OT impedances?

[Dingleberry]

Hi.

I was measuring output tranformers for my next build, which is gonna be running on 2xEL34/KT66/6L6 in p-p configuration.

I tested two different transformers and got results that I actually made me think.
First was Drake 783-257/7. Not much info on the net about that but what I foud out it's from canadian market 2204 50W Marshall. It has only two secondary taps. 4 and 8 ohms. No 16 ohm tap and that was actually the most common hit from google that peope were complaining about the lack of it.
I calculated the impedances with the basic 1khz/0,5v sine wave method and found out that those taps should probably be 8 and 16 ohms.
Correct me if I'm wrong.

With the 4ohm tap I got the impedance ratio to be 627,0016:1
and with the 8 ohm tap i got 329,204736:1

Well if you multiply 627,0016 by 4 you get about 2k5
and if you multiply 329,204736 by 8 you get about 2k6
Pretty close match I assume, but ain't that 2k5 / 2k6 kind of low impedance for two EL34's.
Looking at the datasheets it should be something like 3k4 - 5k or even higher depending on the voltage used.

So if the taps aren't actually 4 and 8 ohms but 8 and 16 ohms then it shoud be something like two EL34's would like to see, a bit on the higher side yes but yet in the very useable margin.
Or could it be that the OT is from 100W Marshall? I really doubt because of the size of it.
Am I on the right track or am I missing something?

The other was from Dynacord Eminent 2T.
It has also been running with two EL34's, but with plate voltage about 750 volts giving 80W of power. And man that OT is HUGE! Something like three times bigger and heavier that the Drake. That high plate voltage probably explains the impedance ratio of that transformer.
I measured it also from the three different taps and got close enough results.
4 ohm tap ratio was 2209:1
8 ohm tap ratio was 1075:1
16 ohm tap ratio was 518:1

So that would make, depending on the secondary impedance chosen,
the primary impedance something like 8k2 - 8k8.
Actually a two times higher that two EL34's would like to see if running at "reasonable" plate voltages.
So running that transformer on "normal" plate voltages those taps should probably be 2, 4, and 8 ohms.

My question is Does my calculations make sense?
And hope you understood my explanations 'cause English is not my native language.

And just a little observation from all that:
Measuring the impedance ratio of both OT's different secondary taps gave me slightly different results. Not much but still evident.
Does this actually explain the phenomenon when some of your friends is telling you that running his amp from that specific tap sounds better than running it from other taps.
I mean why some guys that wire their Marshall 4x12's 4 ohms and others wire 16 ohms. They might be onto something and not just BS'ing.
If the impedance ratio really is little bit different depending on the tap used, one might really hear slight differencies because of the altered primary impedance.
Just my two cents.

-T

[tubeswell]

If you measured the voltage* ratios from secondary to primary, the observed ratios should be accurate enough. Its all about ratios.

(* remembering that impedance ratio is the square of the voltage ratio. Also remembering that you need to measure between the two ends of the primary to get the primary VAC for the equation. If you measured between each end and the CT, then you need the sum of both sides to get the primary factor in the ratio)

As for absolutes, a pair of EL34s in PP will run on anything between 1k7 and 5k with anything between 2k and 4k being fine and anything between 2k5 and 3k3 being finer still.

What load you actually choose in the end will be somewhat up to your ears.

[Dingleberry]

It's all about ratios. That's totally true.
And yes, I measured the the voltages from the two ends of the primary, as I've been taught and counted the impedance ratio by that formula.
Whole process is well presented in the Gerald Weber's book and I've been following that method, although I read somewhere that 400Hz sine wave would give a bit more accurate results than 1kHz.
Probably investigate with that tomorrow and take some readings.
Just last week got meself a signal generator, as you might have quessed;)

And you are right about the fact that primary impedance can vary a lot and you still have a completely working amplifier.
In the end it's all about your ears.

-T