|[AMRadio] Mod xmfr?|
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
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
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 -
> >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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