JeffBlue
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I believe I already know the answer, I was hoping to use a 4X6 cab combined with my Vibro Champ speaker.
2.5 Ohm Speaker Load On A 4 Ohm OT?......................
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JeffBlue
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After confirming with some brethren from other sites, I have connected my 4X6 cab with the speaker in my Vibro Champ with amazing results.
corpse
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I wouldn't do that- you might try an 8 ohm load into a 4 ohm amp but not the other way around. There are tons of stories how incredible an amp sounds just before a transformer pops.
Don
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Is the impedance of the cab 2.5 ohms or is that the resistance measured on a DMM?
JeffBlue
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I am running an NOS 5881 which, I am told, wants to see half of what a 6V6 has for impedance and I am not feeling any heat to the touch on the OT. I replaced the OT with a Mercury Magnetics 55 tweed OT.
Don
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In that case, you should ask MM. They'd know for sure.
montesada
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I wouldn't be worried. 2.5 on a DMM, the speaker array would be more like 3.1 in reality. Close enough.
Jeff, Paul at MM is very helpful with technical questions, ime. I think your amp will be fine at that impedance. Maybe one would worry more at double that load????
Fwiw, the OT is not where heat will show up. PT's are the ones that exhibit heat when current draw is too high.
Fwiw, the OT is not where heat will show up. PT's are the ones that exhibit heat when current draw is too high.
JeffBlue
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The DC resistance measured with a DMM is fairly meaningless, it does not tell you what the impedance load is. If your cabinet has 4 x 4 ohm speakers wired series / parallel then it is a 4 ohm cabinet, if that is then connected in parallel to the internal 4 ohm speaker you have a 2 ohm total load.
A bit more info: I just re-read the thread and see that you are using a 5881 in place of the 6V6, as you correctly say this valve (I am British so it is definitely a valve and not a tube! 
) likes to see a lower impedance than the 6V6 so running a lower speaker impedance should be no problem. My main concern would be that your transformers can handle the extra current requirements of the 5881, in particular the heater current. The 5881 will pull twice the heater current of the 6V6, 0.9A as opposed to 0.45A for the 6V6, you need to be sure that your PT can safely supply this.
This is actually incorrect for a valve amp, if you are going to run an impedance miss-match it is safer to run with a lower impedance than a higher one (the reverse is true for solid state amps)
) likes to see a lower impedance than the 6V6 so running a lower speaker impedance should be no problem. My main concern would be that your transformers can handle the extra current requirements of the 5881, in particular the heater current. The 5881 will pull twice the heater current of the 6V6, 0.9A as opposed to 0.45A for the 6V6, you need to be sure that your PT can safely supply this.This is actually incorrect for a valve amp, if you are going to run an impedance miss-match it is safer to run with a lower impedance than a higher one (the reverse is true for solid state amps)
JeffBlue
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Hi all. Thank you for your replies and info. I decided to relinquish the 4X6 cab because the power transformer is running quite warm to the touch while I'm running my air conditioner. I'd hate to feel how warm to the touch without the air running. The VC still sounds beautiful with just the 10 inch speaker I installed.
PaulD made a good observation in his post that preceded this last post of yours. That heat is likely to be caused by the excess current draw of the 6L6 rather than the speaker load. That 6L6 is demanding a lot of that little PT.....it doesn't matter if the load is 2.6ohms or 3.2 ohms.
JeffBlue
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I'm thinking it might be time to switch back to the 6V6 and add the 4X6 cab again. Thanks Wally and Paul.
DrRobert
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CHamps/Vibrochamps, in general, have a weak OT and do badly if it's stressed. A lower impedance isn't a stress to the OT but a higher one is. Every tech I've talked to about it has a story of a Champ that's died running into an 8 ohm speaker load. It would be VERY unusual for an OT to get warm, the problem is in momentary voltages (flyback voltage) burning thru the insulation of the wires and eventually shorting it out. They flyback voltage can also cause arcing in the circuit, but that's not likely at the voltages that Champs run. A too low speaker impedance runs the tubes out of their sweet spot and they can wear earlier but rarely take out the amp catastrophically.
If you're feeling warmth, it's probably the power transformer. And THAT may not tolerate the 5881. But it'll be fine with the wrong speaker load, it's not a stress on the PT in any way.
If you're feeling warmth, it's probably the power transformer. And THAT may not tolerate the 5881. But it'll be fine with the wrong speaker load, it's not a stress on the PT in any way.
JeffBlue
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I have decided to run the 5881 and the 4X6 cab (with the 10 inch speaker). After running the amp at any length of time, the PT isn't actually hot......I think I was just overly cautious. The amp sounds and functions beautifully and I am not having any issues. Thank you all for your input. If there ever is an issue with my set up, I'll definitely inform you.