Beware of the lightweight XFMRs Patrick. As the turns ratio is increased, so the voltage on the input side. This will get you closer to the voltage you want at the top of the modulation XFMR but the insulation of the small XFMR may not hold up. You might be better off with you first idea. Also a lot of experimentation is required. As someone, Brett I think pointed out the more components in the circuit the more phase shifting at the low and high ends of the audio spectrum. Also wire wound resistors will create a lot more phase shift at the higher frequencies than you think. You can see this very easily using a scope with the horizontal input direct from a signal generator and through a voltage divider of the desired resistors for the vertical input. The straight but slanted line means no phase shift but anything that resembles a circle is a shift. A good circle is 90 degrees.
To be honest a good network pile of carbon or other non inductive resistors in a series parallel network works pretty well to lower the voltage which can then be placed on top of a high inductance screen choke, leaving the smaller XFMR out. There or lots of ways and you idea is as good as any. The phase shift thing is dependent on the coupling and residual inductance of the XFMR and the resistors used for dropping the voltage. There is no perfect solution. There is on you best compromise. Good Luck John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of patrick jankowiak Sent: Tuesday, December 02, 2003 12:05 AM To: [email protected] Subject: [AMRadio] Modulation of the screen grid I agree completely, unfortunately my laziness is the culprit here. The plate and screen modulation transformers have no secondary center taps, I was too lazy to re-do the symbol, that is, I am not very good at CAD and don't know how to do this properly yet. But this is more than worthwhile, I will look into using a different transformer for the screen, I have several, and one might make a mutch better match as your calculations clearly show. I think I have a 50W multimatch around somewhere that would do a better job with the turns ratio than that big 125W bugger. Thank you, Patrick > From: "John Coleman, ARS WA5BXO" <[EMAIL PROTECTED]> > > Hi Patrick > It looks as though you have made all the right calculations. > But keep in mind that the purpose of the screen modulation transformer > is to lower the audio voltage going to the modulator and its design > impedance is of little importance. Its turn ratio is of most concern. > And if I may make two assumptions here then there might be some > simplification. > Assuming that the secondary of the modulation XFMR and the > screen modulation XFMR have a 50/50 split, then consider starting with a > lower audio voltage by using the center tap of the modulation XFMR as > the take off point for the screen audio. This voltage should be 1/2 of > the audio voltage of the top point. Then consider using only half of > the secondary in the screen modulation XFMR. You can only do this if the > primary of screen modulation XFMR can handle the higher primary audio > voltage which will of course be twice as high as before. The dropping > resistor is then much smaller and much less heating and loss involved. > > I took the liberty of recalculation all the parameters for using > 1/2 of each XFMR secondary which you can view at > http://wa5bxo.shacknet.nu/HAMPICTURES/mod002.GIF _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio
