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Old 13th August 2004, 05:22 PM   #1
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Default Inverted Interstage Transformer

How different is the construction of these transfromers compared to the normal interstages? Apart from the phasing of secondary is opposite of the primary? is there any other different construction details

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Old 5th January 2011, 11:21 PM   #2
DrRick is offline DrRick  United States
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Hey Nick,
Did you ever get a response to this or learn what, if any, differences exist?
It's an issue I've been wondering about since I bought Shushudo's book 15-20 years ago.

Originally Posted by NickC View Post
How different is the construction of these transfromers compared to the normal interstages? Apart from the phasing of secondary is opposite of the primary? is there any other different construction details

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Old 5th January 2011, 11:53 PM   #3
tinitus is offline tinitus  Europe
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wouldnt it be matter of how you connect it ?
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Old 6th January 2011, 12:19 AM   #4
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Depending on the construction of the transformer, it can behave differently when you inverse the phase of the secondary. The end of the winding which is closer to the core is usually connected to ground while the other end which has less capacitance to the core is used as 'hot'. If you reverse the secondary, high frequency response can suffer, especially if it is a high impedance transformer.

An example for such a case is the Tango NC20. While it is very wide band when connected as shown in the datasheet, it will have severe and very audible high frequency roll of.

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Old 6th January 2011, 12:57 AM   #5
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I mentioned it in some other thread when I remember well, but you can wind 1:1 interstage transformers in different ways.
The Tango NC20 is a nice example; that one is wound to function as a 1:1 transformer as good as possible. With as good as possible I mean HF bandwidth. What happens is that the primary and secondary windings are coupled as close as possible; this can be done by interleaved winding technique, or bifilar. At one end the primary and secondary are connected to B+ and ground respectively; at the other end the primary and secondary are connected to anode and + respectively. This way there is capacitive coupling between primary and secondary which helps HF bandwidth a lot; with driver tubes with Rp of around 2k several hundreds of kHz can easily be reached. When one of the connections is wrong however the capacitance works fully against and there will be HF drop beginning at 2 or 3 kHz.
Tango NC14 is another interstage transformer which can be connected 1:1, but it is wound completely different because it can also function as a phase splitter. Connecting this one 1:1 does not have that HF bandwidth because there is no tight capacitive coupling between primary and secondary. When it had it would fail as a phase splitter.
I don't agree with Thomas that the core-side terminals should be connected to ground for capacitive reasons. Capacities between coil and core are not important here. Actually I prefer to connect the inside of the coil to anode resp. + and the outside to ground.
I never fully understood the reason behind Shishido's inverted interstage transformer.
He mostly used transmitter type triodes in the output stage into class A2. In that case we have grid current which flows "against" the driver stage current in the interstage transformer equalling out some of the DC magnetizing current, so that a smaller transformer could be used. For reasons explained above this inverted interstage transformer cannot be a tight coupled one primary/secondary wise.
It must be noted however that the Japanese school of Shishido and especially Sakuma was not so interested in stellar specifications; they designed very much with their ears and made very good sounding amplifiers.
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