Add a speaker and match load w/o imp switch

[jeff]

I just had a thought. I have a OT with a 8 and 16 ohm tap. It'd be cool if I could use either one or two 8 ohm speaker without having to use an impedance selector and still have a correct load. I think I have a solution but I'd like your opinions if this could work.

 If you use the ground to the 8 ohm tap you need a 8 ohm load and if you use the ground to the 16 ohm tap you need a 16 ohm load but what if you use the 8 ohm to the 16 ohm taps and don't use the ground tap?

So if you ground the 8 ohm tap, the 8 ohm to 16 ohm tap is now a 8 ohm tap.
Also the ground to 8 ohm tap is an 8 ohm tap but it's out of phase becuase the 8 ohm tap is grounded not the ground tap. You could wire the main speaker out of phase between the ground and the 8 ohm tap which is out of phase so now the speaker is in phase.

 Now can you add another speaker between the 8 ohm(now grounded) and the 16 ohm tap. When you use one speaker you're only using half of the secondary winding but if you use two you're the whole winding.

Add a speaker with no need to use a switch. The impedance is a correct match using one or two speakers.

Wadda you think?

[PRR]

> the 8 ohm to 16 ohm tap is now a 8 ohm tap.

No, it is a 1.37 ohm tap.

[jojokeo]

Just use two 8ohm in parallel to the 8ohm tap (one up or doesn't make a whole lot of difference) or better yet make a special chord where it simply plugs into the back of the amp, plug the two speakers into this and it then runs the speakers in series and use the 16ohm tap.

[jeff]

> the 8 ohm to 16 ohm tap is now a 8 ohm tap.No, it is a 1.37 ohm tap.

I see what you're saying. Working backwards from the imp ratio to get the voltage ratio between the primary and 8 and then the primary and 16 ohm taps then getting the imp ratio between the two taps.

For the 8 ohm tap:
If the imp ratio is 8000:8 or 1000:1 then the voltage ratio is 31.62:1 or 1V in .03162V out
For 16 ohm tap:
If the imp ratio is 8000:16 or 500:1 then the voltage ratio is 22.36:1 or 1V in .04472V out

so for 1V on the primary there's .0131V on the secondary between the 8 and 16 ohm taps. The imp ratio would be squared 1:.0001716
multiply by the 8000 load to get 8000:1.37

Thanks, didn't think about that.

Gotta remember if it could be done, someone would have done it by now.

[jeff]

Just use two 8ohm in parallel to the 8ohm tap (one up or doesn't make a whole lot of difference) or better yet make a special chord where it simply plugs into the back of the amp, plug the two speakers into this and it then runs the speakers in series and use the 16ohm tap.

I see what you're saying. I'm just worried if you use two jacks in series, one would have to be an isolated jack. Then the shield of one cord would be grounded and the shield of the other would not be connected to ground, but to the positive of one speaker and the negative of the other. I'm always thinking worst case, anything that can go wrong will go wrong. If something made contact between the to cord's shields(plugs) it would short one speaker. I was trying to come up with a way to use two seperate 8 ohm cabs where each cable's sheild would be connected to ground. I was thinking an OT with a 8 and 16 ohm was like a 16 ohm with a center tap and you could get two 8 ohm taps out of phase. But as PPR pointed out that's not the case.

What I really need is a 4 ohm tap or a two speaker cab with jacks for one or two speakers. Or like you said just run them parallel and mismatch a 4 ohm load to the 8 ohm tap and not worry about it.

I worry too much.

Thanks for talking it out with me.

[DummyLoad]

just use a switch...

 

[jeff]

OK a switch is the answer.

 Not using a switch wasn't really my main concern. I guess I wrote this post in a backwards way. I was trying to come up with a way that both cable's shields would be grounded and my imagination got away with me. Also if it worked it'd be a way to idiot-proof the amp, plug one 8 into either jack or two 8's into both. No switch, and you can't get it wrong. Guess I didn't think it out fully. Thanks for setting me straight.

Should I be concerned about series wiring for two seperate cabs?

 If two seperate cabs are in series one jack must be isolated. If you have a something metal on top of your amp and it falls and makes contact with the two speaker cables it will short out one speaker. Not so bad, you'll still have an 8 ohm load on the 16 ohm tap but very bad if you have two 25W speakers and a 50W amp.

This was just an attempt to come up with a way of safely using two seperate 8 ohm cabs with a 8/16 ohm tap OT that just got away with me turned into not needing a switch. My train of thought got way off the track. I really need two 16 ohm speakers(one or two in parallel) or build one cab for the two 8's, or an OT with a 4 ohm tap.

[sluckey]

Look at page 10. May give you another idea.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[jeff]

Good Idea. That solves the problem on the amp side.
Is it a problem that one cab's plug is connected to the other speakers +?
I can't foresee this ever happining, but if you touch that plug you're in parallel with one of the speakers.

[jeff]

Duh, Shrink wrap tubing.

[PRR]

> there's .0131V on the secondary between the 8 and 16

It may be instructive to use "realistic" numbers.

Handy to know/note that 16 volts in 16 ohms is 16 watts, a typical 2-6V6 output, and nice round numbers.

Then 8 ohms would be 11.3V, 8-16 tap is 4.7V. But is still a 16W amp. To suck 16W from 4.7V takes 3.4 Amperes. 4.7/3.4= 1.38.

> if you touch that plug you're in parallel with one of the speakers.

Speaker voltages are _usually_ "safe".

We saw 10 or 15 Volts above.

Take a big tube amp. 75 Watts. In 8 ohms that's about 24 Volts max.

You probably should not mess with plugs while the amp is ROARing. When playing soft, volts are much lower. When idle, voltage is about zero.

In the NEC, electrocution is not a factor for voltages below 30VAC, steady.

Electrocution is very unpredictable. However death-by-speaker is pretty unlikely.

Don't put your tongue on it.

Don't load-up on bennies and whiskey, dance until sweaty and heart 200 thubs/minute, then touch it. (But in that case there are many-many other ways to go.)