[AMRadio] Mod transformer turns ratio

D. Chester k4kyv at charter.net
Sat Nov 17 16:18:48 EST 2007

Carl KM1H wrote:

"Looking at a modulation xfmr that is marked 4596VCT RMS Primary and 3747V
RMS secondary. This appears to be a 0.82:1 voltage step down or a .64
impedance step down if I did the math right. Question is what sort of wiggle
room is there in both impedance ratios and actual voltages used?

The power rating is way more than sufficient for my needs and it is rated
down to 50 Hz. I can use modulator tubes to supply sufficient power to
overcome a reasonable mismatch but what is reasonable and how does the
mismatch affect distortion? I havent been able to find any formulas or

That would be more normally expressed as 1.23:1 turns ratio, or almost 
exactly 1.5:1 impedance stepdown.

Does it give a power rating or current rating?  Or load impedance rating? 
Without that information it is difficult to determine what the optimum 
impedances are, but that usually isn't critical.  That's the way the popular 
multi-match modulation transformers like the UTC VM series works.  With  a 
given turns ratio, a wide variety of impedances can be used.  The only thing 
that cannot be changed is the impedance ratio.  So you might use a pair of 
modulator tubes working into a 15000 ohm plate-to-plate load to modulate a 
final amplifier at 10000 ohm modulating impedance.  Or your modulator tubes 
may operate into a p-p load of 9000 ohms, so the modulating impedance would 
be 6000 ohms.  A 4000 ohm modulating impedance would yield a 6000 ohm p-p 
That is a good ratio for using a common power supply for modulator and 
final, for achieving good positive peak capability with plenty of  headroom 
beyond 100% in the positive direction.

How big is the transformer and how much does it weigh?  That might give a 
clue to its power rating if that data isn't given.  Given the voltage 
ratings you listed, if the power rating or current rating is known, the 
optimum modulating impedance can be calculated using Ohm's law, and thus the 
power rating if that isn't given. It sounds like broadcast iron if it's 
rated down to 50 Hz frequency response.

That brings up another issue.  Is it desined to carry the DC to the final 
through the secondary?  If  not, you will need a modulation reactor and 
blocking capacitor to go with it.  A good way to find out, if it open frame 
where you can see the core laminations, is to see if there is a gap in the 
core.  If the laminations are stacked in such a way that there is a gap, 
filled with paper or some other kind of insulation, then it is designed to 
carry the DC.  If it is cross-laminated like a power transformer, with 
adjacent sets of laminations reversed so that there is no gap in the iron 
core, then most likely it is not designed to carry the DC.  Most broadcast 
modulation transformers are NOT designed to carry the DC to the final.

Don k4kyv

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