Noob output transformer question

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Noob output transformer question

Postby kegbarnacle » Wed Jan 07, 2009 12:02 pm

Having just got going with amp building, I've read everything I can get my hands on and my first project has been a kit based tube amp. It's coming along well, and I'm already planning the next project - one that ups the ante beyond pure kit level.

I have a book from Tino Zottola that has projects for five popular amps - I'm doing the '59 Bassman. His parts list calls for an output transformer with a 4000 ohm primary / 4 ohm secondary rated at 50 watts.

That doesn't seem to be a real common off-the-shelf item, but there are several that are close. 4000 primary / 8 secondary seems to be out there, as are transformers with maybe something like 4200 primary.

So my question is - what is the implication of this? It seems to me like an 8 ohm secondary would just mean I'd need to use an 8 ohm cabinet - which is fine, I already have a few of those. Also from what I've read, going from 4000 to 4200 primary would be no big deal. Is there any sort of tradeoff?

Thanks

-Toby
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Postby cartoonweirdo » Wed Jan 07, 2009 1:51 pm

Hi Toby,
The short answer to your questions is that you have the right idea already. There is no problem using an 8 ohm cab instead of a 4 ohm one to match the secondary of your OPT, and the extra 200 ohms of impedance will not make too much difference in the end.

Keep in mind that the impedance of a speaker gets reflected via the OPT to the power tubes, and that speaker impedance changes with frequency. Provided you don't play only one note all the time that means that this is an inexact science (perhaps an art?).

If memory serves the '59 Bassman was a 4x10 combo with all four speakers in parallel for a TWO ohm impedance. A good source for a suitable OPT would be Magic Parts (www.MagicParts.com). They sell to repairmen and guitar stores, so unless you want to do some paperwork to "start a business" (read: buy amp parts at wholesale cost) you may have to purchase through a middle man. Another idea for sourcing parts is that Fender recently "re-isued" the '59 bassman. Perhaps stock transformers from the re-issue ones could be obtained. If you want to go whole hog (e.g. expensive, and you usually get what you pay for) try mercury magnetics.

I hope your project turns out great!

Carl
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Postby EWBrown » Thu Jan 08, 2009 10:57 am

4000 ohms CT doesn't have to be exact, 4200 or 4300 ohms is OK, too.

A few hundred ohms in either direction is fine, as there is usually a 5 to 10 percent tolerance, anyway...

The primary impedance is actually a "reflection" of the secondary load (speaker) impedance, which isn't constant across the audio spectrum, anyway. In "real world" terms, a 4300 ohm OPT would have approximately 23:1 turns ratio for an 8 ohm secondary, and about 33:1 turns ratio for a 4 ohm secondary (impedance is a squared function, and not a linear function).


Last year, someone on e-bay was sellig off massive quantities of Schumaker (sp?) 60 watt PP guitar amp OPTs for a pretty cheap price , these looked pretty decent, fairly large size, and open frame construction. 4300 Ohms CT primary, and at least 8 ohm secondary, if not 4 and 8 ohm taps.

Triode Electronics and AES www.tubesandmore.com both sell Fender transformer "clones" of most of the more popular items.

/ed B
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O.T. choices

Postby Camster » Wed Mar 11, 2009 3:22 pm

Hammond makes great transformers and the Fender replacement they offer has 2 / 4 / 8 ohm out - 4200CT model 1750M . a little pricey but good quality stuff .

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Postby chrish » Sun Apr 26, 2009 4:36 am

Edcore, www.edcorusa.com are getting a great reputation amongst us hi-fi amp builders for great quality for the price. This looks like it fits your needs very well http://edcorusa.com/Products/ShowProduct.aspx?ID=561 priced at around $63. What is really great about Edcor is that they are happy to build something exactly to your specification (for a small additional design fee of $20-30 if I remember correctly). They are also a great source of power transformers. They have a large range of power transformers that are not listed on their site. The quality is FAR superior to Hammond at a lower cost.

Hope this is of some help.

Chris
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