Output transformer primary impedance

[xtian]

I just got a good deal on fleabay, I hope, for this Weber OT:

http://taweber.powweb.com/store/ot100sch.jpg

I'd like to use it in a "plexi" (1987) 50 watt build. So, two questions: 100 watt OT should work fine for lower wattage amp, right? Just possibly won't achieve similar OT distortion at full tilt?

Second, Weber OT has primary impedance of 1.9k ohms, versus my target of 3.8 or 4k ohms for a Marshall build. Merlin seems to suggest that I can double the effective primary impedance by using higher or lower secondary taps…can't quite figure this out:

It's easy to see that a transformer that exhibits a voltage and current ratio of 20 will have an impedance ratio of 400. This means that connecting a speaker of 8 ohms to the secondary will reflect an impedance of 3.2k to the primary. Doubling the speaker impedance to 16 ohms would also double the primary to 6.4k. (This is a useful trick that may allow you to obtain your ideal primary impedance by connecting the transformer to a speaker impedance other than the rated value.)

http://www.valvewizard.co.uk/se.html

So, if I connect a 16 ohm speaker cab to the 8 ohm tap, am I achieving what Merlin suggests?

[JazzGuitarGimp]

Yes, all of your observations are correct. If the OT is reflecting a primary load of 1.9K with an 8-ohm speaker, you will double the primary impedance to 3.8K by connecting a 16-ohm speaker to the same (8-ohm) secondary tap. And, in the process, you will likely get a bit of a beefier sound by using an overrated transformer.

[renshen1957]

Yes, all of your observations are correct. If the OT is reflecting a primary load of 1.9K with an 8-ohm speaker, you will double the primary impedance to 3.8K by connecting a 16-ohm speaker to the same (8-ohm) secondary tap. And, in the process, you will likely get a bit of a beefier sound by using an overrated transformer.

+1, What he said. Saved me five minutes of typing.

Steve

[Kagliostro]

Correct

K