[AMRadio] Mod Transformer

John Coleman jc at pctechref.com
Mon Jan 26 13:07:49 EST 2009

	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?


- Paul

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