Impedance/tube Tolerance Question

[xtjdx]

So, I've got a Musicman HD130. It has a 4/8 ohm selector and two outputs wired in series. So I could set it up for 8 ohms and run two 4 ohm loads, correct? Well, I'm going to be rewiring a cab of mine to be able to use two. Right now it's wired in parallel/series for an 8 ohm load. All speakers are 8 ohms. According to my calculations, I could wire two speakers in parallel and then put the other two in series for a 3.2 ohm load. Would it be safe to run this in this setup? I love this amp and really don't want to damage it by being ignorant about loads and all that fun stuff so any help would be appreciated.

Oh and here's the setup I'm thinking of illustrated.

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|---O---O---|

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Apologies if I'm getting all of this totally wrong.

Edited August 9, 2006 by xtjdx

[mammoth guitars]

It would be highly unusual for an amp to have outputs that are truly wired in series. This would require both outputs to be used or no sound would come out of the speakers.

More likely is that when one or both outputs are used the minimum load is 4 ohms when the selector is set to 4 and a min of 8 ohms when set to 8. This would allow you to connect two 8 ohm cabinets or speakers when the selector is set to 4 ohms.

Generally 4x12 guitar cabs are wired series/parallel to achieve a specific rating of 4, 8 or 16 ohms. A 3.2 ohm load would be to low in this instance and could cause damage to the amp as it would be lower than the minimum of 4 ohms.

[JoeAArthur]

Actually, some of the Musicman amps DO have their speaker jacks wired in SERIES. I believe the HD series is one of those animals. Both do not have to be used - a switching jack simply shorts across it if it isn't being used.

xtjdx - while your calculations are probably correct (I didn't check them but they seem in the ballpark) what you might be forgetting is the unequal distribution of power across the speakers in the cab. In other words, you will have two loud ones and two very un-loud ones.

3.2 ohms isn't that much different than 4 ohms. It won't cause you a problem. Heck people run 6 ohm cabs off either 4 or 8 ohms using the selector as an additional "tone switch".

If you are going to use two cabs - I would wire each cab to 2 ohms by placing all four drivers in parallel and setting the selector to 4 ohms. At least that way all 4 drivers in each cab will be receiving the same amount of power and will sound equally as loud.

Edit: otherwise, just wire up two of the speakers in parallel and leave the other two disconnected for 4 ohms.

Edited August 9, 2006 by JoeAArthur

[mammoth guitars]

Both do not have to be used - a switching jack simply shorts across it if it isn't being used.

This is why the reference to "truly series". They are only in series if both are used.

[xtjdx]

Thanks for the replies, guys. One more question: I do plan on butchering my cab and only using two in parallel. However, these are 50 watt speakers and the head is 130 watts. I have the master around 8 and the volume around 3-4. Should I be worried about blowing them up or is that basically not even close (they're Celestion Rocket 50s). If it's a possibility that they'll blow, I'll just build another cab and use the 2x12 for a different amp and it will sound neat.

[mammoth guitars]

There are two ways the 4 speakers can be wired in series/parallel

1) wire each pair of speakers (2) in parallel (4 ohms) then wire them in series with each other (adds to 8 ohms)

http://home.earthlink.net/~iktoblikto/series_parallel.pdf

2) wire each pair of speakers in series (16 ohms) then parallel (8 ohms)

http://home.earthlink.net/~iktoblikto/parallel_series.pdf

In either case with 4 50 watt speakers you will have a 200 watt power handling capacity.

[xtjdx]

Yeah, I know about those wiring schemes but they don't help since I need a 4 ohm load. What I said was a 2x12 cab for 100 watt handling and 4 ohm load.

[mammoth guitars]

2 8 ohm speakers in parallel will provide a 4 ohm load.

[xtjdx]

2 8 ohm speakers in parallel will provide a 4 ohm load.

Yes. I understand this. But my question is, at the levels I specified above, is there a large risk of blowing up these speakers as it is a 130 watt head?

[JoeAArthur]

Both do not have to be used - a switching jack simply shorts across it if it isn't being used.

This is why the reference to "truly series". They are only in series if both are used.

No, they are in series. I could make the same "truly" argument about parallel jacks.

Series and parallel jacks are a configuration - the configuration doesn't change if you only use one. In a series connected set of jacks, the current does flow through both even if none are being used (NOT recommended).

2 8 ohm speakers in parallel will provide a 4 ohm load.

Yes. I understand this. But my question is, at the levels I specified above, is there a large risk of blowing up these speakers as it is a 130 watt head?

There is always a risk of blowing speakers - even if the amp is underpowered compared to the rating of the speakers (some would even say this is the greater risk).

But no, not a large risk in your case. I've been using a 2x12 40 watt enclosure for maybe 15 years with the original 1968 vintage speakers with amp heads ranging from 65 to 150 watts. Ain't blown any speakers yet.

[xtjdx]

Cool, thanks.