Push-Pull Output Transformer question

[mresistor]

I am wondering how balanced most guitar amplifier push-pull output transformer primary windings are. I assume that the better they are balanced the more balanced the plate current of the respective power tubes would be. I measured an ot I have that is for two 6L6GCs for say a Super Reverb amp..  and one winding has a resistance reading of 44 ohm with the other being 46.3 ohms. This doesn't seem to be extremely balanced to me and I wonder if it can cause problems with getting plate current balanced in an amp even using matched tubes?  Is it critical that the ot's plate windings be as close to equal as possible?  Or is being a few ohms different acceptable? What is the tolerance ?   

[sluckey]

Wire resistance is not a good indicator of how well the primary winding is balanced. A better indicator is the number of turns. You can check the balance by connecting a small AC voltage (I like to use 1V) to the secondary then measuring the voltages for each half of the primary. Perfect balance means exactly the same voltages.

[mresistor]

Thank you Sluckey I'll do that.. could be a while though -  travel plans ..

[PRR]

> one winding has a resistance reading of 44 ohm with the other being 46.3 ohms. This doesn't seem to be extremely balanced to me

Seems exceptionally good to me.

First: Why? Well say you wind 1/2 primary, secondary, 1/2 primary. The outside half of the primary, with the same number of turns, runs 20%-30% longer, so will be a higher resistance. We see this in some cheap iron. You have a 5% diff so we know they used a more sophisticated lay-up.

You have a 2.3 Ohm difference. Taking 50mA idle 200mA peak this is a 2.3V or 10V difference. Compared to a 400V supply, 0.7% to 3% difference. All tube-work is +/-20% makes hardly-any difference. Few-% is nothing, lost in tube tolerances.

The audio action is not about the 44-46 ohms but the audio range 4K impedance. 2.3 Ohm differences are totally negligible.

While you could check actual turns ratios, turn-counting is a BASIC skill in transformer winders. It is very unlikely to be wrong, especially across two "identical" windings.

[Backwoods Joe]

Do you know what brand it is? That seems VERY close to me!

[2deaf]

Maybe very close DC readings indicates a way off turns count. (just kidding).

[mresistor]

It is a Classic Tone http://www.classictone.net/40-18001.html     


thanks for the replies guys..   sounds like I am overly fretting about it.. 

[tubeswell]

The impedance reflected through an OT (from the speaker back to the output tubes) is all about the Pri:Sec turns ratio.


For a PP OT, both halves of the primary have the same number of turns. This means (invariably) that the length of wire for one half is going to be longer than the other, because it has to be wound 'over the top of' the other. This (of course) affects DC resistance by a few tens-of-ohms usually. But as PRR said, when you have interleaved windings, this difference is not quite as big due to the better balance of wire length achieved in the interleaving.


But the wire length itself doesn't affect OT impedance (which is different from DC-resistance), its the number of turns that does.

[jjasilli]

Yes, this is a recurrent topic.  And it ultimately encompasses other balancing issues:  i) the power tubes; and ii) the PI.  Note that these things will vary as tubes age; and how warmed-up they are.