Hi Bow, Linears are tuned for peak power. The idea is that the system is set up so that the tube conducts and pulls the plate voltage down to minimum, a few hundred volts, on negative RF waveform peaks at full power. This is where you get maximum efficiency and maximum output.
When you tune, you apply the necessary drive and tune the tank to load the tube(s). You can do it with a steady carrier, or with a pulsed or modulated waveform, but with a steady waveform, a linear is easier to adjust with simple tools (grid and plate meters and maybe an output wattmeter). Basically you apply the right amount of drive for the desired peak output, and tune and load for the right plate current and output at the peak output level. (With a pair of 813s this will be around 1 KW, but for AM with a pair of 813s you might want to start out at 500 watts peak because of plate dissipation.) Usually people load a linear a little heavily, which reduces efficiency slightly but improves linearity. Then back off on the RF drive, and RF output falls. Set the carrier level around 50% to 40% of the plate current compared to the peak level you just set. This allows for about 100% to 150% modulation respectively. That also means carrier output is 1/4 to 1/6 of peak power, respectively. If you tuned for 600W peak, and you backed off to 50%, then your carrier output will be about 150 watts for an input of about 450 watts, for a carrier dissipation of 150 watts per tube - a bit beyond spec, but probably doable with a fan or two. If you tuned for 600W peak and backed off to 40%, then your carrier output will be about 100 watts for an input of about 400 watts input, for about 150 watts dissipation per tube. AM linear is not too efficient. To be within the plate dissipation spec for 813s, you would tune for 500W peak and run about 125W of carrier output at 100% modulation. Linears with better carrier efficiency are very complex. 60% and higher carrier efficiency is possible with trick designs, but they are difficult to tune. Plate modulation is much simpler, and a pair of 813s can produce 500 watts or more of carrier output. You can certainly set up a linear and convert it to plate modulation when you get the additional parts for that. That's what a lot of people do. Bacon, WA3WDR ______________________________________________________________ AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Partner Website: http://www.amfone.net Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
