Universal transformer help

[mrn1ngstr]

Forgive my ignorance here but... Looking at this, is the aim to find out the output impedance of the output stage, and then use the tap that is the closest? Or do I choose the load impedance and then tweak the power amp output impedance to match?


http://www.mouser.com/ds/2/177/5C_125SE-475075.pdf

[M Fowler]

If you were building a Single Ended amp with primary of 10k and wanted to use an 8 ohm speaker you would select the black 0 as ground to speaker - and green wire as 10k primary at 8 ohm speaker load +

[mrn1ngstr]

Assume were talking about an amp thats already built. So guess my question is, because nothing is ever nice and neat, if my primary is 6.5k or something else not in their listing, what then?

[Stevem]

Are we talking about a SE output stage here , or a A/B type, and with either what output tube(s) does the amp have, and what do you want to use?

[M Fowler]

Assume were talking about an amp thats already built. So guess my question is, because nothing is ever nice and neat, if my primary is 6.5k or something else not in their listing, what then?

You would use the 5k primary of that 125 OT

[mrn1ngstr]

So just matching as close as possible. Thats what I was looking for. Thanks!

[M Fowler]

Your welcome

[mrn1ngstr]

Ok next question. Lets say output impedance is 3.7K. Would you pick the 2.5K or the 5K tap? Would I want to be picking the closest lower or closest higher?

[Colossal]

Ok next question. Lets say output impedance is 3.7K. Would you pick the 2.5K or the 5K tap? Would I want to be picking the closest lower or closest higher?

That very much depends on the plate and screen voltage you intend to use. The general adage is "tubes are very forgiving", but I prefer to calculate the load lines and it is a worthy exercise to do so. There are tonal consequences to the choice of load as well. In your case however, if we assume a higher plate voltage than the typical pure Class A examples given on most datasheets, I would go up to 5k. This will rotate the load line counterclockwise, keeping screen current more manageable.

[mrn1ngstr]

Let's say we go with 6L6 308V on the plate, 295V on the screen.

[Colossal]

Let's say we go with 6L6 308V on the plate, 295V on the screen.

That's pretty much pure Class A conditions (and almost verbatim off the available datasheets) so I would just stick with the 5k tap. Your cathode bias resistor will probably be around 220-250ohm and give you about 7W or so.

[mrn1ngstr]

Cool man. Im going to try a couple different tubes in this amp, but Im trying to understand some of the points that Im not so familiar with. I always have to know "why" and "how" when Im doing something.

[Colossal]

Asking why and how is great. You should look into how to cast a load line if you are not familiar with the practice. It is very insightful and will teach you a lot. When you hear the results of your work, it adds to the reward of learning. When I mentioned the adage of "tubes are forgiving", they are, so long as you are careful operate them within their physical limits. By choosing a specific load line that may be "off the reservation", you might choose to sacrifice power for instance, for a tonal goal.

[mrn1ngstr]

I do understand load lines, I was mainly curious how this particular transformer would work. This isn't the my first amp, although Im lightyears from a pro. Im learning all the wonders of the SE world right now.