Hi Geoff, The inductances add arithmetically, because there is no mutual coupling between them. So you have 10 + 10 + 10 + 16 = 46 Henries with the four of them in series.
I used 30 uF with 50 Henries back in the day. I had an 810 (triode) modulator, a VM-5 mod transformer, and the exact same as you have, 1500V at 300 mA on the RF final (5000 ohm load impedance). A 30uF coupling capacitor and a 50 henry modulation inductor worked fine. I had 30 Hz furnace rumble like you wouldn't believe. I could pass 15 Hz through the thing. (All I had was a test record with tones on it. I played it at half speed to get 15 Hz.) 46 Henries will work just about the same as 50. I had the mod transformer secondary at B+, and the coupling capacitor was just three 10uF 600V oil caps in parallel. But if you use the high-pass filter peaking technique, you need a higher voltage capacitor, because the resonant effects will put a lot of low frequency audio voltage on the cap, even if you keep the DC off of it. It's a high-pass filter issue. You have Z(source) after the mod transformer - Inductance of the mod transformer to audio ground - Coupling Capacitor - Inductance of the modulation inductor to audio ground - 5K load to ground. The inductance of the mod transformer would be the inductance you would measure on the secondary, with no load on the primary. If you can model the high pass filter, you can play with part values and see what will happen. I thin the plate resistance will transform to four times the plate resistance of one tube, transformed by the impedance ration of the mod transformer. So if the tubes have 10K plate resistance and the mod transformer has a 2:1 impedance ratio, then the source impedance will be 40K/2 = 20K. Somebody check me if I'm wrong on this. Bacon, WA3WDR ----- Original Message ----- From: "Geoff" <[EMAIL PROTECTED]> To: "Discussion of AM Radio" <[email protected]> Sent: Wednesday, August 17, 2005 12:30 PM Subject: Re: [AMRadio] coupling cap on the moduation transformer secondary > 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) [EMAIL PROTECTED], and a [EMAIL PROTECTED] 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 > > > _______________________________________________________ _______ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] >
