Deluxe outputs

[angelodp]

Hi, I have a deluxe amp with both 4 ohm and 8 ohm outputs. The OT is built with both taps.

I have a great 8ohm speaker that I am using but i want to experiment a bit.

Here is what i want to try.

Take the 8 ohm out to a 16 ohm speaker ( celestion ) and the 4 ohm tap to the cab 8 ohm speaker. Both at the same time.

For the sake of understanding. if I were to take each tap to their proper designation. 8 to 8 and 4 to 4 at the same time what is the total output to the OT.

Ange

[ChrisK]

If I were to take each tap to their proper designation. 8 to 8 and 4 to 4 at the same time what is the total output to the OT.

It's not the "output" to the output transformer, but the load presented to the output stage of the amp that is of most concern. In this case, twice the rated load would be presented to the output stage. Also, the output winding on the output transformer is a tapped winding and you would exceed the ratings of the wire gauge used as well as the power rating of the core.

You will overload/overheat the output transformer as well as overload the output stage.

From a theoretical basis, presenting half the load (twice the impedance) to each tap may present the proper load to the output stage and transformer, but I'd wait for sumgai to give his opinion as empirical data is.

[sumgai]

Two answers, both worth cogitation time.....

First, the theoretical aspects. By hooking up twice the load (a higher-Ohmage speaker) to a jack rated for XΩ, you decrease the current draw from the power amplifier stage. Not by a factor of 2, there are several factors involved, but some small amount.

Now, you want to do that twice, on two different taps.... Well, the first thing is, you're still not making the amp draw as much current as it would if you were to use "properly" rated speakers in each jack, but consider what Chris said above - a "tap" is a shared connection. That means that if you draw X amount of current through the transformer's coil, and then draw more current for another speaker through that same coil portion, then what are you doing to the doubly-used portion of the coil? If it's within design specs, then you're not hurting anything. However, as Chris points out, many transformers are not made to be used in this manner - it's one or the other, baby, and that's the whole story.

To answer that conundrum, I say, consult with your amp's maker - the manual, or online, or at least a factory-authorized service center. If they all say it's OK to do this, then go for it. If they say "one or the other", then think you must, before The Dark Side you go.


Now, in practical terms.

First argument:
Many reputable amp makers use over-rated transformers, just for this reason.... they know that sometimes "experiments" will occur, or that in an emergency, a mis-rated cabinet will be called to fill-in for a blown 'proper' cabinet. Not all of them, mind you, but most of the Brand Names makers are in this camp. It's part of why a tube amp is so heavy, and a large part of why they cost an arm and a leg.

Does that mean that if you've got a Marshall, that I'm giving you permission to perform your mad scientist project? Nope, not at all. I'm saying that you'd be in the same boat as beaucoup other musicians who've done this, at one time or another, but that doesn't mean that you've got a green light, a clean bill of health, a note from your mother, and 25¢ - it only means that if you do this, and you create an environmental hazard (lots of smoke.....), then don't blame me.

Second argument:
Recall that we've discussed wattage, sound output levels, etc. over the life of this forum, and those topics are debated (often heatedly) almost daily across the innerwebs. The bottom line here is, in order to make the amp draw current near its limits, you need to crank it pretty hard. Otherwise, it's just loafing along about 1/100^th to 1/50^th of it's total output power capability. That does give you cart blanche to hook up whatever speakers you choose, in whatever combination you choose, WITH A CAVEAT! Whatever you do, don't crank the volume up past bedroom levels. If you are fluffing the pillows, then you're too bleeping loud for this kind of experiment!! It may sound 'real soulful' to you, but I invoke all the warnings above (and for the sake of completeness, I'll invoke all of Chris's warnings too) - you are at high risk of major wallet-deflation, not to mention the doorbell being rung by someone in blue who is not there to sell you tickets to the Policeman's Ball.

I've done what you've outlined, and I'd do it again, but only at lower volume levels, period. Or in an absodamnlute emergency, like at a paying gig. But I'd turn it down more than a bit - no sense in blowing up two cabinets, or worse....

HTH



sumgai

[angelodp]

Very good gentlemen thanks for the rundown.

Looks like an extension out might be the way to go, and stick to the single tap at the appropriate rating for the speakers.

ange

[sumgai]

Aug 27, 2009 14:54:59 GMT -5 angelodp said:

.....Looks like an extension out might be the way to go, and stick to the single tap at the appropriate rating for the speakers.

I think you're on the right track. Combining the cabinets such that they present a load (impedance) to the amp that is approximately equal the output impedance rating of any one single jack is the proper way to go.

You've got the part about series and parallel and all that stuff down, right?

HTH




sumgai

[angelodp]

paraseries and sparallel