[AMRadio] Mod Transformer

D. Chester k4kyv at charter.net
Tue Jan 27 00:07:17 EST 2009

> From: "John Coleman" <jc at pctechref.com>

> The output voltage of a modulator is determined by its plate supply 
> voltage
> and the modulation XFMR turns ratio.

> 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.

That is true irrespective of the nominal impedance of the transformer, the 
p-p load impedance of the modulator tubes or the modulating impedance of the 
final.  Of course, the tubes have an optimum p-p load impedance at any given 
plate voltage, and a transformer has an optimum primary and secondary 
impedance at which it works best.  But the actual transformation is based on 
turns ratio.  The impedance ratio stamped on the nameplate of the 
transformer is a nominal value, and a good transformer should be able to 
work at up to twice the nominal value and as low as half the nominal value 
with little degradation in performance.  The impedance transformation ratio 
is the square of the turns ratio.  For example, a transformer with a 2:1 
turns ratio has a 4:1 impedance ratio.

> 100% modulation occurs when the audio voltage from the modulation XFMR is 
> 2
> X the plate supply.

Actually it's when the peak output voltage, the combined audio and DC 
voltage from the winding adds up to 2 X the plate supply voltage.  The peak 
a.c. output voltage from the transformer is equal to 1 X the DC plate supply 
voltage.  When it is in the same polarity as the DC voltage, the two 
voltages add together to produce a sum that is 2 X the plate voltage at the 
positive modulation peak.  At the opposite peak of the audio cycle the 
polarity is reversed and the two voltages cancel, leaving zero volts on the 
plate of the final.  This is the negative modulation peak.

> 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.

That is theoretical.  In actual practice, there is nothing you can do to 
pull the instantaneous plate voltage below about 20% of the power supply 
voltage.  In the case of screen grid modulators, the plate voltage can never 
be pulled to a lower voltage than the DC screen voltage.  As one tube 
reaches maximum conduction, the instantaneous voltage on the plate of the 
other tube will reach about 1.8 X the DC plate supply voltage.

> 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).

Using a higher step-down ratio of 1.6:1 will just barely allow you to reach 
close to 100% modulation with no headroom whatever, but the modulator tubes 
will run more efficiently.  Somewhere between 1.2 and 1.4 will allow more 
headroom at the expense of efficiency.  But that extra headroom is needed 
for minimum distortion and splatter, since driving a modulator or linear 
amplifier (exactly the same thing except the modulator amplifies audio while 
the linear amplifies rf) right to the saturation point results in more 
distortion.  But watch the modulator plate current and make sure you don't 
exceed the tube ratings.  If so, double up to use a pushpull parallel 
modulator.  However, this may increase the audio driver requirements.

>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.

Taking the DC off the secondary greatly reduced the talk-back when I was 
using a UTC VM-5 modulation transformer.  With the DC going through the 
secondary, the thing sounded like a small speaker inside the transmitter 
cabinet.  With the reactor, it was totally quiet.  This will also reduce the 
distortion from the modulator, since it will reduce the magnetic saturation 
of the core over the audio cycle.

A modulation transformer designed to run DC through the secondary will 
usually have a gap in the core, filled with paper or some composition 
material.  A transformer designed to be used with a reactor will usually 
have a core stacked like a power transformer, with no gap.  But this is not 
strictly true.  I have seen transformers designed for DC with an extra large 
core and no gap, while I have seen ones designed for use with a reactor that 
had a very narrow gap, not much more than .001 inch.

Don k4kyv

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