Transformer question wrt A Quick and Not So Dirty Path...

[KX5JT]

http://www.amwindow.org/tech/htm/ssmod.htm

I've been looking at Steve's document.  I'm thinking about building this "add on" hi-fi modulation scheme to the Viking II.  I like the idea of minimal invasion on the rig to do this add-on... I can see how it could easily be switched in and out.

I think an audio transformer that can handle 60 watts should be sufficient. They aren't exactly cheap.  

Here is a 100 watt 70v audio xformer
http://cgi.ebay.com/Altec-EV-15100-Audio-100-Watt-70-Volt-8-ohm-Transformer-/220492062559?_trksid=p5197.m263&_trkparms=algo%3DSIC%252BSI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%252BDDSIC%252BRTU%26otn%3D10%26pmod%3D350262673011%252B260799604144%252B270760509979%26po%3DLVI%26ps%3D63%26clkid%3D589597748541111442

8 ohm to 70v gives an Impedance ratio of  1:6.7
8 ohm to 100v gives an Impedance ratio of 1: 13.6

Or maybe I can bridge a stereo amp for 4 ohms

for 70v Z ratio of 1:13.7
     100v             1:27.4


Is that enough step up to qualify as "Hi-Z" ??  

I still wrestle with impedance concepts.

[W2PFY]

Those speaker matching transformers will not have enough or a turns ratio to give you the voltages you need. You can use toroidal power transformers to do the job and they are not all that expensive. Look up AB2EZ who has written extensively on this BB about the subject. If I were going to do anything like you are planing, it would be the way I would do it.

I have already done the reversed output transformer  configuration with fine results. Those transformer may be found in old guitar amps or organ amps.

You should be able to get almost 3X the ratings from a transformer when used in a hysing configuration. I am able to modulate a Apache well over 100 percent with a 60 watt audio transformer.

[W8ACR]

Does modified Heising configuration really allow one to run a mod transformer at 3X power ratings? I can understand that there is less stress on the secondary of the mod transformer without the B+ current. So yeah, maybe the secondary can do it.

But the primary is still running in standard configuration, and it seems unlikely to me that it would stand up to 3X  power ratings for very long unless it was rated very conservatively. Perhaps with some luck, and the fact that hams do short bursts of transmitting rather than continuous, the transformer will usually hold up. Or perhaps most likely, I'm missing something

I'm running a UTC S-22 (rated 250W) in modified heising with a 50H Dahl reactor, I'm not gonna ask it for more than 200W of modulating audio. Considering the price of big mod iron these days, I'm gonna play it safe. But that's just me.

Ron

[W2PFY]

But the primary is still running in standard configuration, and it seems unlikely to me that it would stand up to 3X  power ratings for very long unless it was rated very conservatively. Perhaps with some luck, and the fact that hams do short bursts of transmitting rather than continuous, the transformer will usually hold up. Or perhaps most likely, I'm missing something Huh

Yep, Been doing it for years. A 250 watt transformer will modulate a KW. This is intermittent service and we're only talking voice.

I'm running a UTC S-22 (rated 250W) in modified heising with a 50H Dahl reactor, I'm not gonna ask it for more than 200W of modulating audio. Considering the price of big mod iron these days, I'm gonna play it safe. But that's just me.

That is your choice but it will probably work just fine.  Each operator will have their own opinion about this but I have no problem with it no matter how old the transformer is. Maybe I feel this way because I have stocked up on mod transformers but at the same time, I would be very unhappy to loose one.

[KE6DF]

I don't think a 70 v line to voice coil transformer would work well for this.

The reason is you wouldn't get enough voltage swing to modulate a 700 v final.

You would need a lot higher step up ratio.

A p-p plate to voice coil output transformer is more what you would need.

Perhaps a transformer with a plate-plate impedance spec of 6K ohms to voice coil and a 75W or so power capability.

This would be an output transformer for a P-P pair of 6L6, 6550, or 807 tubes to speaker voice coil, for example.

Dave

[W2PFY]

I don't think a 70 v line to voice coil transformer would work well for this.

The reason is you wouldn't get enough voltage swing to modulate a 700 v final.

You would need a lot higher step up ratio.

I already said that but thanks for the back up

[KE6DF]

I already said that but thanks for the back up

Well that makes me feel like I'm in good company. :-)

BTW, is there a way to delete a post? I can see how to modify one, but I don't see how to delete it.

Dave

[W2PFY]

BTW, is there a way to delete a post? I can see how to modify one, but I don't see how to delete it.

Well unless it's really embarrassing, forget about it. But there are two ways to delete your post, contact any moderator and they may delete it for you or just modify your post by deleting all that you typed and leave a (.) or a ~.~ or

<.> or just a .

Personally I don't understand why members cannot delete their own post? It must be in case someone calls you an assh() ()e. If someone complains about such a posting, the moderators or owners of the board can see what the fuss was all about. Then make a judgement weather to come to your home and kill you or something

[KX5JT]

Awesome.  Thanks everyone.  Torroid hmm.... I might look into that.

[w3jn]

Another thing to consider is those 70V line transformers do NOT like DC current flowing thru 'em.

[Opcom]

One reason they may do the canonical 3X the power is because as the lowest frequency goes up, so can the volts per turn, to a vague limit. A 250W audio transformer rated for 44V RMS to an 8 Ohm winding at 40Hz may handle 132V RMS on the same winding with the same current (750W) at 120Hz or higher. The rough rule for that is increasing the voltage on the winding but keeping the same current. 3X is a lot of voltage for the secondary. This is almost the same scheme as the 60Hz transformer vs the 400Hz one, and being able to get away with using a 400Hz variac for public address matching if it is oversized a bit.

If the current to voltage ratio is not changed but the power is increased to 3x, 1V becomes only 1.73V and 1A becomes only 1.73A, so when will it saturate and at what frequency? The I2R loss in the windings is probably tolerable (and always unavoidable) in these experiments so it is the core that has to be considered if current is increased.

Without knowing the load the RF stage presents, it's not possible to decide which taps to use. That brings in the mentioned duty cycle issue and also the possibility of saturation during the half-cycle. No guarantees not to destroy the transformer.

The toroid seems like  a better way since they are available and not too costly or precious compared to tube audio transformers.

[KX5JT]

Yeah the torroid is looking more and more attractive to me.. what's that company that makes them... Antek?  I need to google them I guess.

[W1FRM]

Be aware that if you go the toroidal transformer route, it is mandatory that you eliminate the DC component from the secondary (via heising choke).

Guy - W1FRM