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Question about head/cab ohms

beebassdude

beebassdude

Supporting Member
Sep 29, 2008
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Ashburn, VA
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#1
i recently purchased a SVT VR (which completely rules by the way) and im running it with a 2005 ampeg 810E. the head is 4ohms and the cab is 4 ohms as well. im looking at getting a 2x15 cab to go with it, preferably an old sunn 215S, but the ohm rating on that cab is 8ohm/300 watts.

my question is, if i use this 4ohm head with an 8ohm cab, is it going to mess one of them up? or will the sunn cab only be getting half power from the head? im a noob with ohm/wattage stuff so if one of you guys can explain this id be forever grateful. thanks!
 
#5
1. Combining 4 and 8 ohm cabs on one amp will get you 2.67 ohms. If the amp can't handle 2 ohms, you're out of luck.

2. An 8 ohm cab will always draw half the power of a 4 ohm cab when they're both running together.

3. That Sunn 215 is from an era where band volume was far lower than today and cabs were not designed as they are today. It may not perform well at all when compared with today's cabinets.
 
#6
Rick Auricchio said:
1. Combining 4 and 8 ohm cabs on one amp will get you 2.67 ohms. If the amp can't handle 2 ohms, you're out of luck.

2. An 8 ohm cab will always draw half the power of a 4 ohm cab when they're both running together.

3. That Sunn 215 is from an era where band volume was far lower than today and cabs were not designed as they are today. It may not perform well at all when compared with today's cabinets.
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the amp has a switch for 2 ohms and 4 ohms. so if i did this, i would have to switch the button to two ohms? is that going to affect the volume?

in your opinion, if i want to run an 810 and 215, what ohms should each cab be for me to be safe doing this?

thanks for replying man, i apprecite the help. :bassist:
 
#7
Your VR has a switch that allows operation with a 4 ohm speaker load (one 4 ohm cabinet or two 8 ohm cabinets) or a 2 ohm speaker load (two 4 ohm cabinets or 4 8 ohm cabinets). Using a 4 ohm cabinet and an 8 ohm cabinet together (in series) yields an equivalent impedance of 2.67 ohms. I don't have any direct experience with this particular head, but I would suggest just setting the switch for a 2 ohm load and then using both cabs. And yes, when you use a 4 ohm cabinet and an 8 ohm cabinet in series, the 8 ohm cabinet dissipates half as much power as the 4 ohm cabinet.
 
#8
scottbass said:
Your VR has a switch that allows operation with a 4 ohm speaker load (one 4 ohm cabinet or two 8 ohm cabinets) or a 2 ohm speaker load (two 4 ohm cabinets or 4 8 ohm cabinets). Using a 4 ohm cabinet and an 8 ohm cabinet together (in series) yields an equivalent impedance of 2.67 ohms. I don't have any direct experience with this particular head, but I would suggest just setting the switch for a 2 ohm load and then using both cabs. And yes, when you use a 4 ohm cabinet and an 8 ohm cabinet in series, the 8 ohm cabinet dissipates half as much power as the 4 ohm cabinet.
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but the 810E i use currently says on the inputs that they are either 4 or 8ohms. if i got this sunn cab at 8ohms, used the VR at 4 ohms, how do i get the 810 to be in 8ohm mode instead of 4ohm mode?
 
#9
The VR is rated at 300 watts. If you use an 8 ohm cabinet with a 4 ohm cabinet, the 8 ohm cabinet will be "getting" (but not really) 100 watts and the 4 ohm cabinet will be "getting" (but not really) 200 watts. You said that old Sunn cabinet is rated for 300 watts - it should be OK if the amp is only "giving" it 100 watts.

The real danger is if you drive the amp too hard and create clipping. That will probably fry up that old Sunn cabinet pretty quickly. But if you keep the volume at reasonable levels everything should be OK.

If it were me, I would look for a 4 ohm 2x15 cabinet. That would match up with your 4 ohm 8x10 and create a total equivalent impedance of 2 ohms.

But everything should be OK with that 8 ohm Sunn cab. It will just be quieter than your 8x10 when you use them both together.
 
#10
scottbass said:
The VR is rated at 300 watts. If you use an 8 ohm cabinet with a 4 ohm cabinet, the 8 ohm cabinet will be "getting" (but not really) 100 watts and the 4 ohm cabinet will be "getting" (but not really) 200 watts. You said that old Sunn cabinet is rated for 300 watts - it should be OK if the amp is only "giving" it 100 watts.

The real danger is if you drive the amp too hard and create clipping. That will probably fry up that old Sunn cabinet pretty quickly. But if you keep the volume at reasonable levels everything should be OK.

If it were me, I would look for a 4 ohm 2x15 cabinet. That would match up with your 4 ohm 8x10 and create a total equivalent impedance of 2 ohms.

But everything should be OK with that 8 ohm Sunn cab. It will just be quieter than your 8x10 when you use them both together.
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then i will look for a 4ohm 2x15 instead. i play at stupid volume levels and dont wanna fry anything lol.

so when i get whatever 2x15 i get, ill have two cabs at 4ohms each. so do i switch the head to 2 ohms at that point, or leave it at 4 ohms?

and if you know, do those ampeg 215Es come in 4 ohms?
 
#11
Yup, switch to 2ohm duty and fire away. Even though the 2x15 will only get half as much power as the 8x10, it'll probably kick @$$.
If you aren't sure, check the manual for your 8x10, or download another from Ampeg. It explains how to use the stereo/mono input jacks properly. Basically you just want to avoid the uppermost 1/4" jack-that one splits the cab into stereo, and raises the impedance to 8ohms, because only the upper 4 speakers will operate.
 
#12
+1 on getting another 4 ohm cab.

Or running your 810 at 8 ohms, to have a total impendance level of 4ohms

Someone can correct if I am wrong, but you should only match impendance levels on tube amps. If I remember correctly, tube amps really don't like 2.67 ohms because its not an exact impendance, like 4 or 2 ohms.

This is a good question for JimmyM, but I am going on my best memory, and also my experience with my friends with tube amps.
 
#13
beebassdude said:
then i will look for a 4ohm 2x15 instead. i play at stupid volume levels and dont wanna fry anything lol.
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IMO you're better off with an 8ohm 2x15. They can't handle the abuse that the 8x10 can.
so when i get whatever 2x15 i get, ill have two cabs at 4ohms each. so do i switch the head to 2 ohms at that point, or leave it at 4 ohms?
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2ohms either way.
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#15
nightcityburn said:
+1 on getting another 4 ohm cab.

Or running your 810 at 8 ohms, to have a total impendance level of 4ohms

Someone can correct if I am wrong, but you should only match impendance levels on tube amps. If I remember correctly, tube amps really don't like 2.67 ohms because its not an exact impendance, like 4 or 2 ohms.

This is a good question for JimmyM, but I am going on my best memory, and also my experience with my friends with tube amps.
Click to expand...
Nah, it's more of a question for Johnk 10.
 
#17
When used in combination with your existing 4 ohm cabinet, an 8 ohm cabinet will get LESS power than will another 4 ohm cabinet:

4-ohm 8x10: 200 watts
8-ohm 2x15: 100 watts

as compared to:

4-ohm 8x10: 150 watts
4-ohm 2x15: 150 watts

What I don't have any experience with is setting a VR for 2 ohm output and then loading it with a total equivalent impedance of 2.67 ohms, as you would be doing if you run a 4-ohm cabinet with an 8-ohm cabinet. My intuition tells me it would be OK to do this, but I have not actually done this myself and I don't know the intricacies of the VR well enough to tell you it would be OK.
 

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