output transformer info

[dscottguitars]

Hello gurus and others,

What are the 250 ohms, 500 ohms, 70v and 25v tap for? How crucial is the primary impedance rating, 3.5k to 5k for a 4k transformer? I have a 4k/4 ohm transformer in a schematic I want to change to have 4/8/16 ohm taps. Lastly, will a 40 watt rated transformer work for a 50 watt amp? I have one but I'm not sure if I should use it, any advice would be greatly appreciated.

Thanks,

Daniel

 

[HollowState]

Hi Dan,

What are the 250 ohms, 500 ohms, 70v and 25v tap for?

These outputs are for driving long distance speaker lines in public address and sound reinforcement systems. You need the higher voltage level to help prevent signal loss and maintain efficiency. A matching transformer is placed at each speaker to mate with the voice coil impedance.

How crucial is the primary impedance rating, 3.5k to 5k for a 4k transformer? I have a 4k/4 ohm transformer in a schematic I want to change to have 4/8/16 ohm taps.

It's only somewhat critical. A lower impedance match will produce a little more power with a little more distortion. Conversely, a higher impedance match will produce a little less power with an accompanied lower distortion. But don't go much beyond the range stated, although it really depends on the tubes used.

Lastly, will a 40 watt rated transformer work for a 50 watt amp?

Assuming this is for a guitar amplifier, generally yes it will. The power rating of an output transformer refers mainly to the lowest frequencies. If you don't need from 60 Hertz down, then a 40 watt transformer will pass more power without distorting. Sometimes as much as 50% more depending on the quality of the individual transformer.

As an example, I use Acrosound TO-600 transformers from a type UL-II amplifier that are rated at 60 watts. Since I bi-amp my system and cross over at 60 Hz, I can get a clean 100 watts out of them. (That's on a good day 😀)

If this is for a home stereo system, then no, you don't want to use a lower rated transformer unless you don't need the bottom octave.

 

[dscottguitars]

Thanks...

Thanks Hollowstate,

Great answers are those that are understood, and I understand now. A couple of clarifications if you don't mind.

This is for guitar, an acoustic channel and an electric one. I wouldn't think the low E for each of them would go beyond the frequency limit you stated, yes?

And this would be at full output also? Would those lower frequencies be available at lower volumes?

Thanks so much for your help...

 

[HollowState]

I believe the low E is around 80 Hz according to tubelab in another post. And yes, you'd get the lower frequencies too. There is no sharp cutoff point. It's just that at very high power the transformer would not provide the clean undistorted bass notes that a larger one may. Remember, we're talking extremes.

 

[dscottguitars]

Thanks for the information, I really appreciate your time... Those distorted bass notes at extremes may be an advantage.