Help with an OPT question

[Drewaudio]

Hi,

My brain hurts so hopefully someone can help me. 😱

I'm building a 6v6 push pull guitar amp. I have a 20w 4k3 PP output transformer. If I put a 16 ohm speaker load on the 8 ohm transformer tap, the tubes will see 8k6 but the inductance will be half what it should be right? Does this mean a 20hz to 20kHz response become more like 40hz to 20kHz?

Am I on the right track? I'm just trying to save some money and use what I've got...

Thanks in advance!
Andrew

 

[TonyTecson]

no, it's the same opt with fixed number of turns that is the basis of inductance,

 

[danFrank]

Yes the 16 ohm load will reflect 8.6k to the primary when using a 16 ohm load on an 8 ohm tap.
The bass response of the transformer will be less, configured this way. How much less, I can't say but I'm sure there's formulas to calculate the difference in frequency response.
The OT you're using has a specified frequency response when used at 4.3k primary impedance, because of the primary's specified inductance. 8.6k primary impedance needs higher inductance to get the same frequency response. Winding ratio/turns ratio doesn't play a role in inductance values, more turns in the primary affects this. See RDH4 for this.
Use the transformers you have, it will probably sound good!

 

[trobbins]

It's a guitar amp, so even a mediocre OPT is likely to give you an acceptable bass end response with a 16 ohm speaker on an 8 ohm tap - but that can depend on a lot of things like the speaker/enclosure you could use, and if the amp uses feedback from the OPT secondary, and if you play bass, and what tone settings you like, ....

It sounds like your OPT has a 20Hz to 20kHz spec - but that also can be suspect unless the response spec comes with a power level over that 20Hz to 20kHz range.

 

[JMFahey]

Dear Tony, you are right talking absolute inductance, the OP asks :

the inductance will be half what it should

and he´s righyt.

building a 6v6 push pull guitar amp. I have a 20w 4k3 PP output transformer. If I put a 16 ohm speaker load on the 8 ohm transformer tap, the tubes will see 8k6 but the inductance will be half what it should be right?

Right but no big deal.
And given Guitart >82Hz, you probably won´t ven notice anything.

Does this mean a 20hz to 20kHz response become more like 40hz to 20kHz?

Something like that, but practically inaudible.

Am I on the right track? I'm just trying to save some money and use what I've got...

Use it with no concerns.😀

 

[danFrank]

Yeah, I forgot to ask that... I had a notion that it might be for a guitar amp. In that case, the OT is overkill for the job at hand.

Edit: I don't read too well, it was in the OP's post.

Hey, I was born in Argentina... Aconcagua!

 

[TonyTecson]

impedance ratio is just the turns ratio squared....the number of turns, the winding geometry does not change with whatever you put in the secondary...

as to how the frequency response go, that is dependent on primary voltage excitations mostly...inductance also depended on the ac voltage applied...

bottomline, there is no physical change to the taffo whatever and how ever you use it unless you drive it to saturation and high voltages and currents that make it burn out....

 

[Drewaudio]

Thanks everyone, that's been very helpful feedback! It's obvious (now) that the transformer windings and therefore inductance remain unchanged!

I will try it out and see how it goes... 🙂

Andrew

 

[PRR]

> 20w 4k3

That's a very odd spec. 4K is more likely a 40+ Watt 400+V amp or a 10W 120V amp. Where did you get it from?

Do you have any frequency spec? If not, this is all hot air.

 

[Drewaudio]

Hi PRR,

I went and dug out the OPT and there was some data with it. I suspect you are right, these are good for 40 watts.

I've attached a photo of the ''label' and some photos for the size.