Modulation auto-transformers

[KE6DF]

I have a Wilcox modulation transformer out of a Wilcox 99 aviation transmitter.

It's an autotransformer.

It occurs to me that one could connect any normal modulation transformer as an auto-transformer.

For example, I also have a CVM-4. If you connect one side of the secondary to one of the plate connections on the primary (modulator tube side), then you have the same configuration as the Wilcox transformer.

This approach would put 1/2 the primary winding in series with the secondary and give you more inductance in the final plate circuit.

It would also change the ratio and would give some more options to a modulation transformer with a limited number of taps.

What are the tradeoffs -- advantages and disadvantages of this approach?

Has anyone tried it?

Dave

[The Slab Bacon]

Dave,
        I also have one of those, it is a pretty strapping unit rated 350w. the diagram on the bottom sez it is a transformer, but others tell me that it is a tapped autotransformer. I have no way of telling without destroying it.

Everyone seems to think it is an autotransformer. The only disadvantage that I can tell is that it is designed to work with a common power supply for the PA and modulator.

I also have a little cube (about 3" x 3" x 3") out of some kind of aeronautical transmitter that is definately aan autotransformer. It only has 3 terminals and uses an 829B to mod whatever the final was. (I dont have the whole unit, just the mod deck) It uses the 829B in shove-yank with one of the 829B plates and the lead out to the final tied to one leg.

An autotransformer will work just fine for a mod transformer. 5-watt piss- weaker uses a shove-yank 6V6 outpoot transformer with the B+ tied to the center tap, one end to the plate of a  single 6V6 for the modder and the other end lead feeding the 2E26 final. It works great. (It was all that I Had at the time I bult it) Makes 100% with Hi-Fi audio.

[kb3ouk]

That's the same transformer Surplus Sales of NE has listed on their website, I was looking at one as possible mod iron for a project here. description said it was an autotransformer, but the picture makes it look like it is a regular transformer, 3 terminals for the primary and two for the secondary. if it were built as an autotranformer, wouldn't they have made the secondary to primary connection internally? there's an article on the AM Window site where Timtron describes how to hook a TV power transformer as an autotransformer, then either modify it to use like normal mod iron, or hook up in a modified Heising setup.

[W1RKW]

Dave,
My CVM4 in the 813 rig is wired as an auto-former in a Heising arrangement. It works and works well.

[KE6DF]

That's the same transformer Surplus Sales of NE has listed on their website, I was looking at one as possible mod iron for a project here. description said it was an autotransformer, but the picture makes it look like it is a regular transformer, 3 terminals for the primary and two for the secondary. if it were built as an autotranformer, wouldn't they have made the secondary to primary connection internally?

The Wilcox really is an auto-transformer -- and it's connected just like I said in the post above. The reason there are two terminals on the final side is because it has taps for 5.75K and 6.6K plate impedance on the final.

Checking with a DVM shows all the windings are in series and are connected internally.

[KA8WTK]

Hey Dave,
  Can you post a picture of that? Maybe with which "secondary" tap is which imoedance? I think I have one of those.

Thanks,
         Bill

[KE6DF]

OK, here it is:

In the top view, the terminals 1,2, and 3 are numbered starting with 1 in the bottom right, and 4 and 5 in the row at the top of the picture going from right to left.

B+ goes on terminal 2, terminals 1 and 3 are the plates of the modulator tubes.

Terminal 4 is the 5.75K final plate tap and terminal 5 is the 6.6K tap.

If it's like mine, the terminal numbers are stamped next to the terminals.

It's just like the one in the Surplus Sales picture. The sizes are given in the picture and are the same as mine.

http://www.surplussales.com/Transformers/modulation.html

A pair of triode connected 813s moding a 250TL. I think it would work fine with a pair of 813s x 813s also.

[KA8WTK]

I'll be darned! I think I realy have one of those. I did not know it was an auto-transformer, but thought that it had a short inside between the windings causing what I saw at the connections.
Right now it is buried in the garage. I am going to dig it out tomorrow and confirm.

THANKS!

[kb3ouk]

the bad thing about using that transformer then would be you would have to use a common power supply. but, could you use seperate supplys if you conect it through a cap and choke like in modified heising?

[KE6DF]

the bad thing about using that transformer then would be you would have to use a common power supply. but, could you use seperate supplys if you conect it through a cap and choke like in modified heising?

Yes, that would work if you wanted to run the final on a lower voltage than the modulator -- to get over 100% modulation.

Dave

[k7yoo]

I ran a 99 on the air for a couple of years. That trans works very well with a pair of triode connected  813's modulating a 250TL--looked good on the scope.
Skip

[kb3ouk]

ok, that's what i wanted to know.

[K3YA]

Back in the Dark Ages there were a couple AMers using plate transformers as modulation auto-transformers.  Each side of the secondary went to an modulator tube plate.  B+ went the the secondary CT. And the unused primary was connected in series with one side of the secondary to provide higher audio voltage to feed the final.  The final amp B+ current had to be kept off the secondary with a choke and capacitor.

If you connect the secondary in series with the primary to increase the inductance, you will also reduce the current that the transformer will handle before the core saturates from the DC going through the windings to the final.  That may be acceptable in some cases, but you need to take this into account.

[kb3ouk]

but by using a cap and choke, you can keep the current off the secondary, and won't saturate the core. i'd say if you are gonna be hooking transformers up as autotransformers, use a cap and choke for modified heising, no matter whether you are using a old power transformer, or a real modulation transformer.

[The Slab Bacon]

That is exactly the one that I have!! A pretty strapping unit.

IIRC there is a diagram on the bottom (top) of it that shows it like a transformer with one end of the secondary and the primary center tap tied together. If memory serves me. However it is buried on the iron rack and I haven't looked at it in a few years.

[k4kyv]

With a modulation autotransformer you can use separate plate supplies for modulator and final by using a blocking capacitor and modulation reactor.

I have a large autotransformer taken from a Wilcox unit with a pair of 450TLs in the modulator.  I once used it as a modulation reactor with another mod transformer.  I used  the two taps that normally serve as secondary.  The entire winding had more inductance, but didn't take much DC to saturate it.  When pulling PA plate current through it, I actually had more inductance just using the secondary taps than I did using the whole winding.