Determining Output Transformer

[dujuarez]

I made the mistake of removing the Output Transformer and not taking any notes or pictures, as I did with the Power Transformer. I know that it had output for 4 ohm and 8 ohm load. I want to use only the 8 ohm load. My problem is two fold, how do I determine the which wires to use for output (blue, red and brown) and which wire runs to the board and output tubes (white, black and black/yellow). I have resistance readings but don't know if this helps as it does with Power Transformers.




Primaries
White - Black - 0.7 Ohms
White - Black/Yellow - 0.4 Ohms
Black - Black/Yellow - 0.6 Ohms


Secondaries
Red - Brown - 109.5 Ohms
Red - Blue - 83.4 Ohms
Brown - Blue - 192.5 Ohms


Thanks

[Paul1453]

Actually the primary windings on your OT should have the higher resistance readings.

So your labels need to be switched.
Your primary center tap appears to be the Red wire.
Brown and Blue measures across the complete primary windings.

I would guess that if this OT only had 2 secondary taps of 4 and 8 ohms,
then White and Black would be your 8 ohm secondary taps.   

[HotBluePlates]

I agree with Paul.

You can't always count on "standard wiring colors". However, for the primary, Brown & Blue are standard for running to output tube plates, while Red is standard for the primary center-tap.

On the secondary side, Black is typical for a "Common" lead, often connected to ground. That makes Black/Yellow and White the other leads for your 2 taps. Because the highest resistance is between Black & White, and you noted the original secondaries where 4Ω and 8Ω, it follows that Black & White are the wires for your 8Ω tap.

[PRR]

What they said.

Further: the DC (ohm meter) resistance should probably be "small" compared to what it connects to. "Small" may be 5% or 10%.... any more, the transformer is wasting a lot of precious power.

0.7 Ohms is silly-small next to a many K-ohm plate-side impedance, but reasonable for an 8 Ohm load (8% loss).

Likewise 0.4 Ohms is reasonable for a 4 Ohm winding. (A bit higher than we might like, but why pay more?) (Also plenty-fat secondary wire is hard to wind, while very-thin primary wire breaks; we tend to favor the primary for manufacturing reasons.)

100 Ohms is absurd for a speaker-impedance winding (92% loss!) but reasonable for a few-K plate side.

In some other universe, both sides of the push-pull winding have equal Ohms. But if the loss is small, a little difference makes no difference in the end. 83:109 or 26 Ohm difference is fine. Same number of turns (what really counts) but one side is wound over the other, longer path, more Ohms. Your plate-to-plate load is more than 100X bigger than this 26 Ohm difference, and 1% is nothing.

[2deaf]

They're probably right, but I don't trust DMM readings for resistances that low.

The way that PRR told you to use 6Vac to determine the windings in your power transformer works for the OT as well.  Hook the 6Vac to brown and blue and see what comes out the other end.

[dujuarez]

Thanks Paul 1453 for pointing out my mistake with the primaries and secondaries. 2deaf, I'll try that before I hook everything up. Thanks PRR
for bringing your understanding, it helps to what is going on and not just " I can hook this up here and hook this up there". Thanks Hotplateblues, I need to be more cautious and not "assume".






Thanks all!