Mod transformer question

[Bill, KD0HG]

Modulation transformers..Designed for plate modulation use *without* a mod reactor for DC current..Are they designed with a core gap to prevent core saturation from the DC current on the secondary..Or are they designed to function properly with a large DC current on the secondary without a gapped core?

Tnx, OM

Bill

[k4kyv]

Usually the ones designed to carry the DC are designed with a gap in the core.  But I have seen both filter chokes and mod transformers designed to carry the DC, that had no gap. They did have a lot of iron in the core.

I once used a large one with no gap, designed to pass the DC.  I used it with a reactor and got very good low frequency response, even though it was a "communications grade" transformer designed to go into a ship-to-shore radiotelephone transmitter. It was about a cubic foot in size and weighed over 150 lbs, and rated for 1000 watts of audio.

[The Slab Bacon]

Every transformer that I have ever seen that was designed to work with a DC bias on one of the windings (mod, interstage, audio outpoot, etc) has had a gapped core.

Don,
       If you have seen them without a gap, you are one up on me!

A few years back, I wound a choke for a small Heising modulated transmitter. I did a little experimenting while winding it and found some rather interesting results while playing with the core gap. As you close the gap up the amount of inductance increases, but it saturates much more easily. (at a lower current)
As you open the gap up, the amount of inductance decreases, but the amount of current for saturation increases dramatically.

With some of the newer steel alloys this could cause some really interesting results. (IE, hypersil iron, etc) allowing for much smaller transformers.

[WA1GFZ]

So a transformer designed to handle DC current has higher zero DC current inductance. It needs to handle the DC offset and low frequency response

[k4kyv]

The ungapped transformer had about 5 sets of laminations together facing one direction, then the same in the other direction.  Can't see how that would have made much difference from alternating ever other one.  Made by Thordarson.  I got it brand new, and still have the blueprints here somewhere.  The chokes, also by Thordarson I believe, came from an old "composite" AM broadcast transmitter.  Something like 5 Hy @ 1 amp each.  The ungapped core surprised me too. I still have the chokes.

[WA1GFZ]

an ungapped core just uses less turns of wire to get inductance. As long as the flux densidy doesn't drive it to saturation you are fine.

[w1vtp]

Usually the ones designed to carry the DC are designed with a gap in the core.  But I have seen both filter chokes and mod transformers designed to carry the DC, that had no gap. They did have a lot of iron in the core.

I once used a large one with no gap, designed to pass the DC.  I used it with a reactor and got very good low frequency response, even though it was a "communications grade" transformer designed to go into a ship-to-shore radiotelephone transmitter. It was about a cubic foot in size and weighed over 150 lbs, and rated for 1000 watts of audio.

I think that's what I have except it's for 500 watts

[WA1GFZ]

It is pretty easy to test a mod transformer for inductance with DC offset.
You just need a LV DC supply rated for the mod operating current. A cap a resistor equal to operating Z, a scope and signal generator.
Measure the reactance of the winding with dc current flowing through it.

[KM1H]

You lost me at "a cap"

[Carl WA1KPD]

OK Trying to make this simple, I have a question. You need 5 watts of audio to modulate 10 watts of carrier. So if I buy a mod xfrmr for my rig that can run 10 watts  AM do I buy one rated for 10 watts, or 5 watts?
Thanks
Carl
/KPD 

[W7TFO]

Buy by the lower number.  They are rated at what they will carry audio-wise, not the RF output.

73DG

[wd4icj]

So, if I want to make a CW xmtr do plate AM, all I have to do is insert a mod. xfrmr that's designed to handle the large DC voltage, and I don't have to use a blocking cap, or reactor?  Sounds too easy! 

[W7TFO]

When it comes to HI-FI AM, one must use the cap & coil to get the frequency response out to say 50Hz to 15kHz. 

Individual modulator plate current balancing,  beefy drivers or cathode followers for class "B", and global audio feedback is usually the other half of that "HI-FI" AM. 

Along with a brute force power supply running at least half again more than you might even think of needing.  (Broadcast is nothing but "old buzzard" length transmission, 24/7).

For hambone voice grade, a straight mod trans will do nicely. 

Biasing and tube operating class characteristics, loading, and RF drive will change going from CW to AM as well in any case.

73DG

[KM1H]

Id always go for the larger mod transformer as just about all hammy versions are very skimpy on the steel which limits both the low end as well as peak power. Many factory rigs will not make 100% without saturating and distorting.

The 1/2 power rule is BS also as thats purely theoretical and doesnt consider losses. If you have say a 100W carrier shoot for 75W of audio and then build a good driver section. Use the gain control for what the scope tells you is needed, not some hammy at the other end on his second 6 pack

Carl

[The Slab Bacon]

The "official" rule of thumb has always been that you need audio power equal to 50% of the DC inpoot to make 100% modulation.

My (and many others) rule of thumb has always been to have audio capability equal to 100% of the RF outpoot. This way you dont have to beat the BAs off of the modulator to make your 100%. Therby having less distortion and some headroom too boot.

This is also a situation where overkill is your friend. Both for the modder and the mod transfoma. What is important is to get a transfoma with a usable turns ratio for your application. Also with an overdesigned modder, the turns ratio of the tranny is a little less critical as it doesnt need quite as much leverage to swing the final current.