Thanks John, good info - Paul
At 10:07 AM 1/26/2009, you wrote: >Paul: > Modulation XFMRs have a lot of different ratings. > >1. Insulation breakdown voltage ratings. > >2. Audio wattage (can be misleading as this rating is often at 1000hz and it >may not hold true for 100hz). > >3. Max secondary current if any. Some XFMRs assume that you will be using a >modulation reactor as well. > >4. Turns ratio of course. (A confusing issue to a lot of people). > >The output voltage of a modulator is determined by it's plate supply voltage >and the modulation XFMR turns ratio. > >100% modulation occurs when the audio voltage from the modulation XFMR is 2 >X the plate supply. > >You need to think in terms of voltage transformation. If you are using a >common power supply on the final and modulators, or more exact, the >modulator plate supply and the final plate supply have the same voltage, >then the ratio that you use is what determines the maximum modulation. > >At maximum drive the modulator tubes conduction (assuming they or big >enough) takes the plate voltage close to 0 Volts at the peak of the audio >for that conduction cycle. Nothing you can do will take the voltage lower >than zero. As one tube hits the Zero volt peak then the other tube will hit >the 2 X plate voltage point. > >The peak to peak plate voltage max is 2 X the plate supply on one modulator >tube. If plate supply is about 600 volts then the modulation max is 1200V >PTP. In push-pull circuits the output voltage would be 2 this or 2400 volts >Peak to Peak. Since the power supply voltage is the same on the final this >would be twice the amount of audio voltage required to modulate it. This is >too much of an over kill on a 1:1 XFMR. > >Taking a step down, and using a 2:1 turns ratio XFMR and the audio drops to >1200 Volts. This is the exact amount of voltage required with no head room >for error or loss in the XFMR. You will not be able to actually hit 100% >and distortion will be fairly high. > >Something between 2:1 and 1:1 is what is needed. You need a little extra to >make up for the fact that the modulators will use some power in plate >dissipation and you will want a little head room for voice lopsidedness >(everything is not a perfect sine wave). > >Experience information from Don, K4KYV, indicates that between 1.2:1 and >1:4:1 is generally a good choice. 1.2:1 will give you more head room but >will require more modulator current perhaps larger tubes. 1:4:1 will >probably just be enough audio with very little head room, but will require >less modulator current and lighter demand on the modulator tubes. If you >chose 1.2:1 for plenty of head room then choose modulators with a little >more current capability or double up (push pull parallel). You may want to >consider a modulation reactor even if your XFMR says it can handle the >secondary current. Keeping the current out of the secondary will greatly >improve the low frequency capability of the XFMR. You want regret it. > >This may be more info than you needed. >Good Luck >John Coleman, WA5BXO > > > > > > >-----Original Message----- >From: [email protected] >[mailto:[email protected]] On Behalf Of Paul Baldock >Sent: Saturday, January 24, 2009 6:36 PM >To: [email protected] >Subject: [AMRadio] Mod Transformer > >Does anybody have a good reference as to how to calculate modulation >transformer requirements? > >Thanks > >- Paul > >______________________________________________________________ >Our Main Website: http://www.amfone.net >AMRadio mailing list >Searchable Archives: http://www.mail-archive.com/[email protected]/ >List Rules (must read!): http://w5ami.net/amradiofaq.html >List Home: http://mailman.qth.net/mailman/listinfo/amradio >Help: http://mailman.qth.net/mmfaq.html >Post: mailto:[email protected] >To unsubscribe, send an email to [email protected] with >the word unsubscribe in the message body. > >______________________________________________________________ >Our Main Website: http://www.amfone.net >AMRadio mailing list >Searchable Archives: http://www.mail-archive.com/[email protected]/ >List Rules (must read!): http://w5ami.net/amradiofaq.html >List Home: http://mailman.qth.net/mailman/listinfo/amradio >Help: http://mailman.qth.net/mmfaq.html >Post: mailto:[email protected] >To unsubscribe, send an email to [email protected] with >the word unsubscribe in the message body. ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body.
