The AM Forum

I have a modulation transformer labelled 5500 ohm primary/5500 ohm secondary. Obviously a 1:1 turns ratio and 1:1 impedance ratio. I assume that this transformer is just as good at other 1:1 impedance ratios, for example 7500 ohm/7500 ohm, etc. Is this correct? and are there limits to how far away from the design impedance one may go?

Or to ask it in another way, is a 5000 ohm 1:1 transformer designed differently 10000 ohm 1:1 transformer?

One other thing - thanks to this website, my knowledge base for homebrewing has increased exponentially. So a big thank you to everyone who has contributed. I was first licensed in 1975 at age 17. That first year was like magic. The magic has returned. Now if I could just get back on the air after this stupid move...............

Ron

Ron,

   You ask some good questions.

A lower impedance 1:1 modulation transformer will have less turns, and a higher impedance 1:1 modulation transformer will have more turns.

  So if you go much higher in impedance than the design center, you will suffer high frequency loss from all the winding capacitance, and if you go too low in impedance the low frequencies will saturate the core unless you roll them off at a lower level stage. Edit: I think I got this at least half right, but the more I think about it the more I get turned around.  ???

  With that 1:1 tranny you can effectively vary the turns ratio by staggering the modulator B+ from the RF AMP B+. So the tranny being 1:1 can be versatile. Keep in mind that modulation transformers typically don't have a lot of insulation between the primary and secondary windings.

  Also this thread pasted below needs a good re-read:

http://amfone.net/Amforum/index.php?topic=27244.msg207633;topicseen#msg207633

73,
Jim
WD5JKO

A good way to get a handle on the issue of what impedances will work is to look at the connections tables for multimatch modulation transformers.

Usually the same jumper and pin configureations are listed for a variety of impedances over a 2:1 range or sometimes more.

So based on that, the 5000 ohm transformer would range from 7500 ohms down to 3750 ohms or so.

Mostly what matters is the ratio.

If you run the transformer at a higher impedance than nominal, you will lose some low frequency response, since under those conditions, in both primary and secondary you will have less inductance per unit of load impedance (fewer Henries per kilohm). If you run it at lower impedance than nominal, depending on the construction of the transformer, you may have noticeably less high frequency response, since you will  have more leakage inductance per unit of load impedance. Leakage inductance acts like a choke in series with each winding.

For otherwise identical transformers with identical turns (and therefore impedance) ratios, one with a lower nominal impedance will have fewer turns (and heavier wire) in each coil than one with higher nominal impedance.

Taking this to extreme, compare an rf choke to a modulation reactor, both designed to operate at the same load impedance and power level within their intended frequency ranges.