistor as one switches between CW and phone, or, if fixed bias were used, a means of changing the bias voltage. 3. Is the common way to change a triode's bias between CW and phone in order to maintain proper operating conditions, merely to change the RF drive when tuning up? 4. If so, would not the conduction angle change as one switched between CW and phone? Thus, if one designed for an optimum conduction angle in one mode, the conduction angle could be non-optimum in the other mode. 5. Am I missing the boat here somewhere? The fact that I’ve never yet seen an amplifier designer throw just one more relay and one more resistor at his transmitter design, makes me wonder whether it's they or I, with the unsound technical background. What's worrisome, is that there're so many of them and so few of me! :) 6. Is there any literature that comprehensively addresses the matter? Title: Re: Re Triode RF Amplifier Biasing Post by: Ralph W3GL on September 21, 2009, 03:39:01 AM The answer from most of us old timers is behind door 3 ... Title: Re: Re Triode RF Amplifier Biasing Post by: W2PFY on September 21, 2009, 10:03:18 AM I don't know if this will help but here is the way I operate my Westinghouse MW-2 and BC-610 for both CW & Fone. I drive the given final tube with as much drive as I can produce. Some may disagree but with most power triodes, it will be almost imposable to exceed the control grid dissipation with the driver that was supplied as original equipment.
I am just guessing at the possible fact that transmitters designed to do CW & Fone were biased at some happy medium for class C conduction angle. Title: Re: Re Triode RF Amplifier Biasing Post by: KM1H on September 21, 2009, 10:12:18 AM Build it so it works best on AM according to your scope. Use the same settings for CW.
The old buzzard terms were ''Deep Class C" for AM but all that means is you need more drive to overcome the higher bias. CW doesnt care. Carl KM1H Title: Re: Re Triode RF Amplifier Biasing Post by: w1vtp on September 22, 2009, 09:37:17 PM It's late for me but here's my opinion. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
I prefer to use just enough fixed bias to cut off the final. Then I calculate for - given the desired grid drive in ma - for the rest of the voltage so that I'm in class "C" I just think it's worth the extra work to protect these finals (today made of impossibletogetium) for damage should I lose rf drive. I agree with the comment that grid bias should be set up for good AM performance and use the same setup for CW. My 2 cents worth Al |