Yes I have always been told by G3UUR that an ideal turns ratio is around 1.4:1
Now would that typically be with a centre tap on the primary side for the P-P mod tubes? In other words the 1.4 side gets a centre tap? I am also looking at voltage ratios of suitable ready-made old transformers... Encore un fois, s'il vous plait, Don! (By the way Don, you had a BODACIOUS signal down into central Florida on 160m a couple of weeks back calling CQ but I could not raise you with my PW station) 73 - Brian On 11 Oct 2004 at 17:10, Donald Chester wrote: > > >Considering the RF amp with 2 x 813's and the modulator with 2 x 813's - > >what > >are the equations and math to work out the required wattage, impedance, > >step > >ratio etc. > > > >And, just how much can one depart from the ideal arrangement and with what > >effects? ( should I not be able to find the right xmfr for this amp I'm > >planning > >to build ). > > Regardless of tube or impedances, if you are running a common power supply > supplying the same voltage for modulator and final, you need a modulation > transformer with a somewhere between 1.3:1 and 1.6:1 turns ratio. The > lower ratio (1.3:1) will give higher positive peak capability and more > headroom before flat-topping. The higher ratio (1.6:1) will allow the > modulator tubes to run cooler, but your modulation percentage will be > limited to just a little over 100% both positive and negative, leaving > little headroom in the positive direction before peak clipping sets in. > > Most modulation transformers with a given turns ratio will match a wide > variety of actual impedances. For example, a 2:1 impedance ratio (1.4:1 > turns ratio) could match 8000 ohms plate-to-plate to a 4000 ohm modulating > impedance, or the same transformer could be used to match 16000 ohms > plate-to-plate to o 8000 modulating impedance. Modulating impedance is > final amp plate voltage divided by final amp plate current. > > Just be sure that the ratings of the transformer (maximum voltage and > current) are not exceeded. Running a given transformer at higher impedances > tends to result in some low frequency rolloff, and running it an lower > impedances tends to roll off some of the highs, due to inductances and > internal capacitances of the windings. > > Optimum turns ratios will vary if the plate voltage on the modulator is > different from that of the final. > With lower modulator plate voltage you need less step-down in the mod xfmr > and vice versa. Also, if you are interested in extended positive peaks, you > will need less stepdown. > > Don k4kyv > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected]
