Info modulation transformer

[IW3FZQ]

Hello all! Where I can find a suitable transformer of modulation for a transistors transmitter for long and/or medium wave frequencies? After have built some transmitters with the classic system (audio that sets the collector current) now I want to try (in my opinion) the true AM mode: class C RF stage with a suitable audio trafo between the main voltage and the collector of the final stage. I tried many years ago with unknow spare transformers with bad results...

Thanks in advance!

Enrico.

[Opcom]

At first look, not knowing the power level, a simple thing to try might be one of those 70V line transformers because they have many taps for different power levels, corresponding to a variety of turns ratios. It could be used as an autotransformer.

Beware passing DC through the windings of an audio transformer that isn't designed for it, because of core saturation from the DC of your class-C stage.  A modified heising arrangement could be done and let the DC therefore pass through an inductor, such as a 100mH to 1H one rated the proper current. Inductance depends on the voltage and the class-C current. The coupling cap could be an electrolytic or even a plastic type.

Someone here knows the formula for this, the formula for specifying the modulation choke.

An alternative for higher powered stages is a power transformer. There are many with a 120V primary and a 40-120V CT secondary, rated several amps, meaning 100-500W or more. DC should not be passed through these either. (low current from a tube like 250mA may be OK in a 300W transformer as often used for cathode modulation, but 1A or 5A to modulate a transistor's collector will probably make saturation trouble)

A transformer either audio or power will do better if the E-I style laminations are reassembled so there can be a gap. This will reduce saturation if DC is passed. It is always seen on tube type modulation transformers. Toroids are great for audio but are more easily saturated by DC and usually not easy to gap because it's core is a solid strip wound into a ring or some other inconvenient construction.

Please mention the DC input in watts to the class-C stage in carrier conditions, its current, and its operating voltage as well.
If you can provide the requested info, we can help better.

[IW3FZQ]

Hello! Many thanks for your precious informations... I can try to build "at home" a suitable transformer, classic or with toroid, knowing the correct formulas, however, the transmitters have a single stage as final (not push-pull or parallel devices) and usually I use an IRF510 or 2SC1969 for a 5-10 Watts/carrier, no more. I seen some schematic diagrams of home made transmitters that use "Speco" trafo but not available in our local sellers. It will be good thing to use a similar device but with just impendance of primary-secondary values. The inductor you mean, I think, it is used in tubes LPB transmitters...

Enrico

[Opcom]

Speco, that is a popular brand of line-to-speaker unit. There is nothing secret about the Speco so I am sure you have equivalent units available.

The inductor is used (for tube units) to avoid saturation of the modulation transformer core. Even though you use a transistor, saturation could still happen with a small transformer. That is why I mentioned a gap or an inductor. But maybe it is not necessary, as you say others have done the same.