Mixing my ohms

[bassjamm]

Hi all...

Weird one and i've just done a quick search...

I've got an SVT head that can go down to 2 ohms, it has a 4 ohm/2 ohm toggle on the back. I've been using a single 8 ohm cab on the 4 ohm setting with no problems, but i'm considering getting a 4 ohm cab. Not i know it'll work with on it's own, but what if i were to use an additional 8 ohm cab, thus production a load of 2.67 ohms in total? Would i simply need to toggle the switch on the back to 2 ohms and because i'm not exceding the 2 ohm limit, i'll be fine?

If that is fine, then how would the cabs fair, would the 8 ohm cab be delivered too much power? In other words, when you use to 8 ohm cabs, does the power get split evenly? If so, when i use an 8 ohm and a 4 ohm, would the same split occur, and if it would, would the 8 ohm cab be ok with it? Or does this not really matter...? Hope that makes sense.

Thanks guys and gals
Jamie

[Alien]

I'd have thought it's best to use the 2 Ohm setting.

Mixing a 4 and an 8 will send 2/3 of the power to the 4 Ohm, 1/3 to the 8 Ohm . With 2 cabs of the same impedance, the split is 50/50.

Andy

[obbm]

According to [url="http://www.geofex.com/tubeampfaq/taffram.htm"]this[/url] it would appear that it would be safer using the 4-ohm tap.

[quote]The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

[i]It's almost never low impedance that kills an OT, it's too high an impedance. [/i]

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high.

For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load.[/quote]

[bassjamm]

[quote name='obbm' post='226400' date='Jun 25 2008, 07:54 AM']According to [url="http://www.geofex.com/tubeampfaq/taffram.htm"]this[/url] it would appear that it would be safer using the 4-ohm tap.[/quote]

Hmm...just read through that but didn't see that bit. I saw a bit about mixing ohms but it didn't make any sense. I read there that you shouldn't exceed the amps load though as it can cause it to short out. Surely if i were to use the 2.67 overall load of the 2 cabs, on the 4 ohm switch, then i would in effect be putting too much of a load on the amp when in that mode...? But not so if i used the 2 ohm switch?

[obbm]

[quote name='bassjamm' post='226444' date='Jun 25 2008, 08:59 AM']Hmm...just read through that but didn't see that bit. I saw a bit about mixing ohms but it didn't make any sense. I read there that you shouldn't exceed the amps load though as it can cause it to short out. Surely if i were to use the 2.67 overall load of the 2 cabs, on the 4 ohm switch, then i would in effect be putting too much of a load on the amp when in that mode...? But not so if i used the 2 ohm switch?[/quote]

It's not about exceeding loads as with a solid state amp. You mustn't think about a valve amp in the same way as a solid-state amp. In a valve amp the output transformer is there to provide a virtual load for the output valves. There are separate output taps so whether you use, in your case, 2-ohm or 4-ohm speakers, the load on the output valve anodes remains the same and that the amp is operating on the optimum part of its load-line. If you put a mismatch on then you are altering the performance characteristics. This phenomenon does not occur in solid-state amps.

What he is saying is that if you set your tap to 4-ohms and use a speaker of impedance of equal to or less than that then it should be OK.

If you are unsure about this the contact Ampeg technical Support and get the word from them. They designed it so they should be able to advise you according. The other alternative is to get another speaker 4-ohm or 8-ohm cab and do it properly.

[bass_ferret]

proceed with extreme caution - it would be silly to f*** up an ampeg doing this. You wont hear anything from the 8 ohm speaker anyway unless it is more efficient than the 4 ohm one.

[gypsymoth]

as you may have deduced by not smoking your amp running an 8 ohm cab, valve heads are typically OK seeing a slighly higher load than expected - and not so happy with a lower than expected load. severe mismatches in either directions can cause severe problems.
as noted, twice the watts will go to the lower load cab, so make sure those speakers are up to the task and unless the other speaker is more efficient - don't expect to hear it much.

2.6 ohms, use the 2 ohm setting. BUT for all the reaons everyone has noted, you and the amp would be way better off with matched impedance.

Edited June 25, 2008 by gypsymoth

[bassjamm]

Right oh chaps...that's answered that

I've been running the 8 ohm cab without any issues, and then doubled up with another 8 ohm to create a 4 ohm load when needed.

From what i've gathered then, only put a 2 or 4 ohm load into the amp, whether that be a pair of 4 ohm cabs or a pair of 8 ohm cabs, rather than mixing about with un-matched loads!!! Sorted!