AMPS & OHMS QUESTION.....

Geetarguy

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I need your help...tell me how to hook this up.
I have a Crate Club Vintage Head (50watts) and two (2) Marshall 4x10 cabinets. Each Marshall cab is rated at 8 ohms. My question is....Do I set the head to 8 ohms or 16 ohms when using both cabinets? I will be running both speaker cables from the head into each individual cabinet.
Thanks in advance for your help.
 

Geetarguy

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No switch on the head for 4 ohms.....only 8 ohms and 16 ohms........maybe I can't use both cabs????
 

b-squared

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Ummm...if the head has two speaker outs, then setting the head to 8 ohms means that both outputs are 8 ohm taps, right?

It wouldn't make much sense to put two outputs and only make one usable at that setting.

If the cabs were daisy-chained I'd set it at 4, for sure.

BB
 

Geetarguy

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Good point BB. I just wanna make sure I don't damage my amp......I'm looking more into this and thanks for your help...
 

Big John

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If your head doesn't have a 4 ohm setting, do not use both cabs with standard speaker cables. One way to use both cabs safely in your situation would be to fabricate a series speaker cable, which would put a load of 16 ohms to your head. Here you go...

seriesspeakercable.jpg


Another way would be to build a series splitter box, which would allow you to use standard speaker cables.
 

Geetarguy

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Big John...
thanks much my friend.....however, I just found out that if I use the 8 ohm setting on this particular amp then both speaker outputs are 8 ohms. The diagram that you have suggested is great too...cause I can use something like that with my sovtek head (which is the tone I really want)...sooooo.....thanks again for your input I will definitely use it....
g
 

Bowhunterwt

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You could rewire the cab to 16 and then run both cabs and set the amp to 8~!!!!! :fingersx::hmm:
 

Big John

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Hold on!!

According the manual, your amp will not run at 4 ohms. It runs at either 16 or 8. The speaker outputs jacks on your amp are parallel jacks. If you want to run both Marshall cabs (8 ohms each) with each cab getting plugged into it's own jack on the amp, that means the total ohms at the amp will be 4 ohms. I wouldn't do it if I were you. Either make the series cable or build a series box...

seriesbox.jpg
 

b-squared

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Ah...he didn't say it had a MAIN output plus an EXTENSION cab output.

That makes it a different story.

BB
 

Geetarguy

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Ah...he didn't say it had a MAIN output plus an EXTENSION cab output.

That makes it a different story.

BB

Yeah BB...thats exactly what it has.......a MAIN output plus and extension cab output.....it is even labeled like that on the back of the head...its says "MAIN use this first"...... Then the other output say "extension cab"
what do you guys think????
 

dtube

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In order to have dual 8-ohm outputs, your tube amp would need to have an 8-ohm load hanging on both at all times. A tube output section goes up in smoke without a load connected to it. Extension cab outputs are always parallel jacks. So, your only choice is to make the series connection per Big John's advice above and switch to 16-ohm.
-Darren
 

Geetarguy

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In order to have dual 8-ohm outputs, your tube amp would need to have an 8-ohm load hanging on both at all times. A tube output section goes up in smoke without a load connected to it. Extension cab outputs are always parallel jacks. So, your only choice is to make the series connection per Big John's advice above and switch to 16-ohm.
-Darren

dtube...point well taken. However, I have a question for you. I am looking at the back of another amp head (blackheart little giant) and there are speaker outputs as follows: (2) 4 ohm (2) 8 ohm and (1) 16 ohm. I am using the 16 ohm output on the head into the 12" little giant cab (which is a 16 ohm cab). If what you are saying is true (not doubting here just trying to understand) then the other speaker outputs on my little giant head would not have a load connected to them......wouldn't this be bad for the amp also? Am I missing something? Again, thanks for all of your help (all of you guys)!!! This ohm thing has had me mystified ever since I started playing tube amps.
thanks again,
g
 

dtube

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dtube...point well taken. However, I have a question for you. I am looking at the back of another amp head (blackheart little giant) and there are speaker outputs as follows: (2) 4 ohm (2) 8 ohm and (1) 16 ohm. I am using the 16 ohm output on the head into the 12" little giant cab (which is a 16 ohm cab). If what you are saying is true (not doubting here just trying to understand) then the other speaker outputs on my little giant head would not have a load connected to them......wouldn't this be bad for the amp also? Am I missing something? Again, thanks for all of your help (all of you guys)!!! This ohm thing has had me mystified ever since I started playing tube amps.
thanks again,
g

Good question. Output transformers can have multiple taps on the secondary for various output loads. But, they're all coming off the same secondary winding. So, the correct load connected to its designated output jack will "satisfy" the transformer. If you have (2) 16-ohm cabs, you would plug them into the (2) 8-ohm outputs because they will be in parallel, thus the transformer sees a total 8-ohm load. What you can't do with that head is connect a 4-ohm load to the 4-ohm tap, an 8-ohm to the 8-ohm, and a 16-ohm to the 16-ohm jacks at the same time; what you could do though is plug a 16-ohm load into the 8-ohm jack and an 8-ohm load into the 4-ohm jack to run both cabinets. Why the correct load on the output transformer is critical is because of "reflected impedance". That means that if a transformer has an 8-ohm load on the 8-ohm output the secondary will reflect the correct load to the primary which will in turn show the output tubes the correct load. If the secondary load is cut in-half, then the load to the tubes is cut in-half was well. Think of that as your car having its cruise-control set at 50-mph and suddenly half the weight of the car is removed, causing the car to go 100-mph. Through this example, you can see what would happen if there was no load connected to your amp and it was turned on. Generally speaking, most tube amps can handle their load being cut in-half (unless we're speaking about the Super Reverb or Bassman that has a 2-ohm load). But doubling the load can result in "flyback" which can destroy an output transformer. Please note that these rules are pretty much the exact opposite of what you would do with a solid-state output section.

I hope this makes sense - I haven't had any coffee yet...
-Darren
 

Geetarguy

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Good question. Output transformers can have multiple taps on the secondary for various output loads. But, they're all coming off the same secondary winding. So, the correct load connected to its designated output jack will "satisfy" the transformer. If you have (2) 16-ohm cabs, you would plug them into the (2) 8-ohm outputs because they will be in parallel, thus the transformer sees a total 8-ohm load. What you can't do with that head is connect a 4-ohm load to the 4-ohm tap, an 8-ohm to the 8-ohm, and a 16-ohm to the 16-ohm jacks at the same time; what you could do though is plug a 16-ohm load into the 8-ohm jack and an 8-ohm load into the 4-ohm jack to run both cabinets. Why the correct load on the output transformer is critical is because of "reflected impedance". That means that if a transformer has an 8-ohm load on the 8-ohm output the secondary will reflect the correct load to the primary which will in turn show the output tubes the correct load. If the secondary load is cut in-half, then the load to the tubes is cut in-half was well. Think of that as your car having its cruise-control set at 50-mph and suddenly half the weight of the car is removed, causing the car to go 100-mph. Through this example, you can see what would happen if there was no load connected to your amp and it was turned on. Generally speaking, most tube amps can handle their load being cut in-half (unless we're speaking about the Super Reverb or Bassman that has a 2-ohm load). But doubling the load can result in "flyback" which can destroy an output transformer. Please note that these rules are pretty much the exact opposite of what you would do with a solid-state output section.

I hope this makes sense - I haven't had any coffee yet...
-Darren

dtube,
thanks for the info...it's gonna take me awhile to digest all of this info. Gonna read it over ..and over... again till I get it right....lol.... One more question ....Do you know (or any of you other guys know) if they make a series speaker cable or series splitter box (like what Big John is talking about) that I can purchase....and where I might be able to purchase something like this??? If not I guess I'll have to try and make one myself (not too good with a solder iron here). Again..many thanks to all who took the time to help with this. You guys rock!!! :dude:
guy
 

LPV

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dtube...point well taken. However, I have a question for you. I am looking at the back of another amp head (blackheart little giant) and there are speaker outputs as follows: (2) 4 ohm (2) 8 ohm and (1) 16 ohm. I am using the 16 ohm output on the head into the 12" little giant cab (which is a 16 ohm cab). If what you are saying is true (not doubting here just trying to understand) then the other speaker outputs on my little giant head would not have a load connected to them......wouldn't this be bad for the amp also? Am I missing something? Again, thanks for all of your help (all of you guys)!!! This ohm thing has had me mystified ever since I started playing tube amps.
thanks again,
g

As stated, on the LG head its a little confusing. Those jacks are parrell wired.
So on the LG, 2x8 ohm cabs have to use both 4 ohm jacks.
 

dwagar

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Hold on!!

According the manual, your amp will not run at 4 ohms. It runs at either 16 or 8. The speaker outputs jacks on your amp are parallel jacks. If you want to run both Marshall cabs (8 ohms each) with each cab getting plugged into it's own jack on the amp, that means the total ohms at the amp will be 4 ohms. I wouldn't do it if I were you. Either make the series cable or build a series box...

seriesbox.jpg

that's a cool idea. Thanks.
 

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