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16th January 2005, 12:11 AM  #11 
diyAudio Moderator

That's exactly right. Go poke around sites like Sowter's or Jensen's to see what the selection and purchasing process is like. And Jensen's white papers are quite educational, too.
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16th January 2005, 01:50 AM  #12  
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Join Date: Jun 2003
Location: Dallas,TX

Quote:
John 

16th January 2005, 02:09 AM  #13 
diyAudio Member

Yes, like rdf, I'm still confused. Furthermore upon reflection on the concept of secondary reflection, since an IT's secondary is conected to the grid of an output tube, doesn't it in theory has infinite impedance to dc, since it's not connected to anything? This is not so for a.c., but how is this impedance calculated?
Regards, David 
16th January 2005, 02:43 AM  #14  
diyAudio Moderator

Quote:
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16th January 2005, 02:45 AM  #15 
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It might be helpful if I draw out some equivalent circuits for midband, low freq and high freq. Give me a little while to scan, convert, and take care of putting my kid to bed and I'll try to make the mysteries clearer.
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16th January 2005, 05:26 AM  #16 
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Here's a quick sketch of some equivalent circuits, very simplified (if you really want to understand this more rigorously and in more depth, the Radiotron Designer's Handbook is the place to go).
The top sketch shows the midband model, idealized. The source is a voltage source in series with its output impedance, Vin and Rs. The load is assumed to be a pure resistance, too, R_{L}. The impedances (in this case, pure resistances) are transformed ny the square of the turns ratio. The bottom sketch introduces the first level of nonideality. The source and load impedances are still assumed to be pure resistances, but there are a couple of new twists. The first new twist is the finite inductance of the primary. In a practical transformer, the inductive impedance at midband and high frequencies is very large compared to N^{2}R_{L} so can be neglected. As we go down in frequency, the source starts to be loaded by the inductive impedance, which shunts more and more of the source current as the frequency goes down. By the time you reach DC, the voltage across the primary (idealized again!) is zero the transformer doesn't transform DC. The other new twist is the leakage inductance, represented by L_{leakage} in series with the load. At midband and low frequencies, its inductive impedance is negligible. As we go higher in frequency, we start to see its effect; its impedance gets larger with frequency and, acting as a voltage divider in series with the load, it starts dropping more and more voltage. At the extremes of high frequency, effectively all the output voltage is across it, so the load's share drops to zero. Now, to complicate things more, there are various capacitances that need to be added to that model to show different forms of peaking and rolloffs (as shown in the simplified model, the HF rolloff is first order, whereas in a real transformer, it's third order). The goal of the transformer designer is, knowing the nature of the source and load impedances, balance off the winding geometry, core, number of turns, and wire resistances to get a flat response over the desired range. Deviate from the recommended source and load impedances and you'll see a nonflat response compared to running the transformer with optimized loads. Does this make it any clearer?
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16th January 2005, 02:17 PM  #17  
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Quote:
For example, a 5687 driving a 2A3: Rp = 2000 ohms X = 6000 ohms at 50 hz L = X / (2pi)(50) = 19 H This is really a minimum requirement as setting X at 1020 hz will give more headroom and cost more or you can save money by using tubes with lower plate resistance such as 6C45pi. John 

16th January 2005, 04:24 PM  #18 
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Join Date: Jun 2004
Location: big smoke

Going back to the Hammond 804's, measurements show driving transformers with too low an impedance can also cause problems. It's been a long while but I recall that too much source impedance rolled of the high end too quickly and too little leads to peakiness. That's what's bothersome about ambiguous turns ratio specs. Companies like Hammond, and Sy's example Jensen, are very careful to specify precise, optimum conditions for their product such as I/O impedance and operating levels for best performance. It wouldn't surprise me if the dislike for interstage transformers I sometimes read about on the forum comes from simple misapplication of a product based on lack of information.

16th January 2005, 04:39 PM  #19 
diyAudio Member
Join Date: Jun 2004
Location: big smoke

Thx to Sidewinder, a manufacturer's response right here!
http://www.audioasylum.com/forums/Ka...ages/3395.html 
16th January 2005, 08:36 PM  #20 
diyAudio Member

User name
Hello,
TerribleT is Sidewinder. I had difficulty in registering using Sidewinder so I used TerribleT. Both are names of jazz tunes by my favorite trumpet player, Lee Morgan. Again, thanks for all the replies. I am beginning to understand. I need to further digest all the information given. But it seems, everytime I gain knowledge, it become evident I know less than what I thought I didn't know before. Here's another post: http://www.audioasylum.com/forums/ma...ages/1832.html[/URL] 
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