Looking for 4K/8K OT

[ChrisM]

Looking for a OT (around 50W) that has dual primaries, both 4K and 8K. Want to be able to run 6V6 and 6L6 (not at the same time!)

I know Brownote has an OT like this but I want to see if there are other options out there.

Thanks...

[mlp-mx6]

I think you mean dual primaries.

[ChrisM]

Yes thank you, my bad.

[Sonny ReVerb]

If you have multiple secondaries, you can just swap the speaker connection.

I have a single ended amp that I switch between 6V6/6L6. The 6L6's like to see a reflected load of about 4kΩ. The OT has 4Ω and 8Ω taps which relate to impedance ratios of 1000:1 (4000/4) and 500:1 (4000/8). I normally use an 8Ω speaker, so I plug into the 8Ω tap.

When I put in the 6V6's they want to see about twice the reflected load (8kΩ) vs. the 6L6's. To achieve that, I plug the 8Ω speaker into the 4Ω tap with the 1000:1 impedance ratio. 1000:1 = 8000/8. The taps effectively become 8Ω and 16Ω taps with the 6V6's.

Check out this back plate of an Allen Accomplice which is designed to use 6V6's and 6L6's:

[img:251:157]http://img412.imageshack.us/img412/167/taps.jpg[/img]

This may be an easier solution than switching the OT primaries. YMMV.

[ChrisM]

If you have multiple secondaries, you can just swap the speaker connection.

I have a single ended amp that I switch between 6V6/6L6. The 6L6's like to see a reflected load of about 4kΩ. The OT has 4Ω and 8Ω taps which relate to impedance ratios of 1000:1 (4000/4) and 500:1 (4000/8). I normally use an 8Ω speaker, so I plug into the 8Ω tap.

When I put in the 6V6's they want to see about twice the reflected load (8kΩ) vs. the 6L6's. To achieve that, I plug the 8Ω speaker into the 4Ω tap with the 1000:1 impedance ratio. 1000:1 = 8000/8. The taps effectively become 8Ω and 16Ω taps with the 6V6's.

Check out this back plate of an Allen Accomplice which is designed to use 6V6's and 6L6's:

[img:251:157]http://img412.imageshack.us/img412/167/taps.jpg[/img]

This may be an easier solution than switching the OT primaries. YMMV.

Great!

So I should go with a 8R speaker.
A 50W OT with a 4K primary. With 4R and 8R secondaries.
Use 4R for 6V6s and 8R for 6L6s.

Correct?

[Sonny ReVerb]

I believe that would do the trick.

[tubeswell]

Yeah its all about the OT's Impedance Ratio - as Sonny Reverb said.

An OT with a 1000:1 (Pr:Sec) impedance ratio will give an 8k reflected load on the primary with an 8R speaker on the secondary, and a 4k reflected load with a 4R speaker and so on.

The impedance ratio itself is the square of the windings ratio, and the winding ratio is the same as the voltage ratio. So for example that 1000:1 impedance ratio = 31.62:1 winding ratio (Pr:Sec) or 31.62:1 voltage ratio (Pr:Sec). (31.62 x 31.62 = 1000).

And so (in a perfect world) if you fed a 10VAC signal onto the secondary, you'd get 316.2VAC across the primary - and that's how you work it out with a variAC and a couple of VAC meters.