Reading the mod transformer chart

[ka1tdq]

I purchased a UTC S-20 modulation transformer from eBay.  I'm thinking about building an all tube transmitter using a pair of 6L6's to modulate a 7984.  But, I'm having difficult understanding how to read the mod transformer chart on amwindow.org.

In class AB2, a pair of 6L6's is 6600 ohms and my planned plate Z for the 7984 is 2500 ohms. 

If I'm reading the chart correctly, do I connect the B+ for the 6L6's to 9 & 10 with the plates going to 8 and 11?

For the 7984 side, I guess I connect 1 to 3, 4 to 6, and use 1 & 4 for the B+ and 7984 plate?

Jon

[WD5JKO]

Jon,

   I thought you were done with tubes! 

Take a look at this thread:

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


The S-22 is a beast good for 250 watts audio.  Do you want to run it at 1/10 rated power?

Instead of matching impedances, try to use all the windings in series for both primary and secondary, and see what the turns ratio is. Select a turns ratio to deliver a maximum modulation percentage, such as 140%, and go from there. Then if the plate to plate load is too low for the tubes in the modulator, then look at bigger tubes or 4 in push pull parallel.

Jim
Wd5JKO

[w8khk]

//snip

The S-22 is a beast good for 250 watts audio.  Do you want to run it at 1/10 rated power?

 //snip

Jim
Wd5JKO

Jim, I think the transformer Jon is using is the S-20, rated at 55 watts, not the S-22 rated at 250 watts.  I used intermediate, the S-21 (110 watt rating) when re-modeling the modulator in my Valiant to use four 1625s in push-pull parallel.

I recall having to do a bit more study on this, and also on the 500 watt Thordarson CHT mod transformer  with my PP 810s modulating a pair of 250THs.  The charts give a clear picture of the series and parallel combinations of windings to get the proper impedance match, and I agree in most cases it is better to configure the windings in series provide more inductance hence better low-end response.   But I also learned that in some cases the windings must be in parallel in order to handle the DC current, especially in the secondary at a lower impedance.  It is sometimes necessary to juggle turns ratio, impedance, as well as DC resistance to handle the required load current.   I do not have all the spec sheets for the UTC transformer at hand, but it might be a good plan to determine what the current handling capability of the windings, both in series and parallel, in order to get the most life out of the iron.  Hope this helps a bit.  GL on the project.

[KA2DZT]

Most multimatch mod xfmrs have a slight step up or down when using the entire windings on both sides.  So, first figure which side is the higher overall turns and use this for the mod tubes.  Use the entire windings for the mod tubes.  Put the secondary windings in series also.  Probably, putting the B+ on the first tap from the B+ end and top end of the windings to the final.  This would be a good starting point.

With the impedances you stated,  the turns ratio step-down is about 1.6 to 1

[W7TFO]

Jon, the impedances in a multimatch job are at best an approximation. 

Don't let it confuse you, just try it like Fred says, then juggle a tap or two for the best results. 

You won't hurt anything with the power levels you'll be running.

Don't expect great fidelity with any "S" series iron, as they were the cheapest thing UTC built with just enough iron & copper to do a voice-grade job.

Dennis

[ka1tdq]

Yeah, now that I know the secret of class E, I'm not going to use anything tube in the shack for a mainstay transmitter.  I want to build this one as a novelty that I can impress (or bore) people with 30 years from now.  All the tubes will be exposed on the top side of the chassis since the 7984 has the plate on the bottom pin. 

I plan to go with higher peak audio power by using about 275 vdc on the final and about 350 vdc on the modulator tubes, then run heising after the mod transformer. 

However, my wife is telling me to stop buying so many parts, so I may have to delay building this one.  BUT, I have a pretty good start on the parts already. 

Jon

[W9ZSL]

Here is the UTC hook-up chart for the "S"series.  For the primary, the two "B" pins  (the two middle numbers) are connected together and receive the B+. The two "P" pins (outside numbers) feed the plates; one each.