Re: [AMRadio] coupling cap on the moduation transformer secondary
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: amradio@mailman.qth.net 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 + From: Donald Chester [EMAIL PROTECTED] Subject: Re: [AMRadio] AMPX article on mod reactor To: amradio@mailman.qth.net 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:AMRadio@mailman.qth.net
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
Re: [AMRadio] coupling cap on the moduation transformer secondary
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 amradio@mailman.qth.net 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:AMRadio@mailman.qth.net
RE: [AMRadio] coupling cap on the moduation transformer secondary
I think this is correct!!! But don't quote me!! There is a rule of thumb in coupling capacitors that the period of the lowest freq should be equal or less than 1/5 of the RC time constant of the coupling capacitor and load. 4 mf times 4000 ohms = 16000 microseconds or .016sec. 1/5 of that would be .0032sec inverting that would be 312.5 Hz. There would be a gradual roll off below that and a rapid roll off below 62.5HZ (the freq equal to 1 RC time constant). The resonance with the inductor plays a part in this as well and makes the equations more complex. Resonance will cause an asymmetrical wave shape to a more symmetrical wave shape as the fundamental frequency of the wave nears the resonant frequency of the network. If my calculations are correct the resonate freq of 4 mf and 40 henrys would be about 12.5 Hz. This should be well below any frequency that might come out of Don's mouth. So I would say he is not in fear of having his asymmetrical audio smoothed out by resonance at the modulator output. On the other hand if he were to reduce the coupling capacitor to 1 mf, then the resonate frequency would be higher and some symmetrical stuff might be produced that he didn't say. (That's a way of saying it would not be a true reproduction) In some folks that would be an improvement in listening quality HIHI. John, WA5BXO -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Lyles Sent: Tuesday, August 16, 2005 2:22 PM To: amradio@mailman.qth.net 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 + From: Donald Chester [EMAIL PROTECTED] Subject: Re: [AMRadio] AMPX article on mod reactor To: amradio@mailman.qth.net 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:AMRadio@mailman.qth.net
RE: [AMRadio] coupling cap on the moduation transformer secondary
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. But to avoid excessive phase shift distortion, the uniform frequency response should extend at least one octave above and one octave below the intended range of frequencies of the modulation. Therefore if, for example, your target is to actually transmit audio ranging from 100~ to 5000~, the speech amplifier/modulator should be essentially flat from 50 to 10,000~. Even communications quality audio response of 300~ to 3000 would require at minimum a uniform response in the audio section of the transmitter of 150~ to 6000~. UTC ran this recommendation in their LS- series transformer catalogue, and I seem to recall seeing it in Thordarson's Tru-Fidelity catalogue also. Don k4kyv
Re: [AMRadio] coupling cap on the moduation transformer secondary
Donald Chester wrote: 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. But to avoid excessive phase shift distortion, the uniform frequency response should extend at least one octave above and one octave below the intended range of frequencies of the modulation. Therefore if, for example, your target is to actually transmit audio ranging from 100~ to 5000~, the speech amplifier/modulator should be essentially flat from 50 to 10,000~. Even communications quality audio response of 300~ to 3000 would require at minimum a uniform response in the audio section of the transmitter of 150~ to 6000~. UTC ran this recommendation in their LS- series transformer catalogue, and I seem to recall seeing it in Thordarson's Tru-Fidelity catalogue also. --- yeah, I'm thinking the same thing. So, if I don't have enough C in the output of the power supply, I could always add another, which would make them in parallel, which would ADD to the total capacitance in the power supply, and whatever coupling capacitance was used would effectivly be (as I understand it) in series with that power supply output capacitance. Which means if there's 10uF in the supply, and 10uF to be used as coupling, then a total of 5uF would be seen, as far as coupling is concerned and if BC stations are using 1 and 2uF and you're using 4uF, Don, then I should be in no danger of hurting anything here. ;-) Neat discussion - I like it. --- 73 = Best Regards, -Geoff/W5OMR
Re: [AMRadio] coupling cap on the moduation transformer secondary
John E. Coleman (ARS WA5BXO) wrote: I think this is correct!!! But don't quote me!! oops On the other hand if he were to reduce the coupling capacitor to 1 mf, then the resonate frequency would be higher and some symmetrical stuff might be produced that he didn't say. (That's a way of saying it would not be a true reproduction) In some folks that would be an improvement in listening quality HIHI. Hey, I resemble that! I've always been told I had a face for Radio. ;-) --- 73 = Best Regards, -Geoff/W5OMR
