Speaker Ohm and combinations information!

[glennc717]

Hello,
I have a green board Fender Blues Jr. I recently replaced the tube (big difference) and installed an 8 ohm Cannabis Rex (wonderful difference IMHO). If I were to decide to get a 212 cabinet, is there a way to hook up the original 8 ohm speaker and the Cannabis 8 Ohm speaker to the amp's 8 Ohm output. Noticeably lacking in amplifier/speaker knowledge. The change would only be for increasing the sound quality (hopefully). Thanks for any assistance!
Glenn

[NickC]

Two 8 ohm speakers in series presents a 16 ohm load.

Two 8 ohm speakers in parallel presents a 4 ohm load.

Those are the only options. Neither solve your problem.

To get a 8 ohm load with two speakers requires:

(1) Two 16 ohm speakers in parallel, or

(2) Two 4 ohm speakers in series

I'm nervous about using series speaker wiring because a failure of either driver breaks the circuit, thus the output transformer would have no load. That's bad.

With parallel wiring, if either speaker fails, the other one still operates. Even though the 16 ohm speaker load driven by the 8 ohm OT is not optimal, it will get you through the gig.

For series wiring, add the sum of all impedances of the speakers in the series.

Here is the formula for parallel wiring:

(1/r1) + (1/r2) + (1/r3) + ... = (1/rT)

[glennc717]

Howdy NickC,
Well series and parallel I had a feeling about that. Thanks for the series wiring warning!!! Impedance, well I forgot anything I learned about that a long time ago. Of course I have to ask the simple question of what would occur with the amp driving a 4 ohm load? Appreciate your kind assistance!
Glenn

[diagrammatiks]

it would be much safer on the transformer to run a 16ohm load then a 4ohm load.

the 4 ohm load will put stress on the output transformer even though it should be relatively safe.

You might want to check the sensitivity of the original speaker to see if it would even pair well with the crex.

[glennc717]

it would be much safer on the transformer to run a 16ohm load then a 4ohm load.

the 4 ohm load will put stress on the output transformer even though it should be relatively safe.

You might want to check the sensitivity of the original speaker to see if it would even pair well with the crex.

Hello diagrammatiks,
Thanks for the info. I am just snowballing the idea. I'd have to get or make a cabinet and such. I don't want to harm the amp, since as you noted I don't even know what it will sound like. Also considering putting the speaker in a seperate 112 quality cabinet with a removeable panel. How about the speakers in series with a heavy duty 16 ohm resistor in parallel? Guessing it would give the appropriate ohms to the amp and would give an alternate circuit, should one of the speakers burn out and protect the transformer? Just a guess of course... Care to give an opinion?
Glenn

[glennc717]

Is this too inexperienced a question and thus no one wishes to answer? Won't be the first time {:-))!
Glenn

[martin manning]

Sure that would work as far as presenting the correct load, but you will cut the sound output power in half.

[glennc717]

Sure that would work as far as presenting the correct load, but you will cut the sound output power in half.

Hello Martin,
Thanks for responding! Is there a simple way to explain the why of that?
Now since I have a 15W that is too loud at 3 and doesn't really get out of the muddled sound at much below, how in your opinion would that effect this issue? Appreciate any help!!
Glenn

[martin manning]

Sure that would work as far as presenting the correct load, but you will cut the sound output power in half.

Hello Martin,
Thanks for responding! Is there a simple way to explain the why of that?
Now since I have a 15W that is too loud at 3 and doesn't really get out of the muddled sound at much below, how in your opinion would that effect this issue? Appreciate any help!!
Glenn

You have two parallel paths of equal impedance (nominally, at least because the speaker impedance is frequency-dependent), one being the speakers and the other being the resistor. That means about half of the amp's output power will be dissipated as heat by the resistor. This would function like an attenuator and reduce the volume about 3dB, which may not be all that noticable. It will change the sound quality too, since the speaker's varying impedance and electromechanical effects on the amp will be diluted (so to speak). This is a good solution if you want to try running both speakers you have in-hand. I'd say use a 25W resistor.

[glennc717]

Thank you Sir,
I can almost get my head around it. Using my existing speakers was the idea. I had a unconfirmed notion that the stock and a cannabis rex in a separate cabinet might make it sound better. But I guess I won't know until I do it. And if it doesn't sound better than I am out the time and cost. Decisions, decisions..... Is there at least, in your opinion of getting better sound by using those speakers in a quality 212 cabinet with the amp separate? thanks again!
Glenn

[martin manning]

Sometimes a mixed pair of speakers can sound better than a matched pair of either type, but if the stock speaker didn't sound that good I wonder if it's worth the effort.

[glennc717]

I appreciate your honest opinion. It is a 20 year old stock fender blues jr speaker. I can say that IMO the replacement Cannabis Rex sounds much better. I don't think it is worth the time or money. Thanks again!
Glenn