|[AMRadio] coupling cap on the moduation transformer secondary|
w5omr at satx.rr.com
Wed Aug 17 12:30:45 EDT 2005
Bob Bruhns wrote:
>Do whatever works for you.
>You may actually get better low-end response with a
>smaller cap, because the coupling capacitor and
>modulation inductor make a high-pass L-C network that
>can have a peak before rolloff, depending on impedance
>and part values. This peak can supplement a sagging
>low end, although rolloff below the peak will be
>faster. Hopefully it will be low enough in frequency
>that it does not matter.
>A peaky high-pass filter will certainly mess up phase
>response, which would affect low-level negative peak
>clipping and such. And a high-pass filter will also
>leave the modulator unloaded at the bottom end. This
>loading issue should not matter much if the modulator
>tubes are triodes. If your modulator tubes are
>tetrodes or pentodes, you could apply some negative
>feedback from the plates or the transformer output, to
>lower the effective plate resistance and control the
>Mostly, it's the sound that matters. See what works
>best and sounds best. If it doesn't screw things up,
>the right capacitor value could add a few dB of
>response a little bit above low-end rolloff, and that
>could work perfectly well for your voice.
>More capacitance will extend the low end response, the
>low-end phase response will be better. The slight
>low-end peak will be lower in frequency, and it may be
>gone altogether. Less capacitance will make low-end
>response worse, and the peak would rise in frequency
>too. If you want furnace rumble, you might find a
>combination of capacitor and inductor values that
>produces a resonant peak for you down around 20 or 30
Well... only because I'm using what I have available I wonder how the LC
ratio changes, if I'm using multiple inductors in series? Of course,
I'd prefer one 50HY choke at 500mA, but I don't have one of those.
Instead, I've got (3) 10HY at 500mA, and a 16Hy at 450mA all strung in
series. Mutual Inductance calculations? Two of the chokes are potted
and the other two are open-frame. Unfortunatly, the two open framed
chokes are -not- identical.
The total DC resistance is ~300ohms for all chokes in series, and a
total inductance of 46Hy.
The calculations are for 1500VDC @ 300mA in the final.
Final impedeance is therefore 5000ohms. The general rule-of-thumb is
8Hy per 1000ohms Z which is 40Hy. I'm covered there.
Coupling capacitance? This is where I'm stumped, because of the
multiple inductors in series.
73 = Best Regards,
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