RE: [AMRadio] High level modulation of tetrodes
The voltage divider between modulated and unmodulated B+ for a screen supply works wonders on the 32V series transmitters. As a test, I used half the voltage from each, and the positive modulation percentage increased quite a bit on the mod monitor. It worked so well, I left it that way. The rigs are running about 800 volts on the plates had problems getting past about 80% modulation positive when loaded up to 200ma, with the divider it went up to 100% or more positive mod. PEP went up, but of course, no one at the distant end would hear or see any difference in such a small change, I just like it that way. Like Dyson, I just think things should work properly Brett N2DTS -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Bob Bruhns Sent: Tuesday, September 27, 2005 10:13 PM To: Discussion of AM Radio Subject: Re: [AMRadio] High level modulation of tetrodes For screen self-modulation, probably the higher the series impedance, the better. So an inductor in series with a resistor in the screen circuit would be better than either the resistor or the inductor alone. WA1KNX Dino suggested a resistive voltage divider from modulated plate B+ to unmodulated plate B+ feeding the screen. The ratio of resistances was about 6 to 4, for about 40% modulation. The screen would see 40% modulated plate B+ coming through a resistance equal to 1/((1/R1)+(1/R2)). A compensating capacitor was included to tweak the screen modulation at higher audio frequencies. http://www.amwindow.org/tech/htm/scrnmod.htm Bacon, WA3WDR - Original Message - From: John E. Coleman (ARS WA5BXO) [EMAIL PROTECTED] To: 'Discussion of AM Radio' amradio@mailman.qth.net Sent: Tuesday, September 27, 2005 8:54 PM Subject: [AMRadio] High level modulation of tetrodes I changed the subject title here to somewhat match the subject. When a choke or a high value resistor is used in series with the screen supply line the screen grid will self modulate following the voltage on the plate to some degree. This happens because if the plate voltage is increased the plate will attract more electrons to it leaving less for the screen. The reduced screen current will cause the drop across the resistor or screen choke to be less and therefore more voltage on the screen. The opposite thing happens if the plate voltage is reduced as the audio take it down less plate current is drawn and more is available for screen current. The increase in screen current causes a larger drop across the choke or screen resistor resulting in less screen voltage These changes will not be 100% proportionate. But that is generally good because it has been found that when the plate modulated at 100 percent the screen should be modulated at approximately 40 percent. Many times I have found 6146 rigs will have a premature drop in RF voltage at about 85-90 percent on the negative cycle. This is caused by too much audio on the screens. It occurs when the screen dropping resistor supply side is connected to the high side of the modulation XFMR. Connecting the screen resistor to the low side of the modulation XFMR will result in too little audio on the screen. The screen choke with a fixed screen supply works well but there is another fix for small rigs. One fix would be to have a separate winding on the modulation XFMR and a fixed screen supply. This scenario means hard to find parts and complex circuits. I have found that a good fix is to leave the original screen resistor in place and find a value of resistor in series with a audio bypass capacitor that when placed from screen to ground will reduce the audio on the screen to 40% while leaving the DC voltage on the screen at the level it needs to be. See http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.JPG Or http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.bmp John, WA5BXO __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami
[AMRadio] High level modulation of tetrodes
I changed the subject title here to somewhat match the subject. When a choke or a high value resistor is used in series with the screen supply line the screen grid will self modulate following the voltage on the plate to some degree. This happens because if the plate voltage is increased the plate will attract more electrons to it leaving less for the screen. The reduced screen current will cause the drop across the resistor or screen choke to be less and therefore more voltage on the screen. The opposite thing happens if the plate voltage is reduced as the audio take it down less plate current is drawn and more is available for screen current. The increase in screen current causes a larger drop across the choke or screen resistor resulting in less screen voltage These changes will not be 100% proportionate. But that is generally good because it has been found that when the plate modulated at 100 percent the screen should be modulated at approximately 40 percent. Many times I have found 6146 rigs will have a premature drop in RF voltage at about 85-90 percent on the negative cycle. This is caused by too much audio on the screens. It occurs when the screen dropping resistor supply side is connected to the high side of the modulation XFMR. Connecting the screen resistor to the low side of the modulation XFMR will result in too little audio on the screen. The screen choke with a fixed screen supply works well but there is another fix for small rigs. One fix would be to have a separate winding on the modulation XFMR and a fixed screen supply. This scenario means hard to find parts and complex circuits. I have found that a good fix is to leave the original screen resistor in place and find a value of resistor in series with a audio bypass capacitor that when placed from screen to ground will reduce the audio on the screen to 40% while leaving the DC voltage on the screen at the level it needs to be. See http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.JPG Or http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.bmp John, WA5BXO
Re: [AMRadio] High level modulation of tetrodes
In a message dated 9/27/05 6:11:03 PM, [EMAIL PROTECTED] writes: One fix would be to have a separate winding on the modulation XFMR and a fixed screen supply. This scenario means hard to find parts and complex circuits. The military ART-13 does this (813 modulated by a pair of 811s). Special transformer for sure, but the circuitry is otherwise simple and straightforward. Works great. Dennis D. W7QHO Glendale, CA
Re: [AMRadio] High level modulation of tetrodes
For screen self-modulation, probably the higher the series impedance, the better. So an inductor in series with a resistor in the screen circuit would be better than either the resistor or the inductor alone. WA1KNX Dino suggested a resistive voltage divider from modulated plate B+ to unmodulated plate B+ feeding the screen. The ratio of resistances was about 6 to 4, for about 40% modulation. The screen would see 40% modulated plate B+ coming through a resistance equal to 1/((1/R1)+(1/R2)). A compensating capacitor was included to tweak the screen modulation at higher audio frequencies. http://www.amwindow.org/tech/htm/scrnmod.htm Bacon, WA3WDR - Original Message - From: John E. Coleman (ARS WA5BXO) [EMAIL PROTECTED] To: 'Discussion of AM Radio' amradio@mailman.qth.net Sent: Tuesday, September 27, 2005 8:54 PM Subject: [AMRadio] High level modulation of tetrodes I changed the subject title here to somewhat match the subject. When a choke or a high value resistor is used in series with the screen supply line the screen grid will self modulate following the voltage on the plate to some degree. This happens because if the plate voltage is increased the plate will attract more electrons to it leaving less for the screen. The reduced screen current will cause the drop across the resistor or screen choke to be less and therefore more voltage on the screen. The opposite thing happens if the plate voltage is reduced as the audio take it down less plate current is drawn and more is available for screen current. The increase in screen current causes a larger drop across the choke or screen resistor resulting in less screen voltage These changes will not be 100% proportionate. But that is generally good because it has been found that when the plate modulated at 100 percent the screen should be modulated at approximately 40 percent. Many times I have found 6146 rigs will have a premature drop in RF voltage at about 85-90 percent on the negative cycle. This is caused by too much audio on the screens. It occurs when the screen dropping resistor supply side is connected to the high side of the modulation XFMR. Connecting the screen resistor to the low side of the modulation XFMR will result in too little audio on the screen. The screen choke with a fixed screen supply works well but there is another fix for small rigs. One fix would be to have a separate winding on the modulation XFMR and a fixed screen supply. This scenario means hard to find parts and complex circuits. I have found that a good fix is to leave the original screen resistor in place and find a value of resistor in series with a audio bypass capacitor that when placed from screen to ground will reduce the audio on the screen to 40% while leaving the DC voltage on the screen at the level it needs to be. See http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.JPG Or http://wa5bxo.shacknet.nu/screen%20grid%20modulation%20technique.bmp John, WA5BXO __ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net AMfone Website: http://www.amfone.net AM List Admin: Brian Sherrod/w5ami
