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Stupid question of the week:

My TC-15 is working flawlessly and sounds great, but somehow I managed to neglect to label my speaker/ OT transformer impedence switch positions. Because the various impedence choices come from the OT secondary taps, I assume I can figure out how I wired it without opening up the chassis simply by measuring resistance from hot to ground at each of the switch positions. The 4 ohm tap should be the highest resistance (because the 4 ohm tap has the longest run from end to end of the secondary wiring) and 16 ohm the lowest.

Is my reasoning sound or not? At the output jack, my readings for the three switch positions are approx. 0.7 ohms, 1.0 ohms, and 1.8 ohms. I'm guessing these correspond in order to the 16, 8 and 4 ohm selections.

Thanks for bailing me out!

Actually, the 16 ohm winding has the most turns.

I measured my TC-15. I got .7 ohms on the 4 ohm setting, .9 ohms on the 8 ohm setting and 1.2 ohms on the 16 ohm setting.

Thanks for the correction. Strange, but the OT in the TC-15 schematic shows the lower impedence tap across the longer run of wire. That schematic must have the taps labelled backwards. I just looked at a few OT schematics (including for the AC-15 and AC-30) and they show (as you state) the higher impedences across the longer runs (the ends) of the secondary, with lower impedence tap(s) across shorter runs. That would confirm what you state, Mitch. Now I can label my speaker impedence switch, so thanks!

Yes, it's kind of misleading the way the OT is shown on the schematic. Usually, schematics show the wire colors associated with the various impedance windings which I find more important than how the symbol is drawn.

I was going by the formula for output transformer turns ratios and impedance ratios. The turns ratio is the square root of the impedance ratio.

So if the primary impedance is 8K and the secondary is 16 ohms, that's a ratio of 8000/16 or 500:1. The turns ratio will be the square root of that which is about 22:1. Say the primary has 2000 turns. That would mean the 16 ohm winding has 2000/22 or about 90 turns. These probably aren't the actual numbers of turns in the TC-15's OT. This is just to illustrate how the formulas work.

At 8 ohms the primary impedance is still 8K so now the impedance ratio is 8000/8 or 1000:1. The turns ratio is the square root of that which is about 32:1. The primary still has 2000 turns so the secondary will need about 63 turns.

At 4 ohms the primary impedance is still 8K so now the impedance ratio is 8000/4 or 2000:1. The turns ratio is the square root of that which is about 45:1. The primary still has 2000 turns so the secondary will need about 45 turns.

Hope this helps explain things. Here's a website that has a calculator for all this.
http://www.maxmcarter.com/classecalcs/tratiocalc.html

Good points here. Correct info as well. Colours are on the layout but the schematic has now been updated to reflect your feedback!

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Stephen
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That would confirm what you state, Mitch. Now I can label my speaker impedence switch, so thanks!

The link above is SPAM!

I'm guessing these correspond in order to the 16, 8 and 4 ohm selections.


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