Ampeg SVT-CL question

To technically answer your question....
The SVT output transformer plate load is 1.6 K at 4Ω jack and 1.75 K at the 2Ω tap....
So putting an 8Ω cabinet on the 4Ω jack will present a 3.2K plate load...
The plate load is what the six 6550A tubes work into......
Looking at the equivalent of what a pair of 6550A tubes would work into would be 9.6KΩ plate load....
Essentially this will significantly reduce the AC current swing, thus lowering the available power output....
The AC voltage swing will roughly be the same ......
So to answer your question.... The SVT amp should be fine with respect to running it at 8 Ohms..

 

There are some in here that say run it at only the impedance listed. There are some that said an 8 ohm load will not necessarily hurt it. The guy that designed the thing has said it can be run at 8 ohms with no long term damage. I always run mine at the exact load on the switch , usually 4 ohms with an 8 10e cab. I did run it once at a 2 ohm load with two 8 10s. A few hours later , we had the east coast earthquake. Coincidence?
At any rate , can it be run at off loads? Yes. Is that ideal for the head? No. Personally , I wouldn't risk it. It's a $2000 head. Be nice to it. As far as running it at in between loads , I have never researched that so I will withhold any comment.
Oh yeah , never power the thing up without the speakers being connected. The results of that can be catastrophic.

 

This is about the size of it. An 8 ohm cab shouldn't hurt it. I've done it on rental gigs where they provided me a 410he instead of the 810e I asked for (cheap bums!), and never once had a problem. And when Bill Hughes says it's so overbuilt that it can easily take a 2:1 impedance mismatch, you have to listen because he was the co-inventor. I wouldn't worry.

 

Every amp is unique, so each situation needs to be technically evaluated on a case by case basis....I agree that " blanket" statements are not 100% reliable....
In the case of the SVT, 8 ohms wont hurt it.....
The reason it isn't a big deal is because the AC current through the transformer windings is REDUCED when running at 8 Ohms...
There is a limit obviously on how big of a load you can go before you get fly-back and resonant peaking that would make the transformer arc over...
When an amp is run at an impedance LOWER than what it should be at ...you have an INCREASE in AC load current..and that is what can burn up the wire in the Output Transformer...
So depending on the wire gauge in the transformer it may or may not handle the extra current....
I have been designing and winding transformers for close to 43 years.... I know the construction of the SVT output transformers intimately .... 8 Ohms wont bother the windings...

 

In general the cost of running an SVT at 8 ohms is the amp will make less power and the tubes will wear out faster. This is paraphrased from various TalkBass post from Bill Hughes who is largely responsible for designing the original SVTs.

 

So I've heard, but I figure if they're working today, they'll work tomorrow. And if not, then I'll hope it's just a preamp tube

 

You don't present as someone who routinely thrashes his SVTs. The harder you push the amp, the harder the impedance mismatch is on the tubes.

 

Get an HLF410. 4ohms and has balls for days. New tubes are not cheap. I want them to last as long as possible. I have been gigging with this setup for about 6 months. Get compliments on my tone all the time. Enjoy an amazing amp! It truly is the “voice of the gods”!

 

I ran an SVT into an 8 ohm load for a while years ago through ignorance, the power difference was significant to the point of a guitarist telling me I needed new power tubes, speaking of which they did wear out quite a bit faster, I never had a problem with the head though. But when I did realize I had the speakers connect wrong and changed it to the correct 4 ohm load it was like getting a new amp it sounded so much better. A tube head should always be run at the proper impedance. I've had problems with Fenders and Marshall's running them at higher impedances but Ampegs are built like tanks. Your 4 ohm cab will be louder than your 8 ohm cab I'm pretty sure.

 

I agree that it's best to run a tube head into the expected impedance. For a couple of big band gigs I ran an SVT into one of these and it was very loud and HiFi despite running at 8 ohms.

 

I bet it was, I think you showed me those before, cool speakers. When I was a kid before Ampeg made any 410's I cut an SVT cab in half and made the halves into two 8 ohm cabs and plugged each one into the two different outputs on the head. You probably know this but the old heads didn't have a switch for impedance, when you plugged the 2nd cab into the extension jack you ran off the two ohm tap instead of the 4 ohm tap. It never sounded right and didn't really have the oomph it had when the cab was whole so I bought an adapter and plugged them both into main output and what a difference. But I was able to see the difference because it was basically the same cab.

 

That's true...I don't. Only once in a while when nobody else is home.

 

I used to run my SVT 2-PRO through an 8 ohm 2x10, did it for years, gigging heavily with absolutely ZERO issues (other than physical pain lol).
These days on the somewhat rare occasion that I need the SVT I bring my 4x10HLF, or two 8 ohm cabs (2x10 & 4x10HE).
Me personally, I wouldn't mismatch impedances. Probably ok to do once or twice, but I wouldn't risk it. But I have five Ampeg cabinets at my disposal so I never run into that issue.

 

As an SVT-CL owner, the best advice I can give you, is have it gone over by someone you trust, a properly set-up SVT will last many years of normal use, read the instructions, use only a 2 or 4 ohm cabinet, if your going to retube it, it's not cheap, but, what I did, several years ago, was I used Sovtek 6550's, which were burned in, from T&C tubes, and used a balanced phase inverter tube, and a set of JJ's for the driver tubes, I got those from Euro Tubes, I recently swapped out the 2 tubes, in the preamp, for some TungSol's, still playing around, in deciding whether they will stay, but I like what I'm hearing
It's a good amp, just don't abuse it, yes, 8 ohm cabinets will work, but you won't get maximum power out of the amp, and there's the slight chance of shortening your power tube life, but one of the amps designers, had said, 8 ohm load won't hurt it.
To answer you question, on a 2.67 ohm load, just place the switch to the 2 ohm setting, I've done it many times, with my SVT-CL & Super Bassman

 

When I was running an SVT-CL I had two cabs, the 810 classic and a 410HLF. I almost always used the 410HLF because it had better low end and handled the low B better than the 810.