[AMRadio] Mod xmfr?

Brian Carling bcarling at cfl.rr.com
Wed Oct 13 06:41:20 EDT 2004

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
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