[AMRadio] Mod Transformer


Paul Baldock pbaldock at verizon.net
Mon Jan 26 13:37:37 EST 2009


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: amradio-bounces at mailman.qth.net
>[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Paul Baldock
>Sent: Saturday, January 24, 2009 6:36 PM
>To: amradio at mailman.qth.net
>Subject: [AMRadio] Mod Transformer
>
>Does anybody have a good reference as to how to calculate modulation
>transformer requirements?
>
>Thanks
>
>- Paul
>
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