Jim: We might want to clear up something here about plate efficiency. Many new people get confused about all this.
Starting with a triode because it is easier to explain: Suppose we have a triode tube operated in class C service for plate modulation. Let's use 3000 volts and at 86 ma of plate current with an efficiency of 75 percent. This would yield 260 watts input and 195 watts output. If the plate supply is then reduce to 1500 volts (1/2) then the plate current should automatically reduced to (1/2) 43ma. The power input and output should be reduced by 25% which would then yield 65 watts input and 48.75 output power. With this configuration, the plate efficiency should still be 75 percent. If the output power doesn't drop by the same percentage as the input power then the tube is not being operated in the proper class C service for plate modulation. Thus, the RF voltage output will not follow the audio modulation voltage. Now, suppose one wants to maintain the same output power while reducing the plate voltage. This can be done by retuning and reloading, i.e. increasing the plate current in an attempt to get the RF output back up. Under these conditions, the efficiency may change for better or worse. But just reducing the voltage without other adjustments should not change the efficiency. Now, if we are going to use tetrodes to cachieve the same results as before, we will require screen adjustment at the same time as the plate adjustment but generally not at the same percentage. Generally the plate voltage is reduced to reduce output, not to change the circuit operating parameters (Ep, Ip, etc.) If, on the other hand, someone wants to achieve a certain power input and output from a particular tube, then there one should stay within a particular window of operating parameters. If a tube is to be used to produce its maximum output then the window of operation is narrow. But if the expected power output is to be reduced, then the high efficiency operation parameter window becomes wider. A 4-65 has a maximum plate dissipation of 65 watts. While the best plate efficiency window is beyond the scope of this article, we might assume that it is approximately 75 percent for a given voltage and plate current. Given this parameter, the tube can produce 260 watts input and 195 watts output. A tube used in this manner--adjusting a plate current to voltage, while maintaining the same output power-- may prove to over heat, due to the change in efficiency. But if the power input is to be 100 watts and the output to be 75 watts, then the voltage-to-current ratios of Ep-Ip could be changed significantly and yet maintain the same efficiency. John Coleman, WA5BXO ----- Original Message ----- From: "Jim Candela" <[EMAIL PROTECTED]> Subject: Re: [AMRadio] 4-65's The 4-65 is a Higher voltage, and lower current tube. In class C service, the efficiency is really great at 3kv plate voltage, and is diminished at lower voltages. ______________________________________________________________ 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:AMRadio@mailman.qth.net To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.