Need to determine OT Windings with Meter

[silverfox]

I don't really want to take apart the amp to check the wires and determine the switch settings. I have a 3 tap OT. If I measure the windings will I be able to determine the tap setting on the impedance switch?

My thoughts are the readings will be too low. I'm using Mercury Magnetic TW OT.

by now I could probably have taken the chassis out of the shell...

silverfox.

[92Volts]

If you can get a reliable DC resistance reading, you can use it to determine which taps are closer to each other and further away. If you know they're all supposed to be taps on the same winding, you might be able to use this to determine their order.

It might be hard to get enough accuracy to tell, for example, the adjacent taps for ground and 4 ohms, apart from the adjacent windings for 8 ohms and 16 ohms. The spacing between these shouldn't be identical, but DC resistance might not be able to show you the small difference that should exist.

The "right" way to do this is apply an AC voltage across 2 of the windings (say 6.3v). The resulting voltage observed on the others gives both their spacing, more precisely, and their relationship to each other.

[tubeswell]

I use a low output VAC source* to test, and I do this in-situ in the amp (with the tubes removed). The only 'dismantling' you need is unsoldering the primary centre tap (from the filter cap supply node) and unsoldering the OT secondary windings (including the secondary common/ground return) from the speaker terminals. I clip my VAC source between each plate-end of the primary and the CT, and measure the resulting VAC between each secondary tap and the common. (VAC Pr:Sec)^2 = Z Pr:Sec.


*I made a VAC source using a surplus transistor amp PT with a single 50VAC secondary, which I mounted on a project box with a fuse, and mains switch/LED lamp. The secondary is wired to a couple of binding post banana plug terminals, and I use insulated banana/gator clip leads. One VAC meter goes onto the binding posts, and the other meter is used to measure the respective secondary taps directly. FWIW I just acquired a new 25W 1k rheostat that I will 'upgrade' my simple tester with - to provide adjustable AC output and also provide a means of indicating shorting in the transformer being tested. The voltage divider provided by the rheostat will mean that a shorted xformer will short the resistance between the rheostat wiper and the ground, without affecting the source VAC transformer. 25W 1k should be sufficient power rating/resistance for this to work reliably and safely).

[sluckey]

by now I could probably have taken the chassis out of the shell...

Or contacted MM.