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Old 16th January 2005, 12:11 AM   #11
SY is offline SY  United States
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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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Old 16th January 2005, 01:50 AM   #12
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For a 1:1, the primary impedance will look like whatever impedance is loading the secondary.
That's not always the case. An interstage transformer connected directly to the grid of the tube in the following stage sees no load (in Class A). The primary impedence (5K, for example) will indicate the load the driving tube sees terms of an impendence at a certain (usually low end) frequency based on the inductance of the primary winding. In that case you look at turns ratio and primary inductance to see if an interstage transformer is suitable for your purpose.

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Old 16th January 2005, 02:09 AM   #13
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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?
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Old 16th January 2005, 02:43 AM   #14
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Quote:
Originally posted by TerribleT
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?
Well, a transformer doesn't respond down to DC; it's limited at the low end by the primary inductance, which effectively shunts the primary. At midband, the inductance of a properly-speced represents a very high Z, enough so that it can be treated as an open circuit.
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Old 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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Old 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, RL. The impedances (in this case, pure resistances) are transformed ny the square of the turns ratio.

The bottom sketch introduces the first level of non-ideality. 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 N2RL 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 Lleakage 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 non-flat response compared to running the transformer with optimized loads.

Does this make it any clearer?
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Old 16th January 2005, 02:17 PM   #17
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This is not so for a.c., but how is this impedance calculated?
The impedence X = (2pi)fL (where f is the frequency in question and L is the inductance in Henries). A good rule of thumb is that the impedence at 50 hz should be about two to three times the plate resistance of the driving tube or about 7 to 10 Henries of inductance per 1000 ohms of plate resistance.

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 10-20 hz will give more headroom and cost more or you can save money by using tubes with lower plate resistance such as 6C45pi.

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Old 16th January 2005, 04:24 PM   #18
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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 mis-application of a product based on lack of information.
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Old 16th January 2005, 04:39 PM   #19
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Thx to Sidewinder, a manufacturer's response right here!

http://www.audioasylum.com/forums/Ka...ages/3395.html
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Old 16th January 2005, 08:36 PM   #20
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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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