Very well said Don and if I might add, for the newcomers to generating audio. That in any XFMR coupled stage (ie push pull plate modulator), the max audio voltage that can be produce is when the audio tube draws so much current during its half audio cycle that it reduces it's on plate voltage to zero for a moment and on the next half cycle its plate voltage doubles the supply voltage. This is saturation for the circuit. Adding more tubes or bigger tubes to the modulator will not get more audio voltage from the modulation XFMR. The only way to get more from the circuit is to change the turns ratio of the XFMR or to raise the voltage on the modulator plates and once that is done you may need to change the tubes for the safety of the tubes or to handle the increase in current, power and voltage that may be produced by the XFMR change or the voltage change.
Often it is easier to build a bigger modulator with its own power supply of the required voltage than it is to try to find a modulation XFMR with the "turns ratio" that you want. That is when equipment size is not important. John Coleman, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of D. Chester Sent: Tuesday, November 13, 2007 4:09 AM To: [email protected] Subject: Re: [AMRadio] Re:Modulator Plate Voltage Extra modulation headroom can be had two ways. The modulation transformer turns ratio can be reduced (less step-down), or the plate voltage on the modulator can be raised, relative to the final. Many transmitters run the same voltage to the modulator and final, and use a transformer with a turns ratio of about 1.6:1 to 1.7:1. This just barely allows about 95% modulation before the waveform flat-tops. Often this is done intenionally to prevent "overmodulation". The problem is, the flat-topping caused by modulator saturation produces exactly the same kind of splatter and distortion as overmodulation, so this is a bad idea. Since my voice is asymmetrical, I need the extra headroom to accomodate the positive modulation peaks without splatter and distortion. But the modulation transformer is fixed ratio, so I raise the modulator plate voltage well above the final amp plate voltage. I adjust to just below 100% modulation in the negative direction, per the oscilloscope, and let the positive peaks go where they may, making sure they don't flat-top. This same principle has been used since the late 20's, to 100% modulate using a class-A single-ended Heising modulator. Using the same plate voltage resulted in maximum modulation percentage of about 60%. Often the PA plate voltage was reduced using a series power resistor by-passed with a HV oil capacitor. Many, if not most hams have an exaggerated opinion of the efficiency of their transmitters. The efficiency figures given in the tube data sheets are much like the EPA mileage estimates listed on the showroom sticker of a new car. Yours is likely to be much lower. The modulation transformer itself will probably be less than 90% efficient. Then you have losses in the plate tank circuit, the antenna tuner (if one is used), and the antenna feedline. These losses all add up, so that most plate modulated amateur transmitters do well to get much more than about 50% efficiency, when comparing DC input to the final to rf input to the radiating antenna. Comparing rf input to the radiating antenna to the power drawn from the a.c. power mains, you are likely to have a dismal figure of about 20% efficiency (maybe 30% in exceptional cases) with a tube-type transmitter. That is one of the main reasons why the broadcast industry has converted over to high-efficiency solid state transmitters as rapidly as possible. Don k4kyv ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio 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. ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio 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.
