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Using a dummy plug on an early 70s blue line Ampeg SVT

whitespikebass

whitespikebass

Boy Orbision
Feb 19, 2013
205
37
Austin, TX
#1
I touched on this buried in another thread. But I want to get more information so I thought I would start my own.

I have an early 70s SVT and I use an 8 ohm cab. My tech told me that I should use a dummy plug in one speaker jack to make the load 4 ohm. He said that using an 8 ohm cab isn't good for the amp. However, Jimmy (who I know is probably right) says that the dummy plug makes the load 2 ohm.

I've put a considerable amount of time, energy, and money fixing up my SVT and I want to make sure I protect it.

My main questions are:

1. Does the using the dummy plug in one speaker jack and an 8 ohm cab in the other create a 4 or 2 ohm load? The tech says normally it wouldn't work but does with this particular amp.

2. Does using an 8 ohm cab hurt the amp? Normally using an 8 ohm cab with an amp just lessens the power. If that's all it does, that should be fine as this is a loud amp anyhow.

3. Should I just panic and go get a 4 ohm cab just to be sure? If so I would be most interested in the SVT-212AV. Would that be a good match?
 
#3
You are going to get conflicting answers on whether or not you can run an 8 Ω cab with an SVT. Can you do it? People do. Should you do this? I prefer not to. It doubles the impedance that the tubes see. Doing this changes how the amp performs and the tone will change. Depending on the amp, you can damage the tubes and output transformer. It is better for a tube amp to operate with a lower impedance cab than a higher impedance cab. Using the correct impedance with your SVT will optimize power output and headroom.

Your amp is designed to use one 4 Ω cab or two 4 Ω cabs. With a dummy plug, you can connect a 2 Ω cab. You do not want to use a dummy plug and an 8 Ω cab. You would be connecting an 8 Ω cab when the amp is expecting 2 Ω.

Getting a 4Ω cab and doing it right would be a good idea. You have an 8Ω cab. If you had two 8Ω cabs, they could be connected in parallel to give you 4 Ω.
 
#5
I have a 70s V4 4x12 cab meant for guitar. I originally paired it with a V4 head too, but I upgraded to an SVT. I put in Eminence 12a's 8 ohm in series parallel (I think thats what its called) to achieve an 8 ohm load. With the V4 this was fine because the head could switch between 4 and 8 ohms.

The tech told me it was best to use the 8 ohm with a dummy plug, rather than wiring it traditionally at 2 ohms.
 
#7
whitespikebass said:
The tech told me it was best to use the 8 ohm with a dummy plug, rather than wiring it traditionally at 2 ohms.
Click to expand...

Your tech has it backwards. As David said, not only is it bad for an SVT to run at 8 ohms (since the transformer is tapped for 2/4 ohms) the dummy plug in the main speaker output would switch the amp from the 4 ohm tap to the 2 ohm tap. Since the V4 can do 2 ohms, your best choice would be to run the speakers in parallel for 2 ohms and then the cab could be used with either amp.
 
#8
Since you have four 8Ω speakers, they can all be wired in parallel to give you 2Ω. Then with the dummy plug, you can plug the cabinet into the SVT and the impedances will be matched.

Maybe that is what your tech was getting at.
 
#9
You could also wire it to optionally use two 4 ohm inputs similar to what the current 810e's do with two 8 ohm inputs. How to do it, I have no idea, but it can be done. Requires drilling, though.
 
#10
beans-on-toast said:
Since you have four 8Ω speakers, they can all be wired in parallel to give you 2Ω. Then with the dummy plug, you can plug the cabinet into the SVT and the impedances will be matched.

Maybe that is what your tech was getting at.
Click to expand...

Hmmm, the work was done quite a while ago. Surprised I haven't hurt anything. But now I am curious to know if this is what he said.

Can you tell me how to check the wiring? I am almost positive he put it to series parallel. I put the speakers in myself initially. Thats what sucks. The "normal" way I put it in would have been 2 ohms and I paid him to change it. If someone could tell me how I think I could do it myself no?

I would prefer to keep this cab. I really like it. And I already own it!
 
#11
You can check the impedance with a digital voltage meter. Set it to the lowest DC resistance measurement and the meter should read about 3/4 of the cab impedance. Plug a speaker cable into the cab and touch the probes to the free end of the cable, one to the tip, one to the sleeve. If it's wired series/parallel for 8 ohms the meter should read around 6 ohms. If it's been wired in parallel for 2 ohms it will read a little below 2 ohms.
 
#12
whitespikebass said:
Surprised I haven't hurt anything. But now I am curious to know if this is what he said.

Can you tell me how to check the wiring? I am almost positive he put it to series parallel. I put the speakers in myself initially. Thats what sucks. The "normal" way I put it in would have been 2 ohms and I paid him to change it. If someone could tell me how I think I could do it myself no?
Click to expand...

As Corey said, use an Ω meter to measure the DC resistance of the cab.

Series parallel would be 8 ohms [ (8 and 8 in series) parallel to (8 and 8 in series) ]. All parallel would be 2 ohms [8 and 8 and 8 and 8 all in parallel]. In the latter case all the + on the speakers are connected together and that goes to the tip of the ¼" jack; all the other terminal are connected together and that goes to the sleeve of the ¼" jack.

Series-Parallel

Parallel

If you only have one jack on your cab ignore the OUT jack. It's a parallel jack for connecting more than one cab (what they call parallel daisy chaining). If you have a speakon connector on your cab, +1 replaces the jack tip, -1 replaces the jack sleeve.
 
#13
The Ampeg SVT was designed to use two (2) 4 Ohm loads. Pluging an 8 Ohm load into the amp will be fine.

Configuring the amp for two (2) Ohm with the dummy plug is ok also, but you will only realize about 100 Watts.

That dummy plug sets switches inside the amp.

The SVT into 8 Ohm from the 4 Ohm tap will probably make about 250 Watts max, as it is a tube amp and will do its best with that miss-match.

The output transformer as I recall has two secondaries, each 4, 8, and 16 Ohm.

When paralleled inside the amp yields 2, 4, and 8 Ohm.
 
#15
Yes, as I stated there are two secondary windings. Miss-matching 8 on four wont hurt anything.

The two winding configuration was employed to leave future options open.

Also made the transformer easier to design and wind.
 
#16
Interesting comment about the dual winding of the output transformer. I always figured that it was a way to keep the OPT more manageable. I recall that when Marshall first came out with their 100W amp, they used two output transformers wired in parallel because they couldn't find one at the time to handle 100W when the amp was being developed.

David
 
#17
If all four speakers were paralled, and the dummy plug employed, you would probadly burn out the voice coils on those Eminance drivers.

I'd say the situation you have with that miss-match is probably the best situation.

I would leave it alone.
 
#18
I'm pretty confused. So much opposing info I have found on this subject.

I definitely don't want 100 watts. I have heard over and over that an 8 ohm cab will hurt my SVT. I hope you're right bill, 250 watts at 8 ohms is fine. 100 watts at 2 ohms is not! I always thought 2 ohms would produce MORE power.
 
#19
Bill I feel pretty positive that my cab is 8 ohms series parallel. And I have been using that and the dummy plug for a couple years. That being said. I have had to have my amp recapped and tubed during this time. I don't know if thats a coincidence, but sounds like it would be hurting the speakers and not the amp?
 

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