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In Reply to: RE: IT Transformer connection phasing posted by Chip647 on September 17, 2016 at 14:35:13
That diagram shows it correctly!
Has anyone ever measured it hooked up backwards?
Curious about how it creates this shunt capacitance
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In a bifilar transformer, coupling is both magnetic and capacitive. The capacitance is created by the proximity of the primary winding to the secondary winding over the entire length of the bifilar wires. Because of this capacitance, it's important that both wires at one end be connected to high Z, and both wires at the other end be connected to low Z (AC ground). If one or the other winding is connected incorrectly, the high-Z end of one will be adjacent (and capacitively coupled) to the low-Z end of the other. This provides a path for higher frequency signals to be shunted to ground.
If this isn't clear, let me know. I'll post a sketch that shows what I'm trying to describe.
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Buy Chinese. Bury freedom.
It was something I overlooked during both design and build process.
I initially thought the Hammond OPT just sucked that bad until Jack at Electrprint said to check my gain stage. I studied many IT schematics and 90% of them show the IT connections wrong, is this just lack of knowledge or is bifilar not that popular?
I connected it wrong at first too.... sounded pretty awful, no highs.
At some point somebody told me to switch the secondary windings round, and suddenly it sounded normal again. But as I said, I use it as a plate choke - I've read that elsewhere as well.
Mine sounded like someone adjusted the treble knob from 12oclock to 8
Probably sounds better as a plate choke, I have a nicer magnequest choke that performs better. I liked the control better on the IT transformer even though highs were rolled off.
I'm assuming the shunt capacitance acted as a 6db per octave filter.
Wonder if there's a way to calculated this without measuring?I have a Monoloth magnetics IT wound and gapped for the 300b driver if it ever ships.....
Been two months now
Edits: 09/18/16
I think there's a confusion of two issues. (1) If you hook up the IT backwards, the impedance seen by the grid will be way higher than if hooked up the otherway - leading to shitty highs. and (2) The Inverted Interstage where the IT is gapped and wound EXPLICITLY to deal with both grid current from an A2 tube, and plate current from the driver.
In the case of (2), the transformer is not 1:1, it has a lower turns ratio to provide low voltage but high current swings of grid current. That means it not bifilar wound and it means the directions of current must be opposed to provide flux cancelling.
To solve (1), just try it! To solve (2) requires explicit magnetics design and custom winding.
-- Jim
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