What ohms is this 022889 transformer?

[tejastubes]

Is this always a 4 ohm tap or did they make both a 4 and 8 ohm version? How do you check? Or is there any need to if they only made it in 4 ohms?

[IMG685]http://i183.photobucket.com/albums/x34/ ... 10_344.jpg[/img]
[IMG685]http://i183.photobucket.com/albums/x34/ ... 10_343.jpg[/img]

[alvarezh]

Hi Joshua, here is a Fender transformer chart:

http://www.unclespot.com/FenderXFMRchart.html

You can see that 022921 translates to 125A29A. Google that and you get this:

http://www.tube-town.net/ttstore/produc ... everb.html

It has a 4 ohm secondary, according to the Hammond 1750W spec-sheet on that webpage.

Hope this helps.

[tejastubes]

but can it be either 4 or 8 ohms or only 4 ohms?

[alvarezh]

According to the spec-sheet its only 4 ohms. Furthermore, I just checked, I have an identical one I took out from my Fender Silverface Twin Reverb (1974). It has a green and a black wire for the secondaries, that is only a 4 ohm tap.

All the best.

[Luthierwnc]

I'd check it anyway. Run house AC into the primary and measure the voltage on the secondary. Divide your source voltage by the results to get the turns ratio. Square that result to get your impedance ratio.

In this example, say you get a reading of 4 volts output from a 120VAC plug. Divide 120 by 4 and you get 30 -- that's the turns ratio. Square that (30 x 30) and you get 900 -- that's the impedance ratio. For each ohm of Z-out you are looking for around 900 ohms of primary impedance. A pair of 6L6s is going to be around 3600. Divided by 900 that's a 4 ohm secondary.

In this example the 4 volt test looks a lot like 4 ohms impedance but that is purely coincidence. If this was for a pair of 6V6s at 7k, your Z-out would 7.77 ohms. You have to do the math.

I've gotten to test lots of mystery transformers. I always use wire nuts on the primary for safety and bring it up on a variac plugged into a ground-fault plug. Mystery transformers can come with other surprises too

Skip

[tejastubes]

Thanks,,,I am not that sophisticated, but I have confirmed with several folks this is a 4 ohm tap only. I have been told the tranny with an 8 ohm tap actually has a different part #.

[alvarezh]

Thanks Skip, I just checked my O.T. with your math using 1800 ohms for four 6L6s and it came out to 3.75 ohms for the secondary.

Of course, the four tube total plate impedance is an approximate number, so you could say that 3.75 ohms is a close enough value for a 4 ohms labeled tranny.

Again, thank you much.

[Luthierwnc]

I only know the primary impedances because I've read them so many times. I couldn't prove a word. Older tube texts usually have higher primaries than modern ones. It might be the tubes -- might be the much higher voltages we subject them to these days. You try to get it "on" but close counts too. The main thing is not to have a really low secondary.

Another reason to test older ones is to make sure they aren't dead before you stick them in the amp. The one in the photo is nice and shiny so the odds are good it's OK. Transformers that came to a firey (sp?) end range from black to a golden tan. Melted gook leaking out of the laminates is another sign Fenders don't incinerate often but Marshalls are touchier on impedance mismatches.

Skip

[alvarezh]

Thank you for your comments Skip.