[AMRadio] Mod xmfr?

Donald Chester k4kyv at hotmail.com
Mon Oct 11 13:10:34 EDT 2004

>Considering the RF amp with 2 x 813's and the modulator with 2 x 813's - 
>are the equations and math to work out the required wattage, impedance, 
>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 
>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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