[AMRadio] coupling cap on the moduation transformer secondary


Geoff 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
>unloaded output.
>
>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
>Hz.
>

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,
-Geoff/W5OMR





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