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. Bacon, WA3WDR ----- Original Message ----- From: "John Lyles" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Tuesday, August 16, 2005 3:21 PM Subject: [AMRadio] coupling cap on the moduation transformer secondary > I was just about to say this, Don already responded to it. Besides, finding a compact oil filled capacitor of 2-3 kVDC rating is much simpler if you use a few uF. You don't need 10 Hz low end response, thats wasted power since most receiver/speakers won't reproduce that. Also, the capacitor is just another source of stored energy in your radio, to deal with from a safety standpoint. More is not always better. > > > > Message: 3 > > Date: Tue, 16 Aug 2005 03:00:55 +0000 > > From: "Donald Chester" <[EMAIL PROTECTED]> > > Subject: Re: [AMRadio] AMPX article on mod reactor > > To: [email protected] > > > Actually, broadcast transmitters use much less than than, and still boast of > > flat response down to 30~ or lower. RCA transmitters typically use 1 mfd. > > Gates uses 2 mfd. I use 4 mfd in my homebrew rig @ 4000 ohms modulating > > impedance. > > > > 73, Don k4kyv > > > > > _______________________________________________________ _______ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] >
